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Tracking Issue for RFC 3216: "Allow using for<'a> syntax when declaring closures" #97362

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pnkfelix opened this issue May 24, 2022 · 22 comments
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Labels
B-RFC-approved Blocker: Approved by a merged RFC but not yet implemented. B-unstable Blocker: Implemented in the nightly compiler and unstable. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. F-closure_lifetime_binder `#![feature(closure_lifetime_binder)]` S-tracking-needs-summary Status: It's hard to tell what's been done and what hasn't! Someone should do some investigation. T-lang Relevant to the language team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

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@pnkfelix
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pnkfelix commented May 24, 2022

This is a tracking issue for the RFC "Allow using for<'a> syntax when declaring closures" (rust-lang/rfcs#3216).
The feature gate for the issue is #![feature(closure_lifetime_binder)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

Unresolved Questions

None at this time

Implementation history

@pnkfelix pnkfelix added T-lang Relevant to the language team, which will review and decide on the PR/issue. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. labels May 24, 2022
@WaffleLapkin
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I'd like to work on implementing this 👀

@rustbot claim

Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue Jul 14, 2022
…llot

Implement `for<>` lifetime binder for closures

This PR implements RFC 3216 ([TI](rust-lang#97362)) and allows code like the following:

```rust
let _f = for<'a, 'b> |a: &'a A, b: &'b B| -> &'b C { b.c(a) };
//       ^^^^^^^^^^^--- new!
```

cc `@Aaron1011` `@cjgillot`
Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue Jul 14, 2022
…llot

Implement `for<>` lifetime binder for closures

This PR implements RFC 3216 ([TI](rust-lang#97362)) and allows code like the following:

```rust
let _f = for<'a, 'b> |a: &'a A, b: &'b B| -> &'b C { b.c(a) };
//       ^^^^^^^^^^^--- new!
```

cc ``@Aaron1011`` ``@cjgillot``
flip1995 pushed a commit to flip1995/rust-clippy that referenced this issue Jul 15, 2022
Implement `for<>` lifetime binder for closures

This PR implements RFC 3216 ([TI](rust-lang/rust#97362)) and allows code like the following:

```rust
let _f = for<'a, 'b> |a: &'a A, b: &'b B| -> &'b C { b.c(a) };
//       ^^^^^^^^^^^--- new!
```

cc ``@Aaron1011`` ``@cjgillot``
@fmease

This comment was marked as resolved.

@joshtriplett joshtriplett added the S-tracking-needs-summary Status: It's hard to tell what's been done and what hasn't! Someone should do some investigation. label Aug 10, 2022
@fakedrake

This comment was marked as off-topic.

@WaffleLapkin

This comment was marked as off-topic.

@fakedrake

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@WaffleLapkin

This comment was marked as off-topic.

@Jules-Bertholet
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Found an ICE involving this feature: #103736

(Maybe we could use an F-closure_lifetime_binder tag?)

bors bot pushed a commit to stackabletech/operator-rs that referenced this issue Nov 4, 2022
[![Mend Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [Swatinem/rust-cache](https://togithub.com/Swatinem/rust-cache) | action | patch | `v2.0.0` -> `v2.0.2` |
| [clap](https://togithub.com/clap-rs/clap) | dependencies | patch | `3.2.17` -> `3.2.23` |
| [const_format](https://togithub.com/rodrimati1992/const_format_crates) | dependencies | patch | `0.2.26` -> `0.2.30` |
| [darling](https://togithub.com/TedDriggs/darling) | dependencies | patch | `0.14.1` -> `0.14.2` |
| [futures](https://rust-lang.github.io/futures-rs) ([source](https://togithub.com/rust-lang/futures-rs)) | dependencies | patch | `0.3.23` -> `0.3.25` |
| [proc-macro2](https://togithub.com/dtolnay/proc-macro2) | dependencies | patch | `1.0.43` -> `1.0.47` |
| [serde](https://serde.rs) ([source](https://togithub.com/serde-rs/serde)) | dependencies | patch | `1.0.144` -> `1.0.147` |
| [serde_json](https://togithub.com/serde-rs/json) | dependencies | patch | `1.0.85` -> `1.0.87` |
| [serde_yaml](https://togithub.com/dtolnay/serde-yaml) | dependencies | patch | `0.9.10` -> `0.9.14` |
| [snafu](https://togithub.com/shepmaster/snafu) | dependencies | patch | `0.7.1` -> `0.7.3` |
| [syn](https://togithub.com/dtolnay/syn) | dependencies | patch | `1.0.99` -> `1.0.103` |
| [thiserror](https://togithub.com/dtolnay/thiserror) | dependencies | patch | `1.0.32` -> `1.0.37` |
| [tokio](https://tokio.rs) ([source](https://togithub.com/tokio-rs/tokio)) | dependencies | patch | `1.21.1` -> `1.21.2` |
| [tracing](https://tokio.rs) ([source](https://togithub.com/tokio-rs/tracing)) | dependencies | patch | `0.1.36` -> `0.1.37` |
| [tracing-subscriber](https://tokio.rs) ([source](https://togithub.com/tokio-rs/tracing)) | dependencies | patch | `0.3.15` -> `0.3.16` |

---

### Release Notes

<details>
<summary>Swatinem/rust-cache</summary>

### [`v2.0.2`](https://togithub.com/Swatinem/rust-cache/releases/tag/v2.0.2)

[Compare Source](https://togithub.com/Swatinem/rust-cache/compare/v2.0.1...v2.0.2)

-   Avoid calling cargo metadata on pre-cleanup.

### [`v2.0.1`](https://togithub.com/Swatinem/rust-cache/releases/tag/v2.0.1)

[Compare Source](https://togithub.com/Swatinem/rust-cache/compare/v2.0.0...v2.0.1)

-   Primarily just updating dependencies to fix GitHub deprecation notices.

</details>

<details>
<summary>clap-rs/clap</summary>

### [`v3.2.23`](https://togithub.com/clap-rs/clap/releases/tag/v3.2.23)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.22...v3.2.23)

#### \[3.2.23] - 2022-10-24

##### Fixes

-   Upgrade to `textwrap` 0.16

### [`v3.2.22`](https://togithub.com/clap-rs/clap/releases/tag/v3.2.22)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.21...v3.2.22)

##### \[3.2.22] - 2022-09-16

##### Fixes

-   Unify dependencies on `terminal_size` to the 0.2 release

### [`v3.2.21`](https://togithub.com/clap-rs/clap/releases/tag/v3.2.21)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.20...v3.2.21)

#### \[3.2.21] - 2022-09-12

##### Features

-   `TypedValueParser::map` to allow reusing existing value parsers for other purposes

### [`v3.2.20`](https://togithub.com/clap-rs/clap/releases/tag/v3.2.20)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.19...v3.2.20)

#### \[3.2.20] - 2022-09-02

##### Features

-   `ArgMatches::get_count` help for `ArgAction::Count`
-   `ArgMatches::get_flag` help for `ArgAction::SetTrue` / `ArgAction::SetFalse`

### [`v3.2.19`](https://togithub.com/clap-rs/clap/releases/tag/v3.2.19)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.18...v3.2.19)

#### \[3.2.19] - 2022-08-30

##### Fixes

-   *(help)* Ensure required arguments for parent commands aren't shown in their subcommands when using `args_conflicts_with_subcommand`

### [`v3.2.18`](https://togithub.com/clap-rs/clap/blob/HEAD/CHANGELOG.md#&#8203;3218---2022-08-29)

[Compare Source](https://togithub.com/clap-rs/clap/compare/v3.2.17...v3.2.18)

##### Fixes

-   *(help)* `Command::print_help` now respects `Command::colored_help`
-   *(derive)* Improved error messages

</details>

<details>
<summary>rodrimati1992/const_format_crates</summary>

### [`v0.2.29`](https://togithub.com/rodrimati1992/const_format_crates/blob/HEAD/Changelog.md#&#8203;0229)

Added lowercase hexadecimal formatting support.

Breaking: to add lowercase hexadecimal formatting, this crate changed the uppercase hexadecimal formatter from `{:x}` to `{:X}`

### [`v0.2.27`](https://togithub.com/rodrimati1992/const_format_crates/blob/HEAD/Changelog.md#&#8203;0227)

Replacing existing features with these:

-   `"rust_1_64"`: superceeding the soft-deprecated `"more_str_macros"` feature.
-   `"rust_1_51"`: superceeding the soft-deprecated `"const_generics"` feature.
    The new features are enabled by the feature they superceede.

Now the `"fmt"` feature enables the `"rust_1_64"` feature.

</details>

<details>
<summary>TedDriggs/darling</summary>

### [`v0.14.2`](https://togithub.com/TedDriggs/darling/blob/HEAD/CHANGELOG.md#v0142-October-26-2022)

[Compare Source](https://togithub.com/TedDriggs/darling/compare/v0.14.1...v0.14.2)

-   Derived impls of `FromMeta` will now error on literals, rather than silently ignoring them. [#&#8203;193](https://togithub.com/TedDriggs/darling/pull/193)
-   Don't include property paths in compile errors when spans are available. [#&#8203;203](https://togithub.com/TedDriggs/darling/pull/203)

</details>

<details>
<summary>rust-lang/futures-rs</summary>

### [`v0.3.25`](https://togithub.com/rust-lang/futures-rs/blob/HEAD/CHANGELOG.md#&#8203;0325---2022-10-20)

[Compare Source](https://togithub.com/rust-lang/futures-rs/compare/0.3.24...0.3.25)

-   Fix soundness issue in `join!` and `try_join!` macros ([#&#8203;2649](https://togithub.com/rust-lang/futures-rs/issues/2649))
-   Implement `Clone` for `sink::Drain` ([#&#8203;2650](https://togithub.com/rust-lang/futures-rs/issues/2650))

### [`v0.3.24`](https://togithub.com/rust-lang/futures-rs/blob/HEAD/CHANGELOG.md#&#8203;0324---2022-08-29)

[Compare Source](https://togithub.com/rust-lang/futures-rs/compare/0.3.23...0.3.24)

-   Fix incorrect termination of `select_with_strategy` streams ([#&#8203;2635](https://togithub.com/rust-lang/futures-rs/issues/2635))

</details>

<details>
<summary>dtolnay/proc-macro2</summary>

### [`v1.0.47`](https://togithub.com/dtolnay/proc-macro2/releases/tag/1.0.47)

[Compare Source](https://togithub.com/dtolnay/proc-macro2/compare/1.0.46...1.0.47)

-   Fix integer overflow when nesting depth of nested comments exceeds 4 billion ([#&#8203;357](https://togithub.com/dtolnay/proc-macro2/issues/357))

### [`v1.0.46`](https://togithub.com/dtolnay/proc-macro2/releases/tag/1.0.46)

[Compare Source](https://togithub.com/dtolnay/proc-macro2/compare/1.0.45...1.0.46)

-   Make proc_macro2::TokenStream's `FromStr` impl consistent with proc_macro::TokenStream's on strings that begin with a byte order mark `\u{feff}` ([#&#8203;353](https://togithub.com/dtolnay/proc-macro2/issues/353))

### [`v1.0.45`](https://togithub.com/dtolnay/proc-macro2/releases/tag/1.0.45)

[Compare Source](https://togithub.com/dtolnay/proc-macro2/compare/1.0.44...1.0.45)

-   Fix panic on parsing disallowed raw identifiers such as `r#self` ([#&#8203;351](https://togithub.com/dtolnay/proc-macro2/issues/351))

### [`v1.0.44`](https://togithub.com/dtolnay/proc-macro2/releases/tag/1.0.44)

[Compare Source](https://togithub.com/dtolnay/proc-macro2/compare/1.0.43...1.0.44)

-   Expose `span.before()` and `span.after()` to access an empty `Span` located immediately before or after the input span ([#&#8203;348](https://togithub.com/dtolnay/proc-macro2/issues/348), upstream tracking issue: [rust-lang/rust#87552)

</details>

<details>
<summary>serde-rs/serde</summary>

### [`v1.0.147`](https://togithub.com/serde-rs/serde/releases/tag/v1.0.147)

[Compare Source](https://togithub.com/serde-rs/serde/compare/v1.0.146...v1.0.147)

-   Add `serde::de::value::EnumAccessDeserializer` which transforms an `EnumAccess` into a `Deserializer` ([#&#8203;2305](https://togithub.com/serde-rs/serde/issues/2305))

### [`v1.0.146`](https://togithub.com/serde-rs/serde/releases/tag/v1.0.146)

[Compare Source](https://togithub.com/serde-rs/serde/compare/v1.0.145...v1.0.146)

-   Allow internally tagged newtype variant to contain unit ([#&#8203;2303](https://togithub.com/serde-rs/serde/issues/2303), thanks [@&#8203;tage64](https://togithub.com/tage64))

### [`v1.0.145`](https://togithub.com/serde-rs/serde/releases/tag/v1.0.145)

[Compare Source](https://togithub.com/serde-rs/serde/compare/v1.0.144...v1.0.145)

-   Allow RefCell\<T>, Mutex\<T>, and RwLock\<T> to be serialized regardless of whether T is `Sized` ([#&#8203;2282](https://togithub.com/serde-rs/serde/issues/2282), thanks [@&#8203;ChayimFriedman2](https://togithub.com/ChayimFriedman2))

</details>

<details>
<summary>serde-rs/json</summary>

### [`v1.0.87`](https://togithub.com/serde-rs/json/releases/tag/v1.0.87)

[Compare Source](https://togithub.com/serde-rs/json/compare/v1.0.86...v1.0.87)

-   Add `write_i128` and `write_u128` methods to `serde_json::Formatter` to control the formatting of 128-bit integers ([#&#8203;940](https://togithub.com/serde-rs/json/issues/940), thanks [@&#8203;Lucretiel](https://togithub.com/Lucretiel))

### [`v1.0.86`](https://togithub.com/serde-rs/json/releases/tag/v1.0.86)

[Compare Source](https://togithub.com/serde-rs/json/compare/v1.0.85...v1.0.86)

-   Support `arbitrary_precision` feature even in no-std mode ([#&#8203;928](https://togithub.com/serde-rs/json/issues/928), thanks [@&#8203;kvinwang](https://togithub.com/kvinwang))

</details>

<details>
<summary>dtolnay/serde-yaml</summary>

### [`v0.9.14`](https://togithub.com/dtolnay/serde-yaml/releases/tag/0.9.14)

[Compare Source](https://togithub.com/dtolnay/serde-yaml/compare/0.9.13...0.9.14)

-   Implement `Deserializer` for `TaggedValue` and `&TaggedValue` ([#&#8203;339](https://togithub.com/dtolnay/serde-yaml/issues/339))

### [`v0.9.13`](https://togithub.com/dtolnay/serde-yaml/releases/tag/0.9.13)

[Compare Source](https://togithub.com/dtolnay/serde-yaml/compare/0.9.12...0.9.13)

-   Recognize `True`, `TRUE`, `False`, `FALSE` as booleans, `Null`, `NULL` as null ([#&#8203;330](https://togithub.com/dtolnay/serde-yaml/issues/330))

### [`v0.9.12`](https://togithub.com/dtolnay/serde-yaml/releases/tag/0.9.12)

[Compare Source](https://togithub.com/dtolnay/serde-yaml/compare/0.9.11...0.9.12)

-   Support deserializing tagged literal scalar into primitive ([#&#8203;327](https://togithub.com/dtolnay/serde-yaml/issues/327))

    ```yaml
    "foo": !!int |-
      7200
    ```

### [`v0.9.11`](https://togithub.com/dtolnay/serde-yaml/releases/tag/0.9.11)

[Compare Source](https://togithub.com/dtolnay/serde-yaml/compare/0.9.10...0.9.11)

-   Serialize strings on a single line when they do not already contain a newline character, regardless of string length ([#&#8203;321](https://togithub.com/dtolnay/serde-yaml/issues/321), [#&#8203;322](https://togithub.com/dtolnay/serde-yaml/issues/322))

</details>

<details>
<summary>shepmaster/snafu</summary>

### [`v0.7.3`](https://togithub.com/shepmaster/snafu/blob/HEAD/CHANGELOG.md#&#8203;073---2022-10-20)

[Compare Source](https://togithub.com/shepmaster/snafu/compare/0.7.2...0.7.3)

##### Fixed

-   The macro no longer generates invalid code when implicitly-generated
    types (such as backtraces) were used in conjunction with
    `#[snafu(source(from))]` and the type before transformation does not
    implement `std::error::Error`.

[0.7.3]: https://togithub.com/shepmaster/snafu/releases/tag/0.7.3

### [`v0.7.2`](https://togithub.com/shepmaster/snafu/blob/HEAD/CHANGELOG.md#&#8203;072---2022-10-09)

[Compare Source](https://togithub.com/shepmaster/snafu/compare/0.7.1...0.7.2)

##### Added

-   `Report` can be returned from `main` or test functions to provide a
    user-friendly display of errors.

-   A cheat sheet for the most common `#[snafu(...)]` attribute usages
    has been added to the `Snafu` macro's documentation.

-   Optional support for using the standard library's
    `std::backtrace::Backtrace` type via the `backtraces-impl-std`
    feature flag.

-   Optional support for implementing the Provider API using the
    `std::error::Error::provide` method via the `unstable-provider-api`
    feature flag.

-   Optional support for implementing the `core::error::Error` trait
    instead of `std::error::Error` via the `unstable-core-error` feature
    flag.

-   `GenerateImplicitData` has a new method `generate_with_source`.

##### Changed

-   `ErrorCompat::iter_chain` and `ChainCompat` are now available in
    no_std environments.

-   `ChainCompat` now implements `Clone`.

-   The `Debug` implementation for `Location` no longer shows some
    irrelevant internal details.

[0.7.2]: https://togithub.com/shepmaster/snafu/releases/tag/0.7.2

</details>

<details>
<summary>dtolnay/syn</summary>

### [`v1.0.103`](https://togithub.com/dtolnay/syn/releases/tag/1.0.103)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.102...1.0.103)

-   Implement `PartialOrd` for `Cursor` ([#&#8203;1236](https://togithub.com/dtolnay/syn/issues/1236), [#&#8203;1237](https://togithub.com/dtolnay/syn/issues/1237), thanks [@&#8203;CAD97](https://togithub.com/CAD97))

### [`v1.0.102`](https://togithub.com/dtolnay/syn/releases/tag/1.0.102)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.101...1.0.102)

-   More efficient internal representation for `TokenBuffer` ([#&#8203;1223](https://togithub.com/dtolnay/syn/issues/1223), thanks [@&#8203;CAD97](https://togithub.com/CAD97))
-   Fix parsing of a left shift after macro metavariable in type position ([#&#8203;1229](https://togithub.com/dtolnay/syn/issues/1229))

### [`v1.0.101`](https://togithub.com/dtolnay/syn/releases/tag/1.0.101)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.100...1.0.101)

-   Eliminate a bunch of redundant work done by LitStr::parse ([#&#8203;1221](https://togithub.com/dtolnay/syn/issues/1221))

### [`v1.0.100`](https://togithub.com/dtolnay/syn/releases/tag/1.0.100)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.99...1.0.100)

-   Parse closures with explicit empty lifetime parameter list `for<> || -> T {…}` ([#&#8203;1212](https://togithub.com/dtolnay/syn/issues/1212), [rust-lang/rust#97362)
-   Parse `dyn*` provisional syntax ([#&#8203;1213](https://togithub.com/dtolnay/syn/issues/1213), [rust-lang/rust#91611)
-   Parse attributes on the "rest" pattern of a struct in pattern position, `Struct { #[attr] .. }` ([#&#8203;1214](https://togithub.com/dtolnay/syn/issues/1214))
-   Parse `static async` and `static async move` closures ([#&#8203;1215](https://togithub.com/dtolnay/syn/issues/1215), [rust-lang/rust#62290)

</details>

<details>
<summary>dtolnay/thiserror</summary>

### [`v1.0.37`](https://togithub.com/dtolnay/thiserror/releases/tag/1.0.37)

[Compare Source](https://togithub.com/dtolnay/thiserror/compare/1.0.36...1.0.37)

-   Documentation improvements

### [`v1.0.36`](https://togithub.com/dtolnay/thiserror/releases/tag/1.0.36)

[Compare Source](https://togithub.com/dtolnay/thiserror/compare/1.0.35...1.0.36)

-   Documentation improvements ([#&#8203;195](https://togithub.com/dtolnay/thiserror/issues/195), thanks [@&#8203;matklad](https://togithub.com/matklad))

### [`v1.0.35`](https://togithub.com/dtolnay/thiserror/releases/tag/1.0.35)

[Compare Source](https://togithub.com/dtolnay/thiserror/compare/1.0.34...1.0.35)

-   More work on integrating [std::any::Provider](https://doc.rust-lang.org/std/any/trait.Provider.html) for backtrace support
-   Fix *"Multiple applicable `provide` methods in scope"* error when the caller has both std::error::Error and std::any::Provide traits in scope ([#&#8203;185](https://togithub.com/dtolnay/thiserror/issues/185))

### [`v1.0.34`](https://togithub.com/dtolnay/thiserror/releases/tag/1.0.34)

[Compare Source](https://togithub.com/dtolnay/thiserror/compare/1.0.33...1.0.34)

-   Tweak "generic member access" based Backtrace implementation ([#&#8203;184](https://togithub.com/dtolnay/thiserror/issues/184))

### [`v1.0.33`](https://togithub.com/dtolnay/thiserror/releases/tag/1.0.33)

[Compare Source](https://togithub.com/dtolnay/thiserror/compare/1.0.32...1.0.33)

-   Expose backtraces via the new "generic member access" API on the Error trait ([rust-lang/rust#99301, [rust-lang/rust#96024)

</details>

<details>
<summary>tokio-rs/tokio</summary>

### [`v1.21.2`](https://togithub.com/tokio-rs/tokio/releases/tag/tokio-1.21.2)

[Compare Source](https://togithub.com/tokio-rs/tokio/compare/tokio-1.21.1...tokio-1.21.2)

### 1.21.2 (September 27, 2022)

This release removes the dependency on the `once_cell` crate to restore the MSRV of 1.21.x, which is the latest minor version at the time of release. ([#&#8203;5048])

[#&#8203;5048]: https://togithub.com/tokio-rs/tokio/pull/5048

</details>

<details>
<summary>tokio-rs/tracing</summary>

### [`v0.1.37`](https://togithub.com/tokio-rs/tracing/releases/tag/tracing-0.1.37)

[Compare Source](https://togithub.com/tokio-rs/tracing/compare/tracing-0.1.36...tracing-0.1.37)

This release of `tracing` incorporates changes from `tracing-core`
[v0.1.30][core-0.1.30] and `tracing-attributes` [v0.1.23][attrs-0.1.23],
including the new `Subscriber::on_register_dispatch` method for performing late
initialization after a `Subscriber` is registered as a `Dispatch`, and bugfixes
for the `#[instrument]` attribute. Additionally, it fixes instances of the
`bare_trait_objects` lint, which is now a warning on `tracing`'s MSRV and will
become an error in the next edition.

##### Fixed

-   **attributes**: Incorrect handling of inner attributes in `#[instrument]`ed
    functions ([#&#8203;2307])
-   **attributes**: Incorrect location of compiler diagnostic spans generated for
    type errors in `#[instrument]`ed `async fn`s ([#&#8203;2270])
-   **attributes**: Updated `syn` dependency to fix compilation with `-Z
    minimal-versions` ([#&#8203;2246])
-   `bare_trait_objects` warning in `valueset!` macro expansion ([#&#8203;2308])

##### Added

-   **core**: `Subscriber::on_register_dispatch` method ([#&#8203;2269])
-   **core**: `WeakDispatch` type and `Dispatch::downgrade()` function ([#&#8203;2293])

##### Changed

-   `tracing-core`: updated to [0.1.30][core-0.1.30]
-   `tracing-attributes`: updated to [0.1.23][attrs-0.1.23]

##### Documented

-   Added [`tracing-web`][tracing-web] and [`reqwest-tracing`][reqwest-tracing] to related crates ([#&#8203;2283],
    [#&#8203;2331])

Thanks to new contributors [@&#8203;compiler-errors](https://togithub.com/compiler-errors), [@&#8203;e-nomem](https://togithub.com/e-nomem), [@&#8203;WorldSEnder](https://togithub.com/WorldSEnder), [@&#8203;Xiami2012](https://togithub.com/Xiami2012),
and [@&#8203;tl-rodrigo-gryzinski](https://togithub.com/tl-rodrigo-gryzinski), as well as [@&#8203;jswrenn](https://togithub.com/jswrenn) and [@&#8203;CAD97](https://togithub.com/CAD97), for contributing to
this release!

[core-0.1.30]: https://togithub.com/tokio-rs/tracing/releases/tag/tracing-core-0.1.30

[attrs-0.1.23]: https://togithub.com/tokio-rs/tracing/releases/tag/tracing-attributes-0.1.23

[`tracing-web`]: https://crates.io/crates/tracing-web/

[`reqwest-tracing`]: https://crates.io/crates/reqwest-tracing/

[#&#8203;2246]: https://togithub.com/tokio-rs/tracing/pull/2246

[#&#8203;2269]: https://togithub.com/tokio-rs/tracing/pull/2269

[#&#8203;2283]: https://togithub.com/tokio-rs/tracing/pull/2283

[#&#8203;2270]: https://togithub.com/tokio-rs/tracing/pull/2270

[#&#8203;2293]: https://togithub.com/tokio-rs/tracing/pull/2293

[#&#8203;2307]: https://togithub.com/tokio-rs/tracing/pull/2307

[#&#8203;2308]: https://togithub.com/tokio-rs/tracing/pull/2308

[#&#8203;2331]: https://togithub.com/tokio-rs/tracing/pull/2331

</details>

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@Jules-Bertholet
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Suggestion: could it be allowed to elide the return type of an explicitly higher-ranked closure when it is ()? Currently, Rust allows eliding () returns in all other function signatures, even when nothing else can be elided.

@WaffleLapkin
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@Jules-Bertholet I don't think it's a good idea, we may want to allow inferring return type in the future, just like with normal closures.

@Jules-Bertholet
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@WaffleLapkin "Infer return type when it is ()" should be forward-compatible with "infer return type always."

@WaffleLapkin
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@Jules-Bertholet oh, I misunderstood this as "make for<> || { some expression... } return type always (), same as with normal function". Inferring it is probably fine.

@kevincox

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@fee1-dead

This comment was marked as resolved.

gwilymk added a commit to agbrs/agb that referenced this issue Mar 23, 2023
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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [syn](https://togithub.com/dtolnay/syn) | dependencies | major | `1`
-> `2` |

---

### Release Notes

<details>
<summary>dtolnay/syn</summary>

### [`v2.0.8`](https://togithub.com/dtolnay/syn/releases/tag/2.0.8)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.7...2.0.8)

- Treat `try` keyword as 2015-edition identifier in definition of try
macro ([#&#8203;1422](https://togithub.com/dtolnay/syn/issues/1422))

### [`v2.0.7`](https://togithub.com/dtolnay/syn/releases/tag/2.0.7)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.6...2.0.7)

-   Fix parsing of `mut self` inside of Type::BareFn

### [`v2.0.6`](https://togithub.com/dtolnay/syn/releases/tag/2.0.6)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.5...2.0.6)

- Improve error message on missing ';' between statements
([#&#8203;1419](https://togithub.com/dtolnay/syn/issues/1419))
- Keep non-brace macro invocations in trailing expr position as
Expr::Macro
([#&#8203;1420](https://togithub.com/dtolnay/syn/issues/1420))

### [`v2.0.5`](https://togithub.com/dtolnay/syn/releases/tag/2.0.5)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.4...2.0.5)

- Expose `ExprMacro` data structure even when `features="full"` is not
used ([#&#8203;1417](https://togithub.com/dtolnay/syn/issues/1417))

### [`v2.0.4`](https://togithub.com/dtolnay/syn/releases/tag/2.0.4)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.3...2.0.4)

- Improve error reporting when parsing identifiers and paths
([#&#8203;1415](https://togithub.com/dtolnay/syn/issues/1415),
[#&#8203;1416](https://togithub.com/dtolnay/syn/issues/1416))

### [`v2.0.3`](https://togithub.com/dtolnay/syn/releases/tag/2.0.3)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.2...2.0.3)

- Expose `ExprGroup` data structure even when `features="full"` is not
used ([#&#8203;1412](https://togithub.com/dtolnay/syn/issues/1412))

### [`v2.0.2`](https://togithub.com/dtolnay/syn/releases/tag/2.0.2)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.1...2.0.2)

-   Documentation improvements

### [`v2.0.1`](https://togithub.com/dtolnay/syn/releases/tag/2.0.1)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.0...2.0.1)

- Add methods on syn::Meta for reporting error on an incorrect kind of
attribute ([#&#8203;1409](https://togithub.com/dtolnay/syn/issues/1409))

### [`v2.0.0`](https://togithub.com/dtolnay/syn/releases/tag/2.0.0)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.109...2.0.0)

This release contains a batch of syntax tree improvements to incorporate
ongoing Rust language development from the past 3.5 years since syn 1.

It never seems like an ideal time to finalize a syntax tree design,
considering the frankly alarming number of syntax-disrupting language
features currently in flight: keyword generics, restrictions,
capabilities and contexts, conditional constness, new varieties of
literals, dyn revamp such as explicitly dyn-safe traits and dyn-star,
expression syntax in various phases of being added or being torn out
(const blocks, try blocks, raw references), auto traits and negative
impls, generalizations to higher rank trait bounds, async closures and
static async trait methods, postfix keywords, pattern types, return type
notation, unsafe attributes, …

The plan continues to be the same as laid out originally in the 1.0.0
release announcement:

> Be aware that the underlying Rust language will continue to evolve.
Syn is able to accommodate most kinds of Rust grammar changes via the
nonexhaustive enums and `Verbatim` variants in the syntax tree, but we
will plan to put out new major versions on a 12 to 24 month cadence to
incorporate ongoing language changes as needed.

If anything, the takeaway from the 3.5 year longevity of syn 1 is that
this period was tamer from a language development perspective than
anticipated, but that is unlikely to last and I think around 24 months
is still the correct cadence to expect between releases going forward.

<br>

<p align="center"><a href="https://docs.rs/syn/2/syn/">[API
documentation for 2.0]</a></p>

### Breaking changes

-   Minimum required Rust version is raised from rustc 1.31 to 1.56.

##### Expressions

- Support for `box expr` syntax has been deleted, as it has been deleted
recently from rustc.

- Support for type ascription syntax `expr: Type` in expression position
has been deleted.

- Support for unstable `&raw const expr` raw-pointer reference syntax
has been deleted.

- The representation of generic arguments has been unified between
method calls and non-method paths into a single `GenericArgument` type,
which supersedes the previous `GenericMethodArgument` and
`MethodTurbofish`.

- Generic arguments now distinguish between associated types
(`AssocType`) and associated constant values (`AssocConst`). Previously
these would be parsed ambiguously as `Binding`.

- The binary assignment operators in `BinOp` have been renamed to align
with the naming used by the standard library's `core::ops` module's
traits. For example `BinOp::AddEq` is now called `BinOp::AddAssign`.

- `Expr::Struct` struct construction expressions now support structs
which are a variant of an enum associated type of a trait, as in `<Type
as Trait>::Assoc::Variant { ... }`, which has recently been added to
Rust.

- `Expr::Range` now follows the `start` and `end` naming used by the
standard library's `RangeBounds` trait, rather than `from`/`to` or
`lo`/`hi`.

- `Expr::AssignOp` has been merged into `Expr::Binary`, which now
represents both non-assignment and assignment binary operators.

- Stricter parsing of ranges. None of the following are valid
expressions, but were previously accepted by syn: `..=`, `lo..=`, `...`,
`...hi`, `lo...`, `lo...hi`.

- `Expr::Closure` now includes a representation for `for<...>`
lifetimes.

##### Statements

- Variants `Stmt::Expr` (tail-position expression without trailing
semicolon) and `Stmt::Semi` (non-tail expression with trailing
semicolon) have been combined into `Stmt::Expr` with the optional
semicolon represented by `Option<Token![;]>`.

- The syntax tree for `Stmt::Local` has been extended to handle
`let`/`else` syntax.

- Macros in statement position are now uniformly parsed as
`Stmt::Macro`. Previously these would be disambiguated to `Stmt::Item`,
although it was ambiguous whether a macro in statement position would
expand to an item (like `thread_local! { ... }`) vs an expression (like
`println! { ... }`).

##### Patterns

- Pattern parsing for all the different syntactic positions in which
patterns are allowed has been split into `Pat::parse_single` (for
function- and closure-argument position, where top-level `|` is not
allowed), `Pat::parse_multi` (where `|` is allowed) and
`Pat::parse_multi_with_leading_vert` (for the pattern of match arms,
which allow an optional leading `|`). Previously only a single `parse`
behavior was supported and behaved like the new `parse_single`.

- The `Pat` syntax tree now shares more common data structures with the
`Expr` syntax tree where possible, such as for literals, paths, macros,
and ranges in pattern position.

- Parsing of struct field patterns does a better job rejecting bogus
syntax such as `Struct { 0 asdf }` and `Struct { ref mut 0: asdf }`,
which were previously incorrectly accepted.

- `Pat::Range` now supports one-sided ranges by representing the start
and end bound of the range by `Option<Expr>`.

- `Pat::Struct` keeps track of attributes on the optional `..` "rest"
part of the pattern, as in `let Struct { x, #[cfg(any())] .. } = _;`.

- Parsing unary negation now enforces that only literal patterns can be
unarily negated. For example `-self::CONST` and `-const { 0i32 }` are
not valid syntax in pattern position.

- `Pat::TupleStruct` no longer wraps a value of type `PatTuple` but
represents that information in its fields directly.

- A single parenthesized pattern without trailing comma inside the
parentheses is no longer considered a `Pat::Tuple`, it will be parsed as
`Pat::Paren`.

- One-sided range patterns are no longer allowed inside of slice
patterns. `[lo..]` and `[..=hi]` are not considered valid pattern syntax
by Rust.

##### Items

- Typed `self` in a method signature, such as `self: Pin<&mut Self>`,
will now be parsed as `FnArg::Receiver`. This means `self`, whether with
or without an explicit type, is always treated as a `Receiver`.
Previously only the `&self` and `&mut self` shorthand receivers were
parsed as `Receiver`.

- `TraitItem::Method` and `ImplItem::Method` have been renamed to
`TraitItem::Fn` and `ImplItem::Fn`, as they do not necessarily represent
methods if the function signature contains no `self`.

- `Item::Macro2` has been deleted as "macros 2.0" syntax is no longer
considered on track for stabilization.

- Various item kinds now hold `Generics` which didn't used to have them.

- The variadic argument of an extern function signature can now be given
an optional parameter name.

-   `WherePredicate::Eq` is no longer supported.

- `Visibility::Crate` is no longer supported. This syntax has been
removed from rustc.

- Public visibility is now represented by a single `Token![pub]` token
rather than the old `VisPublic` struct.

- `LifetimeDef` is now called `LifetimeParam`. This name makes more
sense in the context of the `GenericParam` enum (which also includes
`TypeParam` and `ConstParam`), and is the name that the Rust Reference
uses.

- Modules and extern blocks (`Item::Mod` and `Item::ForeignMod`) can now
be marked `unsafe`.

##### Attributes

- The syntax tree for `Attribute` has been redesigned. The new API
better accommodates attributes which mix structured and unstructured
content at different levels of nesting.

-   `AttributeArgs` has been removed. Use `Punctuated<Meta, Token![,]>`.

- For parsing attribute contents, `parse_meta()` is superseded by a new
parsing library called `syn::meta`, and the `parse_nested_meta` method
on `Attribute`.

##### Tokens

- In string literals, the handling of non-ASCII whitespace after
trailing `\` now matches what is implemented by rustc. Space, horizontal
tab, line feed, and carriage return are the only 4 whitespace characters
which are supposed to be stripped from the beginning of the next line.

- The delimiter tokens `syn::token::Paren`, `Bracket`, and `Brace` now
store 2 spans (the open and close punctuation separately) rather than
just 1. Use `.join()` to obtain a single `Span` spanning the whole
group.

- Keyword construction now requires a single span; an array of 1 span is
no longer accepted. Use `Token![trait](span)` instead of
`Token![trait]([span])`.

- Some token types have been renamed to conform with terminology used by
the [Rust
Reference](https://doc.rust-lang.org/1.68.0/reference/tokens.html#punctuation).
These are `Add`->`Plus`, `Bang`->`Not`, `Colon2`->`PathSep`,
`Div`->`Slash`, `Dot2`->`DotDot`, `Dot3`->`DotDotDot`, `Rem`->`Percent`,
and `Sub`->`Minus`.

##### More

- Several enums have been made `#[non_exhaustive]` in anticipation of
upcoming language changes. This includes `WherePredicate`, `Lit`, and
`GenericArgument`.

- The `impl Extend<Pair<T, P>> for Punctuated<T, P>` now requires `P:
Default` and will push a default punctuation between the pre-existing
elements and the new ones, if there is not already a trailing
punctuation. Previously it would panic in this situation.

- `ParseStream::parse_terminated` now takes a peek-style punctuation
argument instead of turbofish. Replace `input.parse_terminated::<_,
Token![,]>(Thing::parse)` with `input.parse_terminated(Thing::parse,
Token![,])`.

###
[`v1.0.109`](https://togithub.com/dtolnay/syn/compare/1.0.108...1.0.109)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.108...1.0.109)

### [`v1.0.108`](https://togithub.com/dtolnay/syn/releases/tag/1.0.108)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.107...1.0.108)

- Fix handling of unusual whitespace after escaped newlines in
`LitStr::value`
([#&#8203;1381](https://togithub.com/dtolnay/syn/issues/1381), thanks
[@&#8203;ModProg](https://togithub.com/ModProg))

### [`v1.0.107`](https://togithub.com/dtolnay/syn/releases/tag/1.0.107)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.106...1.0.107)

-   Opt out of `-Zrustdoc-scrape-examples` on docs.rs for now

### [`v1.0.106`](https://togithub.com/dtolnay/syn/releases/tag/1.0.106)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.105...1.0.106)

-   Documentation improvements

### [`v1.0.105`](https://togithub.com/dtolnay/syn/releases/tag/1.0.105)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.104...1.0.105)

- Improve parse errors related to `dyn` and `impl` type syntax
([#&#8203;1245](https://togithub.com/dtolnay/syn/issues/1245))

### [`v1.0.104`](https://togithub.com/dtolnay/syn/releases/tag/1.0.104)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.103...1.0.104)

-   Add `PathArguments::is_none()`

### [`v1.0.103`](https://togithub.com/dtolnay/syn/releases/tag/1.0.103)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.102...1.0.103)

- Implement `PartialOrd` for `Cursor`
([#&#8203;1236](https://togithub.com/dtolnay/syn/issues/1236),
[#&#8203;1237](https://togithub.com/dtolnay/syn/issues/1237), thanks
[@&#8203;CAD97](https://togithub.com/CAD97))

### [`v1.0.102`](https://togithub.com/dtolnay/syn/releases/tag/1.0.102)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.101...1.0.102)

- More efficient internal representation for `TokenBuffer`
([#&#8203;1223](https://togithub.com/dtolnay/syn/issues/1223), thanks
[@&#8203;CAD97](https://togithub.com/CAD97))
- Fix parsing of a left shift after macro metavariable in type position
([#&#8203;1229](https://togithub.com/dtolnay/syn/issues/1229))

### [`v1.0.101`](https://togithub.com/dtolnay/syn/releases/tag/1.0.101)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.100...1.0.101)

- Eliminate a bunch of redundant work done by LitStr::parse
([#&#8203;1221](https://togithub.com/dtolnay/syn/issues/1221))

### [`v1.0.100`](https://togithub.com/dtolnay/syn/releases/tag/1.0.100)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.99...1.0.100)

- Parse closures with explicit empty lifetime parameter list `for<> ||
-> T {…}` ([#&#8203;1212](https://togithub.com/dtolnay/syn/issues/1212),
[https://github.com/rust-lang/rust/issues/97362](https://togithub.com/rust-lang/rust/issues/97362))
- Parse `dyn*` provisional syntax
([#&#8203;1213](https://togithub.com/dtolnay/syn/issues/1213),
[https://github.com/rust-lang/rust/issues/91611](https://togithub.com/rust-lang/rust/issues/91611))
- Parse attributes on the "rest" pattern of a struct in pattern
position, `Struct { #[attr] .. }`
([#&#8203;1214](https://togithub.com/dtolnay/syn/issues/1214))
- Parse `static async` and `static async move` closures
([#&#8203;1215](https://togithub.com/dtolnay/syn/issues/1215),
[https://github.com/rust-lang/rust/issues/62290](https://togithub.com/rust-lang/rust/issues/62290))

### [`v1.0.99`](https://togithub.com/dtolnay/syn/releases/tag/1.0.99)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.98...1.0.99)

-   Add categories and keywords to crates.io metadata

### [`v1.0.98`](https://togithub.com/dtolnay/syn/releases/tag/1.0.98)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.97...1.0.98)

-   Format example code with rustfmt

### [`v1.0.97`](https://togithub.com/dtolnay/syn/releases/tag/1.0.97)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.96...1.0.97)

-   Update examples

### [`v1.0.96`](https://togithub.com/dtolnay/syn/releases/tag/1.0.96)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.95...1.0.96)

- Add a `punct_mut()` method on `syn::punctuated::Pair` to return
`Option<&mut P>`
([#&#8203;1183](https://togithub.com/dtolnay/syn/issues/1183))

### [`v1.0.95`](https://togithub.com/dtolnay/syn/releases/tag/1.0.95)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.94...1.0.95)

- Replace unicode-xid with unicode-ident crate:
https://github.com/dtolnay/unicode-ident

### [`v1.0.94`](https://togithub.com/dtolnay/syn/releases/tag/1.0.94)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.93...1.0.94)

-   Resolve some unused_macro_rules warnings

### [`v1.0.93`](https://togithub.com/dtolnay/syn/releases/tag/1.0.93)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.92...1.0.93)

- Fix compile error when `Some` means something unusual in the
expression namespace of the scope where `custom_keyword` is invoked
([#&#8203;1171](https://togithub.com/dtolnay/syn/issues/1171), thanks
[@&#8203;zakarumych](https://togithub.com/zakarumych))

### [`v1.0.92`](https://togithub.com/dtolnay/syn/releases/tag/1.0.92)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.91...1.0.92)

- Make compatible with Miri's tag-raw-pointers mode
([#&#8203;1166](https://togithub.com/dtolnay/syn/issues/1166), thanks
[@&#8203;saethlin](https://togithub.com/saethlin))

### [`v1.0.91`](https://togithub.com/dtolnay/syn/releases/tag/1.0.91)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.90...1.0.91)

-   impl ToTokens for RangeLimits
-   impl Parse for ExprAwait
-   impl Parse for GenericMethodArgument
-   impl Parse for MethodTurbofish

### [`v1.0.90`](https://togithub.com/dtolnay/syn/releases/tag/1.0.90)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.89...1.0.90)

- Update recommended exhaustive matching idiom to use
`non_exhaustive_omitted_patterns` lint:

    ```rust
    match expr {
        Expr::Array(e) => {…}
        Expr::Assign(e) => {…}
        ...
        Expr::Yield(e) => {…}

        #[cfg_attr(test, deny(non_exhaustive_omitted_patterns))]
        _ => {/* some sane fallback */}
    }
    ```

### [`v1.0.89`](https://togithub.com/dtolnay/syn/releases/tag/1.0.89)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.88...1.0.89)

- Remove support for inner attributes on non-block expressions
([#&#8203;1146](https://togithub.com/dtolnay/syn/issues/1146),
[https://github.com/rust-lang/rust/pull/83312](https://togithub.com/rust-lang/rust/pull/83312))
- Fix panic on comma after `mut self` in a bare fn type
([#&#8203;1148](https://togithub.com/dtolnay/syn/issues/1148))

### [`v1.0.88`](https://togithub.com/dtolnay/syn/releases/tag/1.0.88)

[Compare
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- Parse `..` pattern in pattern of `let`
([#&#8203;1136](https://togithub.com/dtolnay/syn/issues/1136))
- Parse `for<…>` lifetime introducer on closures
([#&#8203;1135](https://togithub.com/dtolnay/syn/issues/1135))
- Reject postfix operators after cast
([#&#8203;1117](https://togithub.com/dtolnay/syn/issues/1117))
- Implement extra-traits for `Nothing`
([#&#8203;1144](https://togithub.com/dtolnay/syn/issues/1144))

### [`v1.0.87`](https://togithub.com/dtolnay/syn/releases/tag/1.0.87)

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- Parse trait bounds containing associated constant constraint
([#&#8203;1138](https://togithub.com/dtolnay/syn/issues/1138))
- Change syntax of where-clauses on generic associated types
([#&#8203;1071](https://togithub.com/dtolnay/syn/issues/1071),
[https://github.com/rust-lang/rust/issues/89122](https://togithub.com/rust-lang/rust/issues/89122))

### [`v1.0.86`](https://togithub.com/dtolnay/syn/releases/tag/1.0.86)

[Compare
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-   Add support for parsing if- and while-let-chains (RFC 2497)

### [`v1.0.85`](https://togithub.com/dtolnay/syn/releases/tag/1.0.85)

[Compare
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- Add `token()` method to each variant of syn::Lit to expose the
underlying token representation with original formatting

### [`v1.0.84`](https://togithub.com/dtolnay/syn/releases/tag/1.0.84)

[Compare
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- Add
[`parse_quote_spanned!`](https://docs.rs/syn/1.0.84/syn/macro.parse_quote_spanned.html)
macro which is a combination `parse_quote!` + `quote_spanned!`

### [`v1.0.83`](https://togithub.com/dtolnay/syn/releases/tag/1.0.83)

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- Fix panic parsing trait impl with qself in trait type: `impl
<Thing<>>::Trait for Type {}`
([#&#8203;1109](https://togithub.com/dtolnay/syn/issues/1109))
- Preserve attributes on let-else stmt: `#[attr] let pat = val else {
return }` ([#&#8203;1110](https://togithub.com/dtolnay/syn/issues/1110))

### [`v1.0.82`](https://togithub.com/dtolnay/syn/releases/tag/1.0.82)

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- Support parenthesized generic argument syntax with `::` disambiguator:
`Fn::() -> !`
([#&#8203;1096](https://togithub.com/dtolnay/syn/issues/1096))

### [`v1.0.81`](https://togithub.com/dtolnay/syn/releases/tag/1.0.81)

[Compare
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- Support arbitrary precision negative literal tokens on rustc 1.56+
([#&#8203;1087](https://togithub.com/dtolnay/syn/issues/1087),
[#&#8203;1088](https://togithub.com/dtolnay/syn/issues/1088))

### [`v1.0.80`](https://togithub.com/dtolnay/syn/releases/tag/1.0.80)

[Compare
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- Parse unstable `~const` syntax in where-clauses
([#&#8203;1083](https://togithub.com/dtolnay/syn/issues/1083), tracking
issue
[https://github.com/rust-lang/rust/issues/67792](https://togithub.com/rust-lang/rust/issues/67792))

### [`v1.0.79`](https://togithub.com/dtolnay/syn/releases/tag/1.0.79)

[Compare
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- Support trailing `+` in `dyn Trait` syntax, including bare (non-`dyn`)
pre-2018 trait object syntax
([#&#8203;1075](https://togithub.com/dtolnay/syn/issues/1075),
[#&#8203;1077](https://togithub.com/dtolnay/syn/issues/1077),
[#&#8203;1078](https://togithub.com/dtolnay/syn/issues/1078),
[#&#8203;1079](https://togithub.com/dtolnay/syn/issues/1079),
[#&#8203;1080](https://togithub.com/dtolnay/syn/issues/1080),
[#&#8203;1081](https://togithub.com/dtolnay/syn/issues/1081))

### [`v1.0.78`](https://togithub.com/dtolnay/syn/releases/tag/1.0.78)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.77...1.0.78)

- Parse trailing `+` in the bounds of `impl Trait` type
([#&#8203;1073](https://togithub.com/dtolnay/syn/issues/1073))

### [`v1.0.77`](https://togithub.com/dtolnay/syn/releases/tag/1.0.77)

[Compare
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- Match Rust 1.57+'s parsing of dot and question mark after a curly
braced macro invocation
([#&#8203;1068](https://togithub.com/dtolnay/syn/issues/1068))

### [`v1.0.76`](https://togithub.com/dtolnay/syn/releases/tag/1.0.76)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.75...1.0.76)

- Parse unstable `let`–`else` syntax
([#&#8203;1050](https://togithub.com/dtolnay/syn/issues/1050),
[#&#8203;1057](https://togithub.com/dtolnay/syn/issues/1057))
- Parse qualified braced variant expressions and patterns: `<E>::V {..}`
([#&#8203;1058](https://togithub.com/dtolnay/syn/issues/1058),
[#&#8203;1059](https://togithub.com/dtolnay/syn/issues/1059))
- Fix precedence of outer attributes in front of an assignment
expression in statement position
([#&#8203;1060](https://togithub.com/dtolnay/syn/issues/1060))

### [`v1.0.75`](https://togithub.com/dtolnay/syn/releases/tag/1.0.75)

[Compare
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-   Improve spans on a method without body inside an impl block
- Documentation improvements
([#&#8203;922](https://togithub.com/dtolnay/syn/issues/922), thanks
[@&#8203;Tamschi](https://togithub.com/Tamschi))

### [`v1.0.74`](https://togithub.com/dtolnay/syn/releases/tag/1.0.74)

[Compare
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- Reject `for<'a> dyn Trait<'a>` syntax; the correct representation is
`dyn for<'a> Trait<'a>`
([#&#8203;1042](https://togithub.com/dtolnay/syn/issues/1042))

### [`v1.0.73`](https://togithub.com/dtolnay/syn/releases/tag/1.0.73)

[Compare
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- Add From\<Ident>, From\<Index>, From\<usize> for Member
([#&#8203;1038](https://togithub.com/dtolnay/syn/issues/1038), thanks
[@&#8203;m-ou-se](https://togithub.com/m-ou-se))

### [`v1.0.72`](https://togithub.com/dtolnay/syn/releases/tag/1.0.72)

[Compare
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- Parse const generics which use value of a previous const generic
parameter as a default value
([#&#8203;1027](https://togithub.com/dtolnay/syn/issues/1027))

### [`v1.0.71`](https://togithub.com/dtolnay/syn/releases/tag/1.0.71)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.70...1.0.71)

- Fix panic deserializing an ItemImpl with a non-path in trait position
([#&#8203;1023](https://togithub.com/dtolnay/syn/issues/1023))
- Parse visibility on impl blocks
([#&#8203;1024](https://togithub.com/dtolnay/syn/issues/1024))
- Fix parsing a type parameter default on the first generic parameter of
an impl block
([#&#8203;1025](https://togithub.com/dtolnay/syn/issues/1025), thanks
[@&#8203;taiki-e](https://togithub.com/taiki-e))

### [`v1.0.70`](https://togithub.com/dtolnay/syn/releases/tag/1.0.70)

[Compare
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- Fix precedence of closure body vs ExprRange rhs: `|| .. .method()`
([#&#8203;1019](https://togithub.com/dtolnay/syn/issues/1019))
- Parse inner attributes inside of structs and enums
([#&#8203;1022](https://togithub.com/dtolnay/syn/issues/1022))

### [`v1.0.69`](https://togithub.com/dtolnay/syn/releases/tag/1.0.69)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.68...1.0.69)

- Improve Parse impls of ExprBox, ExprUnary, ExprLet, ExprClosure,
ExprReference, ExprBreak, ExprContinue, ExprReturn, ExprYield to respect
precedence for parsing a subexpression beginning with the respective
keyword/punctuation
([#&#8203;1007](https://togithub.com/dtolnay/syn/issues/1007),
[#&#8203;1008](https://togithub.com/dtolnay/syn/issues/1008),
[#&#8203;1009](https://togithub.com/dtolnay/syn/issues/1009),
[#&#8203;1010](https://togithub.com/dtolnay/syn/issues/1010),
[#&#8203;1011](https://togithub.com/dtolnay/syn/issues/1011),
[#&#8203;1012](https://togithub.com/dtolnay/syn/issues/1012),
[#&#8203;1013](https://togithub.com/dtolnay/syn/issues/1013),
[#&#8203;1014](https://togithub.com/dtolnay/syn/issues/1014),
[#&#8203;1015](https://togithub.com/dtolnay/syn/issues/1015))

### [`v1.0.68`](https://togithub.com/dtolnay/syn/releases/tag/1.0.68)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.67...1.0.68)

- Preserve span of `LexError` on conversion to `syn::Error`
([#&#8203;1006](https://togithub.com/dtolnay/syn/issues/1006))

### [`v1.0.67`](https://togithub.com/dtolnay/syn/releases/tag/1.0.67)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.66...1.0.67)

- Accept outer attributes in the Parse impl of ExprBlock
([#&#8203;1004](https://togithub.com/dtolnay/syn/issues/1004))

### [`v1.0.66`](https://togithub.com/dtolnay/syn/releases/tag/1.0.66)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.65...1.0.66)

- Parse equality constraints on generic associated types
([#&#8203;979](https://togithub.com/dtolnay/syn/issues/979))
- Parse default value exprs in const generic declarations
([#&#8203;980](https://togithub.com/dtolnay/syn/issues/980))
- Fix infinite loop parsing malformed type ascription expression in
non-`full` mode
([#&#8203;978](https://togithub.com/dtolnay/syn/issues/978))
- Improve Parse impls of ExprArray, ExprAsync, ExprBlock, ExprMacro,
ExprParen, ExprRepeat, ExprStruct, ExprTryBlock, ExprUnsafe to succeed
parsing even in the presence of trailing binary operators
([#&#8203;991](https://togithub.com/dtolnay/syn/issues/991),
[#&#8203;992](https://togithub.com/dtolnay/syn/issues/992),
[#&#8203;993](https://togithub.com/dtolnay/syn/issues/993),
[#&#8203;994](https://togithub.com/dtolnay/syn/issues/994),
[#&#8203;995](https://togithub.com/dtolnay/syn/issues/995),
[#&#8203;996](https://togithub.com/dtolnay/syn/issues/996),
[#&#8203;997](https://togithub.com/dtolnay/syn/issues/997),
[#&#8203;998](https://togithub.com/dtolnay/syn/issues/998),
[#&#8203;999](https://togithub.com/dtolnay/syn/issues/999))

### [`v1.0.65`](https://togithub.com/dtolnay/syn/releases/tag/1.0.65)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.64...1.0.65)

- Parse visibility on macro_rules
([#&#8203;981](https://togithub.com/dtolnay/syn/issues/981), tracking
issue
[rust-lang/rust#&#8203;78855](https://togithub.com/rust-lang/rust/issues/78855))
- Parse leading vert in or-patterns
([#&#8203;982](https://togithub.com/dtolnay/syn/issues/982), matching
the Rust grammar change in
[rust-lang/rust#&#8203;81869](https://togithub.com/rust-lang/rust/issues/81869))
- Parse static with omitted type
([#&#8203;983](https://togithub.com/dtolnay/syn/issues/983))
- Parse placeholder type in type parameter position
([#&#8203;984](https://togithub.com/dtolnay/syn/issues/984))

### [`v1.0.64`](https://togithub.com/dtolnay/syn/releases/tag/1.0.64)

[Compare
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- Avoid `clippy::expl_impl_clone_on_copy` pedantic lint in downstream
custom token types
([#&#8203;976](https://togithub.com/dtolnay/syn/issues/976))

### [`v1.0.63`](https://togithub.com/dtolnay/syn/releases/tag/1.0.63)

[Compare
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- Fix parsing associated types with empty trait bound list after colon
([#&#8203;974](https://togithub.com/dtolnay/syn/issues/974))

### [`v1.0.62`](https://togithub.com/dtolnay/syn/releases/tag/1.0.62)

[Compare
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- Republish to work around docs.rs bug
[https://github.com/rust-lang/docs.rs/issues/1300](https://togithub.com/rust-lang/docs.rs/issues/1300)

### [`v1.0.61`](https://togithub.com/dtolnay/syn/releases/tag/1.0.61)

[Compare
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- Improve assertion failures on invalid use of `Punctuated` API
([#&#8203;970](https://togithub.com/dtolnay/syn/issues/970), thanks
[@&#8203;osa1](https://togithub.com/osa1))
- Add `Lifetime::span` and `Lifetime::set_span` accessors
([#&#8203;971](https://togithub.com/dtolnay/syn/issues/971))

### [`v1.0.60`](https://togithub.com/dtolnay/syn/releases/tag/1.0.60)

[Compare
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- Provide an idiom for testing exhaustiveness of pattern matches on
`Expr`, `Type`, `Pat`, `Item`, `ForeignItem`, `TraitItem`, and
`ImplItem` ([#&#8203;694](https://togithub.com/dtolnay/syn/issues/694))

    ```rust
    match expr {
        Expr::Array(e) => {...}
        Expr::Assign(e) => {...}
        ...
        Expr::Yield(e) => {...}

        #[cfg(test)]
        Expr::__TestExhaustive(_) => unimplemented!(),
        #[cfg(not(test))]
        _ => { /* some sane fallback */ }
    }
    ```

The above is the only supported idiom for exhaustive matching of those
enum. Do not write anything differently as it is not supported.

The conditional compilation on match-arms lets us fail your tests but
not break your library when adding a variant. You will be notified by a
test failure when a variant is added, so that you can add code to handle
it, but your library will continue to compile and work for downstream
users in the interim.

### [`v1.0.59`](https://togithub.com/dtolnay/syn/releases/tag/1.0.59)

[Compare
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- Parse empty supertrait lists consistently with rustc
([#&#8203;952](https://togithub.com/dtolnay/syn/issues/952))
- Fix loss of span information on comma tokens inside of Type::Tuple
during parse
([#&#8203;959](https://togithub.com/dtolnay/syn/issues/959))

### [`v1.0.58`](https://togithub.com/dtolnay/syn/releases/tag/1.0.58)

[Compare
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- Allow literals to parse as a const generic path argument even without
"full" feature enabled
([#&#8203;951](https://togithub.com/dtolnay/syn/issues/951))

    ```rust
    pub struct Struct {
        array: Array<10>,  // used to require `features = "full"`
    }
    ```

### [`v1.0.57`](https://togithub.com/dtolnay/syn/releases/tag/1.0.57)

[Compare
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- Make Punctuated::new available as a `const fn`
([#&#8203;949](https://togithub.com/dtolnay/syn/issues/949))

### [`v1.0.56`](https://togithub.com/dtolnay/syn/releases/tag/1.0.56)

[Compare
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- Add
[`Error::into_compile_error`](https://docs.rs/syn/1.0.56/syn/struct.Error.html#method.into_compile_error)

### [`v1.0.55`](https://togithub.com/dtolnay/syn/releases/tag/1.0.55)

[Compare
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- Preserve None-delimited group around macro metavariable when parsing
`T<$ty>` ([#&#8203;944](https://togithub.com/dtolnay/syn/issues/944),
[#&#8203;945](https://togithub.com/dtolnay/syn/issues/945))

### [`v1.0.54`](https://togithub.com/dtolnay/syn/releases/tag/1.0.54)

[Compare
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- Fix parsing of `impl` items with macro metavariable in the trait path:
`impl $trait for Type {...}`
([#&#8203;942](https://togithub.com/dtolnay/syn/issues/942))

### [`v1.0.53`](https://togithub.com/dtolnay/syn/releases/tag/1.0.53)

[Compare
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- Parse `impl !Trait {...}` syntax
([#&#8203;936](https://togithub.com/dtolnay/syn/issues/936))

### [`v1.0.52`](https://togithub.com/dtolnay/syn/releases/tag/1.0.52)

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- Parse `unsafe extern` block syntax
([#&#8203;918](https://togithub.com/dtolnay/syn/issues/918))
- Parse `unsafe mod` syntax
([#&#8203;919](https://togithub.com/dtolnay/syn/issues/919))
- Parse `const {...}` block syntax
([#&#8203;921](https://togithub.com/dtolnay/syn/issues/921))
- Parse destructuring assignment syntax
([#&#8203;933](https://togithub.com/dtolnay/syn/issues/933))

### [`v1.0.51`](https://togithub.com/dtolnay/syn/releases/tag/1.0.51)

[Compare
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- Allow parsing signatures in which const generic params appear in front
of lifetime params
([#&#8203;920](https://togithub.com/dtolnay/syn/issues/920))

### [`v1.0.50`](https://togithub.com/dtolnay/syn/releases/tag/1.0.50)

[Compare
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- Apply `doc(cfg(...))` on feature gated APIs for docs.rs-rendered
documentation
([#&#8203;925](https://togithub.com/dtolnay/syn/issues/925))

### [`v1.0.48`](https://togithub.com/dtolnay/syn/releases/tag/1.0.48)

[Compare
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-   Rebuild for https://astexplorer.net

### [`v1.0.47`](https://togithub.com/dtolnay/syn/releases/tag/1.0.47)

[Compare
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-   Documentation improvements

### [`v1.0.46`](https://togithub.com/dtolnay/syn/releases/tag/1.0.46)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.45...1.0.46)

- Fix parsing structured attributes that contain nested absolute paths,
such as `#[derive(::serde::Serialize)]`
([#&#8203;909](https://togithub.com/dtolnay/syn/issues/909))

### [`v1.0.45`](https://togithub.com/dtolnay/syn/releases/tag/1.0.45)

[Compare
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- Provide more detailed error messages when parsing specific literal
kind ([#&#8203;908](https://togithub.com/dtolnay/syn/issues/908))

### [`v1.0.44`](https://togithub.com/dtolnay/syn/releases/tag/1.0.44)

[Compare
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- Fix some parsing of patterns inside of None-delimited groups
([#&#8203;907](https://togithub.com/dtolnay/syn/issues/907))

### [`v1.0.43`](https://togithub.com/dtolnay/syn/releases/tag/1.0.43)

[Compare
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-   Add Parse impl for syn::Signature

### [`v1.0.42`](https://togithub.com/dtolnay/syn/releases/tag/1.0.42)

[Compare
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- Fix several bugs involving unusual suffixes on integer and floating
point literal tokens
([#&#8203;898](https://togithub.com/dtolnay/syn/issues/898),
[#&#8203;899](https://togithub.com/dtolnay/syn/issues/899),
[#&#8203;900](https://togithub.com/dtolnay/syn/issues/900))

### [`v1.0.41`](https://togithub.com/dtolnay/syn/releases/tag/1.0.41)

[Compare
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- Fix panic on printing an incomplete (having fewer path segments than
originally parsed with) qualified path (ExprPath, PatPath, TypePath
containing QSelf)
([#&#8203;891](https://togithub.com/dtolnay/syn/issues/891), thanks
[@&#8203;taiki-e](https://togithub.com/taiki-e))
- Fix panic triggered by syntactically invalid overflowing negative
float literal after `.` in a field access position, e.g. `let _ =
obj.-0.9E999999`
([#&#8203;895](https://togithub.com/dtolnay/syn/issues/895), thanks
[@&#8203;sameer](https://togithub.com/sameer))
- Enable using `parse_macro_input!` with a Parser function rather than
type having a Parse impl
([#&#8203;896](https://togithub.com/dtolnay/syn/issues/896), thanks
[@&#8203;sbrocket](https://togithub.com/sbrocket))

### [`v1.0.40`](https://togithub.com/dtolnay/syn/releases/tag/1.0.40)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.39...1.0.40)

- Fix panic on parsing float literals having both an exponent and a
suffix beginning with 'e' or 'E', such as `9e99e999`
([#&#8203;893](https://togithub.com/dtolnay/syn/issues/893))

### [`v1.0.39`](https://togithub.com/dtolnay/syn/releases/tag/1.0.39)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.38...1.0.39)

- Improve compile time by pre-expanding derives
([#&#8203;885](https://togithub.com/dtolnay/syn/issues/885))
- Parse const generic parameters in any order relative to type
parameters ([#&#8203;886](https://togithub.com/dtolnay/syn/issues/886))

### [`v1.0.38`](https://togithub.com/dtolnay/syn/releases/tag/1.0.38)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.37...1.0.38)

- Accept traits with parenthesized path arguments in impls
([#&#8203;880](https://togithub.com/dtolnay/syn/issues/880), thanks
[@&#8203;alecmocatta](https://togithub.com/alecmocatta))

### [`v1.0.37`](https://togithub.com/dtolnay/syn/releases/tag/1.0.37)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.36...1.0.37)

- Handle shebang in a way that matches rustc 1.46+
([#&#8203;876](https://togithub.com/dtolnay/syn/issues/876),
[https://github.com/rust-lang/rust/pull/71487](https://togithub.com/rust-lang/rust/pull/71487),
[https://github.com/rust-lang/rust/pull/73596](https://togithub.com/rust-lang/rust/pull/73596))

    ```rust
    #!//am/i/a/comment

    fn main() {} // ^ shebang
    ```

    ```rust
    #!//am/i/a/comment

    [allow(dead_code)] // ^ not a shebang
    fn main() {}
    ```

- Accept <code>tuple.0.  0</code> as a tuple indexing expression
([#&#8203;877](https://togithub.com/dtolnay/syn/issues/877))

### [`v1.0.36`](https://togithub.com/dtolnay/syn/releases/tag/1.0.36)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.35...1.0.36)

- Add Lit::span, Lit::set_span
([#&#8203;874](https://togithub.com/dtolnay/syn/issues/874))

### [`v1.0.35`](https://togithub.com/dtolnay/syn/releases/tag/1.0.35)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.34...1.0.35)

- Fix parsing of Expr::Field in non-full mode
([#&#8203;870](https://togithub.com/dtolnay/syn/issues/870))

### [`v1.0.34`](https://togithub.com/dtolnay/syn/releases/tag/1.0.34)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.33...1.0.34)

-   Documentation improvements

### [`v1.0.33`](https://togithub.com/dtolnay/syn/releases/tag/1.0.33)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.32...1.0.33)

- Parse `tuple.0.0` as an indexing expression
([https://github.com/rust-lang/rust/pull/71322](https://togithub.com/rust-lang/rust/pull/71322))
- Add `Parse` impls for optional of proc-macro2 types:
`Option<TokenTree>`, `Option<Punct>`, `Option<Literal>`, `Option<Group>`

### [`v1.0.32`](https://togithub.com/dtolnay/syn/releases/tag/1.0.32)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.31...1.0.32)

- Fix parsing $:item macro_rules metavariables containing outer
attributes ([#&#8203;852](https://togithub.com/dtolnay/syn/issues/852))

### [`v1.0.31`](https://togithub.com/dtolnay/syn/releases/tag/1.0.31)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.30...1.0.31)

- Add
[`Expr::parse_without_eager_brace`](https://docs.rs/syn/1.0.31/syn/enum.Expr.html#method.parse_without_eager_brace)
to parse expressions in ambiguous syntactic position.

Rust grammar has an ambiguity where braces sometimes turn a path
expression into a struct initialization and sometimes do not. In the
following code, the expression `S {}` is one expression. Presumably
there is an empty struct `struct S {}` defined somewhere which it is
instantiating.

    ```rust
    let _ = *S {};

    // parsed by rustc as: `*(S {})`
    ```

We would want to parse the above using `Expr::parse` after the `=`
token.

    But in the following, `S {}` is *not* a struct init expression.

    ```rust
    if *S {} {}

    // parsed by rustc as:
    //
    //    if (*S) {
    //        /* empty block */
    //    }
    //    {
    //        /* another empty block */
    //    }
    ```

For that reason we would want to parse if-conditions using
`Expr::parse_without_eager_brace` after the `if` token. Same for similar
syntactic positions such as the condition expr after a `while` token or
the expr at the top of a `match`.

The Rust grammar's choices around which way this ambiguity is resolved
at various syntactic positions is fairly arbitrary. Really either parse
behavior could work in most positions, and language designers just
decide each case based on which is more likely to be what the programmer
had in mind most of the time.

    ```rust
    if return S {} {}

    // parsed by rustc as:
    //
    //    if (return (S {})) {
    //    }
    //
    // but could equally well have been this other arbitrary choice:
    //
    //    if (return S) {
    //    }
    //    {}
    ```

Note the grammar ambiguity on trailing braces is distinct from
precedence and is not captured by assigning a precedence level to the
braced struct init expr in relation to other operators. This can be
illustrated by `return 0..S {}` vs `match 0..S {}`. The former parses as
`return (0..(S {}))` implying tighter precedence for struct init than
`..`, while the latter parses as `match (0..S) {}` implying tighter
precedence for `..` than struct init, a contradiction.

### [`v1.0.30`](https://togithub.com/dtolnay/syn/releases/tag/1.0.30)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.29...1.0.30)

- Parse struct init expressions where the type name is an interpolated
macro_rules metavariable, such as `$struct {}` where $struct:ident
([#&#8203;842](https://togithub.com/dtolnay/syn/issues/842))
- Handle nesting of None-delimited groups
([#&#8203;843](https://togithub.com/dtolnay/syn/issues/843), thanks
[@&#8203;Aaron1011](https://togithub.com/Aaron1011))

### [`v1.0.29`](https://togithub.com/dtolnay/syn/releases/tag/1.0.29)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.28...1.0.29)

- Parse macro call exprs where the macro name is an interpolated
macro_rules metavariable, such as `$macro!()`
([#&#8203;838](https://togithub.com/dtolnay/syn/issues/838))
- Parse paths containing generic parameters where the first path segment
is an interpolated macro_rules metavariable, such as `$seg<'a>`
([#&#8203;839](https://togithub.com/dtolnay/syn/issues/839))

### [`v1.0.28`](https://togithub.com/dtolnay/syn/releases/tag/1.0.28)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.27...1.0.28)

- Recognize empty None-delimited group produced by interpolating a $:vis
macro metavariable when parsing a Visibility
([#&#8203;836](https://togithub.com/dtolnay/syn/issues/836))

### [`v1.0.27`](https://togithub.com/dtolnay/syn/releases/tag/1.0.27)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.26...1.0.27)

- Parse function calls in which the callee is an interpolated macro
variable `$fn(...)`
([#&#8203;833](https://togithub.com/dtolnay/syn/issues/833))

### [`v1.0.26`](https://togithub.com/dtolnay/syn/releases/tag/1.0.26)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.25...1.0.26)

- Parse paths containing an interpolated first component, such as
`$first::rest`
([https://github.com/rust-lang/rust/issues/72608](https://togithub.com/rust-lang/rust/issues/72608),
[#&#8203;832](https://togithub.com/dtolnay/syn/issues/832))

### [`v1.0.25`](https://togithub.com/dtolnay/syn/releases/tag/1.0.25)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.24...1.0.25)

- Parse opt-out `?const` trait bounds
([#&#8203;767](https://togithub.com/dtolnay/syn/issues/767))
- Parse const generics in method generic arguments
([#&#8203;816](https://togithub.com/dtolnay/syn/issues/816), thanks
[@&#8203;yodaldevoid](https://togithub.com/yodaldevoid))
- Parse trait bounds on type alias items
([#&#8203;821](https://togithub.com/dtolnay/syn/issues/821))
- Parse const generics on impl blocks
([#&#8203;822](https://togithub.com/dtolnay/syn/issues/822))
- Fix precedence of attributes on binary expressions to match rustc
([#&#8203;823](https://togithub.com/dtolnay/syn/issues/823))
- Remove parsing of `extern::` paths which were removed from nightly in
January 2019
([#&#8203;825](https://togithub.com/dtolnay/syn/issues/825),
[https://github.com/rust-lang/rust/pull/57572](https://togithub.com/rust-lang/rust/pull/57572))
- Add `Punctuated::clear`, analogous to Vec::clear
([#&#8203;828](https://togithub.com/dtolnay/syn/issues/828))

### [`v1.0.24`](https://togithub.com/dtolnay/syn/releases/tag/1.0.24)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.23...1.0.24)

- Parse `mut self` receiver in function pointer type
([#&#8203;812](https://togithub.com/dtolnay/syn/issues/812),
[#&#8203;814](https://togithub.com/dtolnay/syn/issues/814))
- Parse const trait impls
([#&#8203;813](https://togithub.com/dtolnay/syn/issues/813))
- Improve error reporting inside struct expressions and struct patterns
([#&#8203;818](https://togithub.com/dtolnay/syn/issues/818))

### [`v1.0.23`](https://togithub.com/dtolnay/syn/releases/tag/1.0.23)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.22...1.0.23)

- Parse inner attributes in traits
([#&#8203;803](https://togithub.com/dtolnay/syn/issues/803))
- Parse const underscore in traits and impls
([#&#8203;804](https://togithub.com/dtolnay/syn/issues/804))
- Implement Extend<Error> for Error
([#&#8203;805](https://togithub.com/dtolnay/syn/issues/805))
- Parse Or patterns
([#&#8203;806](https://togithub.com/dtolnay/syn/issues/806))
- Parse outer attributes on Expr\* structs
([#&#8203;807](https://togithub.com/dtolnay/syn/issues/807))
- Parse top level const/static without value
([#&#8203;808](https://togithub.com/dtolnay/syn/issues/808))
- Parse syntactically accepted functions
([#&#8203;809](https://togithub.com/dtolnay/syn/issues/809))
- Parse extern static with value
([#&#8203;810](https://togithub.com/dtolnay/syn/issues/810))

Thanks [@&#8203;taiki-e](https://togithub.com/taiki-e) for all of these.

### [`v1.0.22`](https://togithub.com/dtolnay/syn/releases/tag/1.0.22)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.21...1.0.22)

- Parse literal suffix on byte string, byte, and char literal tokens:
`br#"..."#suffix`, `b'?'suffix`, `'?'suffix`
([#&#8203;799](https://togithub.com/dtolnay/syn/issues/799),
[#&#8203;800](https://togithub.com/dtolnay/syn/issues/800))

### [`v1.0.21`](https://togithub.com/dtolnay/syn/releases/tag/1.0.21)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.20...1.0.21)

-   Documentation improvements

### [`v1.0.20`](https://togithub.com/dtolnay/syn/releases/tag/1.0.20)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.19...1.0.20)

- Improve span of error message when an error during
`syn::Macro::parse_body` is triggered past the last token of the macro
body ([#&#8203;791](https://togithub.com/dtolnay/syn/issues/791))

### [`v1.0.19`](https://togithub.com/dtolnay/syn/releases/tag/1.0.19)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.18...1.0.19)

- Parse a more lenient extern type syntax inside extern blocks
([#&#8203;763](https://togithub.com/dtolnay/syn/issues/763))

### [`v1.0.18`](https://togithub.com/dtolnay/syn/releases/tag/1.0.18)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.17...1.0.18)

- Ignore unparsed empty None-delimited groups at the end of a macro
input ([#&#8203;783](https://togithub.com/dtolnay/syn/issues/783))

### [`v1.0.17`](https://togithub.com/dtolnay/syn/releases/tag/1.0.17)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.16...1.0.17)

-   Expose `syn::Lit` in `default-features = false` mode

### [`v1.0.16`](https://togithub.com/dtolnay/syn/releases/tag/1.0.16)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.15...1.0.16)

- Fix parsing of `&raw` raw reference operator
([https://github.com/rust-lang/rust/issues/64490](https://togithub.com/rust-lang/rust/issues/64490))
to require explicitly specified constness, `&raw mut` or `&raw const`

### [`v1.0.15`](https://togithub.com/dtolnay/syn/releases/tag/1.0.15)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.14...1.0.15)

- Add
[`Punctuated::first_mut`](https://docs.rs/syn/1.0.15/syn/punctuated/struct.Punctuated.html#method.first_mut)
to return a mut reference to the first sequence element

### [`v1.0.14`](https://togithub.com/dtolnay/syn/releases/tag/1.0.14)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.13...1.0.14)

- Produce more helpful error messages from
[Attribute::parse_args](https://docs.rs/syn/1.0/syn/struct.Attribute.html#method.parse_args)

### [`v1.0.13`](https://togithub.com/dtolnay/syn/releases/tag/1.0.13)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.12...1.0.13)

- Allow parse_quote! to parse Vec\<Stmt>, with the same behavior as
Block::parse_within
([#&#8203;741](https://togithub.com/dtolnay/syn/issues/741))

### [`v1.0.12`](https://togithub.com/dtolnay/syn/releases/tag/1.0.12)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.11...1.0.12)

- Reject function signatures with an incorrectly placed receiver
parameter, like `fn f(x: u8, &self)`
- Produce correctly spanned error when parsing punct beyond the end of a
delimited group
([#&#8203;739](https://togithub.com/dtolnay/syn/issues/739))

### [`v1.0.11`](https://togithub.com/dtolnay/syn/releases/tag/1.0.11)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.10...1.0.11)

- Implement quote::IdentFragment for syn::Member and syn::Index so that
spans are preserved when using these types in quote's `format_ident!`
macro

    ```rust
    use quote::format_ident;
    use syn::Index;

    let index: Index = /* ... */;
    let ident = format_ident!("__{}", index); // produces __0, __1, etc
    ```

### [`v1.0.10`](https://togithub.com/dtolnay/syn/releases/tag/1.0.10)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.9...1.0.10)

- Provide `Hash` and `Eq` impls for syn::Member even without
"extra-traits" feature enabled, as this type is commonly useful in a
hashset

### [`v1.0.9`](https://togithub.com/dtolnay/syn/releases/tag/1.0.9)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.8...1.0.9)

- Fix failure to parse tuple struct fields of tuple type starting with
`crate` ([#&#8203;720](https://togithub.com/dtolnay/syn/issues/720),
[#&#8203;723](https://togithub.com/dtolnay/syn/issues/723), thanks
[@&#8203;mystor](https://togithub.com/mystor))
- Fix unexpected tokens being ignored when using Speculative::advance_to
([#&#8203;721](https://togithub.com/dtolnay/syn/issues/721),
[#&#8203;723](https://togithub.com/dtolnay/syn/issues/723), thanks
[@&#8203;mystor](https://togithub.com/mystor))

### [`v1.0.8`](https://togithub.com/dtolnay/syn/releases/tag/1.0.8)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.7...1.0.8)

- Require trailing comma when parsing TypeTuple with one element
([#&#8203;716](https://togithub.com/dtolnay/syn/issues/716), thanks
[@&#8203;8BitMate](https://togithub.com/8BitMate))

### [`v1.0.7`](https://togithub.com/dtolnay/syn/releases/tag/1.0.7)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.6...1.0.7)

- Add a receiver getter to syn::Signature
([#&#8203;714](https://togithub.com/dtolnay/syn/issues/714), thanks
[@&#8203;mystor](https://togithub.com/mystor))

    ```rust
    impl Signature {
/// A method's `self` receiver, such as `&self` or `self: Box<Self>`.
        pub fn receiver(&self) -> Option<&FnArg>;
    }
    ```

### [`v1.0.6`](https://togithub.com/dtolnay/syn/releases/tag/1.0.6)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.5...1.0.6)

- Add conversions from Item{Struct,Enum,Union} into DeriveInput
([#&#8203;711](https://togithub.com/dtolnay/syn/issues/711), thanks
[@&#8203;mystor](https://togithub.com/mystor))
- Add Fields::len and Fields::is_empty
([#&#8203;712](https://togithub.com/dtolnay/syn/issues/712), thanks
[@&#8203;mjbshaw](https://togithub.com/mjbshaw))

### [`v1.0.5`](https://togithub.com/dtolnay/syn/releases/tag/1.0.5)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.4...1.0.5)

- Expose syn::Variadic with just "derive" feature, not "full", because
it appears in Type::BareFn

### [`v1.0.4`](https://togithub.com/dtolnay/syn/releases/tag/1.0.4)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.3...1.0.4)

- Fix size_hint of Punctuated iterators
([#&#8203;700](https://togithub.com/dtolnay/syn/issues/700), thanks
[@&#8203;ExpHP](https://togithub.com/ExpHP))

### [`v1.0.3`](https://togithub.com/dtolnay/syn/releases/tag/1.0.3)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.2...1.0.3)

- Add
[`Path::get_ident`](https://docs.rs/syn/1.0/syn/struct.Path.html#method.get_ident)
([#&#8203;696](https://togithub.com/dtolnay/syn/issues/696), thanks
[@&#8203;infinity0](https://togithub.com/infinity0))

### [`v1.0.2`](https://togithub.com/dtolnay/syn/releases/tag/1.0.2)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.1...1.0.2)

-   Documentation improvements

### [`v1.0.1`](https://togithub.com/dtolnay/syn/releases/tag/1.0.1)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.0...1.0.1)

- Add
[`LitInt::base10_parse`](https://docs.rs/syn/1.0/syn/struct.LitInt.html#method.base10\_parse)
to produce error that has the right span when parsing literal digits

</details>

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [syn](https://togithub.com/dtolnay/syn) | dependencies | major | `1`
-> `2` |

---

### Release Notes

<details>
<summary>dtolnay/syn (syn)</summary>

### [`v2.0.27`](https://togithub.com/dtolnay/syn/releases/tag/2.0.27)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/2.0.26...2.0.27)

- Documentation improvements (thanks
[@&#8203;GuillaumeGomez](https://togithub.com/GuillaumeGomez))

### [`v2.0.26`](https://togithub.com/dtolnay/syn/releases/tag/2.0.26)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/2.0.25...2.0.26)

- Implement `Spanned` for `QSelf`
([#&#8203;1465](https://togithub.com/dtolnay/syn/issues/1465))

### [`v2.0.25`](https://togithub.com/dtolnay/syn/releases/tag/2.0.25)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/2.0.24...2.0.25)

- Support single identifier as unbraced const generic argument
([#&#8203;1483](https://togithub.com/dtolnay/syn/issues/1483))
- Produce error message when LitStr::parse is used on a suffixed string
literal ([#&#8203;1484](https://togithub.com/dtolnay/syn/issues/1484))

### [`v2.0.24`](https://togithub.com/dtolnay/syn/releases/tag/2.0.24)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/2.0.23...2.0.24)

- Fix duplication of braces around const generic argument in non-full
mode ([#&#8203;1482](https://togithub.com/dtolnay/syn/issues/1482))

### [`v2.0.23`](https://togithub.com/dtolnay/syn/releases/tag/2.0.23)

[Compare
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- Preserve attributes on verbatim Item in statement position
([#&#8203;1476](https://togithub.com/dtolnay/syn/issues/1476))
- Support generic_const_exprs where-clauses such as `where [(); {
T::COUNT }]:` in non-"full" mode
([#&#8203;1478](https://togithub.com/dtolnay/syn/issues/1478))

### [`v2.0.22`](https://togithub.com/dtolnay/syn/releases/tag/2.0.22)

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- Parse `c"…"` c-string literals (tracking issue:
[https://github.com/rust-lang/rust/issues/105723](https://togithub.com/rust-lang/rust/issues/105723))

### [`v2.0.21`](https://togithub.com/dtolnay/syn/releases/tag/2.0.21)

[Compare
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- Fix value computed by `LitByteStr::value` in the case of a cooked byte
string literal containing form feed or vertical tab characters following
an escaped newline
([#&#8203;1474](https://togithub.com/dtolnay/syn/issues/1474))

### [`v2.0.20`](https://togithub.com/dtolnay/syn/releases/tag/2.0.20)

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-   Documentation improvements

### [`v2.0.19`](https://togithub.com/dtolnay/syn/releases/tag/2.0.19)

[Compare
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- Improve rendering of compile errors within 2015-edition code calling a
2018+ edition proc macro
([#&#8203;1467](https://togithub.com/dtolnay/syn/issues/1467), thanks
[@&#8203;danielhenrymantilla](https://togithub.com/danielhenrymantilla))

### [`v2.0.18`](https://togithub.com/dtolnay/syn/releases/tag/2.0.18)

[Compare
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- Permit empty attr in syn::meta::parser
([#&#8203;1460](https://togithub.com/dtolnay/syn/issues/1460))

### [`v2.0.17`](https://togithub.com/dtolnay/syn/releases/tag/2.0.17)

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- Enable proc_macro support on wasm targets
([#&#8203;1459](https://togithub.com/dtolnay/syn/issues/1459))

### [`v2.0.16`](https://togithub.com/dtolnay/syn/releases/tag/2.0.16)

[Compare
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- Parse `builtin #` syntax as Expr::Verbatim
([https://github.com/rust-lang/rust/issues/110680](https://togithub.com/rust-lang/rust/issues/110680),
[#&#8203;1454](https://togithub.com/dtolnay/syn/issues/1454))

### [`v2.0.15`](https://togithub.com/dtolnay/syn/releases/tag/2.0.15)

[Compare
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- Ensure `Type::Tuple` of length 1 prints as a tuple even if trailing
comma is not provided in the Punctuated
([#&#8203;1444](https://togithub.com/dtolnay/syn/issues/1444), thanks
[@&#8203;Fancyflame](https://togithub.com/Fancyflame))

### [`v2.0.14`](https://togithub.com/dtolnay/syn/releases/tag/2.0.14)

[Compare
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- Add Punctuated::pop_punct()
([#&#8203;1442](https://togithub.com/dtolnay/syn/issues/1442), thanks
[@&#8203;programmerjake](https://togithub.com/programmerjake))

### [`v2.0.13`](https://togithub.com/dtolnay/syn/releases/tag/2.0.13)

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- Improve spans of Expr::Field parsed from a float Literal
([#&#8203;1433](https://togithub.com/dtolnay/syn/issues/1433),
[#&#8203;1436](https://togithub.com/dtolnay/syn/issues/1436))

### [`v2.0.12`](https://togithub.com/dtolnay/syn/releases/tag/2.0.12)

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- Refer to `compile_error!` by absolute path in token stream produced by
syn::Error::to_compile_error
([#&#8203;1431](https://togithub.com/dtolnay/syn/issues/1431), thanks
[@&#8203;smoelius](https://togithub.com/smoelius))

### [`v2.0.11`](https://togithub.com/dtolnay/syn/releases/tag/2.0.11)

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- Improve error message on empty parens inside parse_nested_meta
([#&#8203;1428](https://togithub.com/dtolnay/syn/issues/1428))

### [`v2.0.10`](https://togithub.com/dtolnay/syn/releases/tag/2.0.10)

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- Fix visibility being parsed incorrectly on macro invocations inside of
a trait

### [`v2.0.9`](https://togithub.com/dtolnay/syn/releases/tag/2.0.9)

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- Disallow `type` items in an extern block, trait, or module from being
marked `default`

### [`v2.0.8`](https://togithub.com/dtolnay/syn/releases/tag/2.0.8)

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- Treat `try` keyword as 2015-edition identifier in definition of try
macro ([#&#8203;1422](https://togithub.com/dtolnay/syn/issues/1422))

### [`v2.0.7`](https://togithub.com/dtolnay/syn/releases/tag/2.0.7)

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-   Fix parsing of `mut self` inside of Type::BareFn

### [`v2.0.6`](https://togithub.com/dtolnay/syn/releases/tag/2.0.6)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.5...2.0.6)

- Improve error message on missing ';' between statements
([#&#8203;1419](https://togithub.com/dtolnay/syn/issues/1419))
- Keep non-brace macro invocations in trailing expr position as
Expr::Macro
([#&#8203;1420](https://togithub.com/dtolnay/syn/issues/1420))

### [`v2.0.5`](https://togithub.com/dtolnay/syn/releases/tag/2.0.5)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.4...2.0.5)

- Expose `ExprMacro` data structure even when `features="full"` is not
used ([#&#8203;1417](https://togithub.com/dtolnay/syn/issues/1417))

### [`v2.0.4`](https://togithub.com/dtolnay/syn/releases/tag/2.0.4)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.3...2.0.4)

- Improve error reporting when parsing identifiers and paths
([#&#8203;1415](https://togithub.com/dtolnay/syn/issues/1415),
[#&#8203;1416](https://togithub.com/dtolnay/syn/issues/1416))

### [`v2.0.3`](https://togithub.com/dtolnay/syn/releases/tag/2.0.3)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.2...2.0.3)

- Expose `ExprGroup` data structure even when `features="full"` is not
used ([#&#8203;1412](https://togithub.com/dtolnay/syn/issues/1412))

### [`v2.0.2`](https://togithub.com/dtolnay/syn/releases/tag/2.0.2)

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-   Documentation improvements

### [`v2.0.1`](https://togithub.com/dtolnay/syn/releases/tag/2.0.1)

[Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.0...2.0.1)

- Add methods on syn::Meta for reporting error on an incorrect kind of
attribute ([#&#8203;1409](https://togithub.com/dtolnay/syn/issues/1409))

### [`v2.0.0`](https://togithub.com/dtolnay/syn/releases/tag/2.0.0)

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This release contains a batch of syntax tree improvements to incorporate
ongoing Rust language development from the past 3.5 years since syn 1.

It never seems like an ideal time to finalize a syntax tree design,
considering the frankly alarming number of syntax-disrupting language
features currently in flight: keyword generics, restrictions,
capabilities and contexts, conditional constness, new varieties of
literals, dyn revamp such as explicitly dyn-safe traits and dyn-star,
expression syntax in various phases of being added or being torn out
(const blocks, try blocks, raw references), auto traits and negative
impls, generalizations to higher rank trait bounds, async closures and
static async trait methods, postfix keywords, pattern types, return type
notation, unsafe attributes, …

The plan continues to be the same as laid out originally in the 1.0.0
release announcement:

> Be aware that the underlying Rust language will continue to evolve.
Syn is able to accommodate most kinds of Rust grammar changes via the
nonexhaustive enums and `Verbatim` variants in the syntax tree, but we
will plan to put out new major versions on a 12 to 24 month cadence to
incorporate ongoing language changes as needed.

If anything, the takeaway from the 3.5 year longevity of syn 1 is that
this period was tamer from a language development perspective than
anticipated, but that is unlikely to last and I think around 24 months
is still the correct cadence to expect between releases going forward.

<br>

<p align="center"><a href="https://docs.rs/syn/2/syn/">[API
documentation for 2.0]</a></p>

### Breaking changes

-   Minimum required Rust version is raised from rustc 1.31 to 1.56.

##### Expressions

- Support for `box expr` syntax has been deleted, as it has been deleted
recently from rustc.

- Support for type ascription syntax `expr: Type` in expression position
has been deleted.

- Support for unstable `&raw const expr` raw-pointer reference syntax
has been deleted.

- The representation of generic arguments has been unified between
method calls and non-method paths into a single `GenericArgument` type,
which supersedes the previous `GenericMethodArgument` and
`MethodTurbofish`.

- Generic arguments now distinguish between associated types
(`AssocType`) and associated constant values (`AssocConst`). Previously
these would be parsed ambiguously as `Binding`.

- The binary assignment operators in `BinOp` have been renamed to align
with the naming used by the standard library's `core::ops` module's
traits. For example `BinOp::AddEq` is now called `BinOp::AddAssign`.

- `Expr::Struct` struct construction expressions now support structs
which are a variant of an enum associated type of a trait, as in `<Type
as Trait>::Assoc::Variant { ... }`, which has recently been added to
Rust.

- `Expr::Range` now follows the `start` and `end` naming used by the
standard library's `RangeBounds` trait, rather than `from`/`to` or
`lo`/`hi`.

- `Expr::AssignOp` has been merged into `Expr::Binary`, which now
represents both non-assignment and assignment binary operators.

- Stricter parsing of ranges. None of the following are valid
expressions, but were previously accepted by syn: `..=`, `lo..=`, `...`,
`...hi`, `lo...`, `lo...hi`.

- `Expr::Closure` now includes a representation for `for<...>`
lifetimes.

##### Statements

- Variants `Stmt::Expr` (tail-position expression without trailing
semicolon) and `Stmt::Semi` (non-tail expression with trailing
semicolon) have been combined into `Stmt::Expr` with the optional
semicolon represented by `Option<Token![;]>`.

- The syntax tree for `Stmt::Local` has been extended to handle
`let`/`else` syntax.

- Macros in statement position are now uniformly parsed as
`Stmt::Macro`. Previously these would be disambiguated to `Stmt::Item`,
although it was ambiguous whether a macro in statement position would
expand to an item (like `thread_local! { ... }`) vs an expression (like
`println! { ... }`).

##### Patterns

- Pattern parsing for all the different syntactic positions in which
patterns are allowed has been split into `Pat::parse_single` (for
function- and closure-argument position, where top-level `|` is not
allowed), `Pat::parse_multi` (where `|` is allowed) and
`Pat::parse_multi_with_leading_vert` (for the pattern of match arms,
which allow an optional leading `|`). Previously only a single `parse`
behavior was supported and behaved like the new `parse_single`.

- The `Pat` syntax tree now shares more common data structures with the
`Expr` syntax tree where possible, such as for literals, paths, macros,
and ranges in pattern position.

- Parsing of struct field patterns does a better job rejecting bogus
syntax such as `Struct { 0 asdf }` and `Struct { ref mut 0: asdf }`,
which were previously incorrectly accepted.

- `Pat::Range` now supports one-sided ranges by representing the start
and end bound of the range by `Option<Expr>`.

- `Pat::Struct` keeps track of attributes on the optional `..` "rest"
part of the pattern, as in `let Struct { x, #[cfg(any())] .. } = _;`.

- Parsing unary negation now enforces that only literal patterns can be
unarily negated. For example `-self::CONST` and `-const { 0i32 }` are
not valid syntax in pattern position.

- `Pat::TupleStruct` no longer wraps a value of type `PatTuple` but
represents that information in its fields directly.

- A single parenthesized pattern without trailing comma inside the
parentheses is no longer considered a `Pat::Tuple`, it will be parsed as
`Pat::Paren`.

- One-sided range patterns are no longer allowed inside of slice
patterns. `[lo..]` and `[..=hi]` are not considered valid pattern syntax
by Rust.

##### Items

- Typed `self` in a method signature, such as `self: Pin<&mut Self>`,
will now be parsed as `FnArg::Receiver`. This means `self`, whether with
or without an explicit type, is always treated as a `Receiver`.
Previously only the `&self` and `&mut self` shorthand receivers were
parsed as `Receiver`.

- `TraitItem::Method` and `ImplItem::Method` have been renamed to
`TraitItem::Fn` and `ImplItem::Fn`, as they do not necessarily represent
methods if the function signature contains no `self`.

- `Item::Macro2` has been deleted as "macros 2.0" syntax is no longer
considered on track for stabilization.

- Various item kinds now hold `Generics` which didn't used to have them.

- The variadic argument of an extern function signature can now be given
an optional parameter name.

-   `WherePredicate::Eq` is no longer supported.

- `Visibility::Crate` is no longer supported. This syntax has been
removed from rustc.

- Public visibility is now represented by a single `Token![pub]` token
rather than the old `VisPublic` struct.

- `LifetimeDef` is now called `LifetimeParam`. This name makes more
sense in the context of the `GenericParam` enum (which also includes
`TypeParam` and `ConstParam`), and is the name that the Rust Reference
uses.

- Modules and extern blocks (`Item::Mod` and `Item::ForeignMod`) can now
be marked `unsafe`.

##### Attributes

- The syntax tree for `Attribute` has been redesigned. The new API
better accommodates attributes which mix structured and unstructured
content at different levels of nesting.

-   `AttributeArgs` has been removed. Use `Punctuated<Meta, Token![,]>`.

- For parsing attribute contents, `parse_meta()` is superseded by a new
parsing library called `syn::meta`, and the `parse_nested_meta` method
on `Attribute`.

##### Tokens

- In string literals, the handling of non-ASCII whitespace after
trailing `\` now matches what is implemented by rustc. Space, horizontal
tab, line feed, and carriage return are the only 4 whitespace characters
which are supposed to be stripped from the beginning of the next line.

- The delimiter tokens `syn::token::Paren`, `Bracket`, and `Brace` now
store 2 spans (the open and close punctuation separately) rather than
just 1. Use `.join()` to obtain a single `Span` spanning the whole
group.

- Keyword construction now requires a single span; an array of 1 span is
no longer accepted. Use `Token![trait](span)` instead of
`Token![trait]([span])`.

- Some token types have been renamed to conform with terminology used by
the [Rust
Reference](https://doc.rust-lang.org/1.68.0/reference/tokens.html#punctuation).
These are `Add`->`Plus`, `Bang`->`Not`, `Colon2`->`PathSep`,
`Div`->`Slash`, `Dot2`->`DotDot`, `Dot3`->`DotDotDot`, `Rem`->`Percent`,
and `Sub`->`Minus`.

##### More

- Several enums have been made `#[non_exhaustive]` in anticipation of
upcoming language changes. This includes `WherePredicate`, `Lit`, and
`GenericArgument`.

- The `impl Extend<Pair<T, P>> for Punctuated<T, P>` now requires `P:
Default` and will push a default punctuation between the pre-existing
elements and the new ones, if there is not already a trailing
punctuation. Previously it would panic in this situation.

- `ParseStream::parse_terminated` now takes a peek-style punctuation
argument instead of turbofish. Replace `input.parse_terminated::<_,
Token![,]>(Thing::parse)` with `input.parse_terminated(Thing::parse,
Token![,])`.

###
[`v1.0.109`](https://togithub.com/dtolnay/syn/compare/1.0.108...1.0.109)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.108...1.0.109)

### [`v1.0.108`](https://togithub.com/dtolnay/syn/releases/tag/1.0.108)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.107...1.0.108)

- Fix handling of unusual whitespace after escaped newlines in
`LitStr::value`
([#&#8203;1381](https://togithub.com/dtolnay/syn/issues/1381), thanks
[@&#8203;ModProg](https://togithub.com/ModProg))

### [`v1.0.107`](https://togithub.com/dtolnay/syn/releases/tag/1.0.107)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.106...1.0.107)

-   Opt out of `-Zrustdoc-scrape-examples` on docs.rs for now

### [`v1.0.106`](https://togithub.com/dtolnay/syn/releases/tag/1.0.106)

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-   Documentation improvements

### [`v1.0.105`](https://togithub.com/dtolnay/syn/releases/tag/1.0.105)

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- Improve parse errors related to `dyn` and `impl` type syntax
([#&#8203;1245](https://togithub.com/dtolnay/syn/issues/1245))

### [`v1.0.104`](https://togithub.com/dtolnay/syn/releases/tag/1.0.104)

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-   Add `PathArguments::is_none()`

### [`v1.0.103`](https://togithub.com/dtolnay/syn/releases/tag/1.0.103)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.102...1.0.103)

- Implement `PartialOrd` for `Cursor`
([#&#8203;1236](https://togithub.com/dtolnay/syn/issues/1236),
[#&#8203;1237](https://togithub.com/dtolnay/syn/issues/1237), thanks
[@&#8203;CAD97](https://togithub.com/CAD97))

### [`v1.0.102`](https://togithub.com/dtolnay/syn/releases/tag/1.0.102)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.101...1.0.102)

- More efficient internal representation for `TokenBuffer`
([#&#8203;1223](https://togithub.com/dtolnay/syn/issues/1223), thanks
[@&#8203;CAD97](https://togithub.com/CAD97))
- Fix parsing of a left shift after macro metavariable in type position
([#&#8203;1229](https://togithub.com/dtolnay/syn/issues/1229))

### [`v1.0.101`](https://togithub.com/dtolnay/syn/releases/tag/1.0.101)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.100...1.0.101)

- Eliminate a bunch of redundant work done by LitStr::parse
([#&#8203;1221](https://togithub.com/dtolnay/syn/issues/1221))

### [`v1.0.100`](https://togithub.com/dtolnay/syn/releases/tag/1.0.100)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.99...1.0.100)

- Parse closures with explicit empty lifetime parameter list `for<> ||
-> T {…}` ([#&#8203;1212](https://togithub.com/dtolnay/syn/issues/1212),
[https://github.com/rust-lang/rust/issues/97362](https://togithub.com/rust-lang/rust/issues/97362))
- Parse `dyn*` provisional syntax
([#&#8203;1213](https://togithub.com/dtolnay/syn/issues/1213),
[https://github.com/rust-lang/rust/issues/91611](https://togithub.com/rust-lang/rust/issues/91611))
- Parse attributes on the "rest" pattern of a struct in pattern
position, `Struct { #[attr] .. }`
([#&#8203;1214](https://togithub.com/dtolnay/syn/issues/1214))
- Parse `static async` and `static async move` closures
([#&#8203;1215](https://togithub.com/dtolnay/syn/issues/1215),
[https://github.com/rust-lang/rust/issues/62290](https://togithub.com/rust-lang/rust/issues/62290))

### [`v1.0.99`](https://togithub.com/dtolnay/syn/releases/tag/1.0.99)

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-   Add categories and keywords to crates.io metadata

### [`v1.0.98`](https://togithub.com/dtolnay/syn/releases/tag/1.0.98)

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-   Format example code with rustfmt

### [`v1.0.97`](https://togithub.com/dtolnay/syn/releases/tag/1.0.97)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.96...1.0.97)

-   Update examples

### [`v1.0.96`](https://togithub.com/dtolnay/syn/releases/tag/1.0.96)

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- Add a `punct_mut()` method on `syn::punctuated::Pair` to return
`Option<&mut P>`
([#&#8203;1183](https://togithub.com/dtolnay/syn/issues/1183))

### [`v1.0.95`](https://togithub.com/dtolnay/syn/releases/tag/1.0.95)

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- Replace unicode-xid with unicode-ident crate:
https://github.com/dtolnay/unicode-ident

### [`v1.0.94`](https://togithub.com/dtolnay/syn/releases/tag/1.0.94)

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-   Resolve some unused_macro_rules warnings

### [`v1.0.93`](https://togithub.com/dtolnay/syn/releases/tag/1.0.93)

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- Fix compile error when `Some` means something unusual in the
expression namespace of the scope where `custom_keyword` is invoked
([#&#8203;1171](https://togithub.com/dtolnay/syn/issues/1171), thanks
[@&#8203;zakarumych](https://togithub.com/zakarumych))

### [`v1.0.92`](https://togithub.com/dtolnay/syn/releases/tag/1.0.92)

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- Make compatible with Miri's tag-raw-pointers mode
([#&#8203;1166](https://togithub.com/dtolnay/syn/issues/1166), thanks
[@&#8203;saethlin](https://togithub.com/saethlin))

### [`v1.0.91`](https://togithub.com/dtolnay/syn/releases/tag/1.0.91)

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-   impl ToTokens for RangeLimits
-   impl Parse for ExprAwait
-   impl Parse for GenericMethodArgument
-   impl Parse for MethodTurbofish

### [`v1.0.90`](https://togithub.com/dtolnay/syn/releases/tag/1.0.90)

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- Update recommended exhaustive matching idiom to use
`non_exhaustive_omitted_patterns` lint:

    ```rust
    match expr {
        Expr::Array(e) => {…}
        Expr::Assign(e) => {…}
        ...
        Expr::Yield(e) => {…}

        #[cfg_attr(test, deny(non_exhaustive_omitted_patterns))]
        _ => {/* some sane fallback */}
    }
    ```

### [`v1.0.89`](https://togithub.com/dtolnay/syn/releases/tag/1.0.89)

[Compare
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- Remove support for inner attributes on non-block expressions
([#&#8203;1146](https://togithub.com/dtolnay/syn/issues/1146),
[https://github.com/rust-lang/rust/pull/83312](https://togithub.com/rust-lang/rust/pull/83312))
- Fix panic on comma after `mut self` in a bare fn type
([#&#8203;1148](https://togithub.com/dtolnay/syn/issues/1148))

### [`v1.0.88`](https://togithub.com/dtolnay/syn/releases/tag/1.0.88)

[Compare
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- Parse `..` pattern in pattern of `let`
([#&#8203;1136](https://togithub.com/dtolnay/syn/issues/1136))
- Parse `for<…>` lifetime introducer on closures
([#&#8203;1135](https://togithub.com/dtolnay/syn/issues/1135))
- Reject postfix operators after cast
([#&#8203;1117](https://togithub.com/dtolnay/syn/issues/1117))
- Implement extra-traits for `Nothing`
([#&#8203;1144](https://togithub.com/dtolnay/syn/issues/1144))

### [`v1.0.87`](https://togithub.com/dtolnay/syn/releases/tag/1.0.87)

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- Parse trait bounds containing associated constant constraint
([#&#8203;1138](https://togithub.com/dtolnay/syn/issues/1138))
- Change syntax of where-clauses on generic associated types
([#&#8203;1071](https://togithub.com/dtolnay/syn/issues/1071),
[https://github.com/rust-lang/rust/issues/89122](https://togithub.com/rust-lang/rust/issues/89122))

### [`v1.0.86`](https://togithub.com/dtolnay/syn/releases/tag/1.0.86)

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-   Add support for parsing if- and while-let-chains (RFC 2497)

### [`v1.0.85`](https://togithub.com/dtolnay/syn/releases/tag/1.0.85)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.84...1.0.85)

- Add `token()` method to each variant of syn::Lit to expose the
underlying token representation with original formatting

### [`v1.0.84`](https://togithub.com/dtolnay/syn/releases/tag/1.0.84)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.83...1.0.84)

- Add
[`parse_quote_spanned!`](https://docs.rs/syn/1.0.84/syn/macro.parse_quote_spanned.html)
macro which is a combination `parse_quote!` + `quote_spanned!`

### [`v1.0.83`](https://togithub.com/dtolnay/syn/releases/tag/1.0.83)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.82...1.0.83)

- Fix panic parsing trait impl with qself in trait type: `impl
<Thing<>>::Trait for Type {}`
([#&#8203;1109](https://togithub.com/dtolnay/syn/issues/1109))
- Preserve attributes on let-else stmt: `#[attr] let pat = val else {
return }` ([#&#8203;1110](https://togithub.com/dtolnay/syn/issues/1110))

### [`v1.0.82`](https://togithub.com/dtolnay/syn/releases/tag/1.0.82)

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- Support parenthesized generic argument syntax with `::` disambiguator:
`Fn::() -> !`
([#&#8203;1096](https://togithub.com/dtolnay/syn/issues/1096))

### [`v1.0.81`](https://togithub.com/dtolnay/syn/releases/tag/1.0.81)

[Compare
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- Support arbitrary precision negative literal tokens on rustc 1.56+
([#&#8203;1087](https://togithub.com/dtolnay/syn/issues/1087),
[#&#8203;1088](https://togithub.com/dtolnay/syn/issues/1088))

### [`v1.0.80`](https://togithub.com/dtolnay/syn/releases/tag/1.0.80)

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- Parse unstable `~const` syntax in where-clauses
([#&#8203;1083](https://togithub.com/dtolnay/syn/issues/1083), tracking
issue
[https://github.com/rust-lang/rust/issues/67792](https://togithub.com/rust-lang/rust/issues/67792))

### [`v1.0.79`](https://togithub.com/dtolnay/syn/releases/tag/1.0.79)

[Compare
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- Support trailing `+` in `dyn Trait` syntax, including bare (non-`dyn`)
pre-2018 trait object syntax
([#&#8203;1075](https://togithub.com/dtolnay/syn/issues/1075),
[#&#8203;1077](https://togithub.com/dtolnay/syn/issues/1077),
[#&#8203;1078](https://togithub.com/dtolnay/syn/issues/1078),
[#&#8203;1079](https://togithub.com/dtolnay/syn/issues/1079),
[#&#8203;1080](https://togithub.com/dtolnay/syn/issues/1080),
[#&#8203;1081](https://togithub.com/dtolnay/syn/issues/1081))

### [`v1.0.78`](https://togithub.com/dtolnay/syn/releases/tag/1.0.78)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.77...1.0.78)

- Parse trailing `+` in the bounds of `impl Trait` type
([#&#8203;1073](https://togithub.com/dtolnay/syn/issues/1073))

### [`v1.0.77`](https://togithub.com/dtolnay/syn/releases/tag/1.0.77)

[Compare
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- Match Rust 1.57+'s parsing of dot and question mark after a curly
braced macro invocation
([#&#8203;1068](https://togithub.com/dtolnay/syn/issues/1068))

### [`v1.0.76`](https://togithub.com/dtolnay/syn/releases/tag/1.0.76)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.75...1.0.76)

- Parse unstable `let`–`else` syntax
([#&#8203;1050](https://togithub.com/dtolnay/syn/issues/1050),
[#&#8203;1057](https://togithub.com/dtolnay/syn/issues/1057))
- Parse qualified braced variant expressions and patterns: `<E>::V {..}`
([#&#8203;1058](https://togithub.com/dtolnay/syn/issues/1058),
[#&#8203;1059](https://togithub.com/dtolnay/syn/issues/1059))
- Fix precedence of outer attributes in front of an assignment
expression in statement position
([#&#8203;1060](https://togithub.com/dtolnay/syn/issues/1060))

### [`v1.0.75`](https://togithub.com/dtolnay/syn/releases/tag/1.0.75)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.74...1.0.75)

-   Improve spans on a method without body inside an impl block
- Documentation improvements
([#&#8203;922](https://togithub.com/dtolnay/syn/issues/922), thanks
[@&#8203;Tamschi](https://togithub.com/Tamschi))

### [`v1.0.74`](https://togithub.com/dtolnay/syn/releases/tag/1.0.74)

[Compare
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- Reject `for<'a> dyn Trait<'a>` syntax; the correct representation is
`dyn for<'a> Trait<'a>`
([#&#8203;1042](https://togithub.com/dtolnay/syn/issues/1042))

### [`v1.0.73`](https://togithub.com/dtolnay/syn/releases/tag/1.0.73)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.72...1.0.73)

- Add From\<Ident>, From\<Index>, From\<usize> for Member
([#&#8203;1038](https://togithub.com/dtolnay/syn/issues/1038), thanks
[@&#8203;m-ou-se](https://togithub.com/m-ou-se))

### [`v1.0.72`](https://togithub.com/dtolnay/syn/releases/tag/1.0.72)

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- Parse const generics which use value of a previous const generic
parameter as a default value
([#&#8203;1027](https://togithub.com/dtolnay/syn/issues/1027))

### [`v1.0.71`](https://togithub.com/dtolnay/syn/releases/tag/1.0.71)

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Source](https://togithub.com/dtolnay/syn/compare/1.0.70...1.0.71)

- Fix panic deserializing an ItemImpl with a non-path in trait position
([#&#8203;1023](https://togithub.com/dtolnay/syn/issues/1023))
- Parse visibility on impl blocks
([#&#8203;1024](https://togithub.com/dtolnay/syn/issues/1024))
- Fix parsing a type parameter default on the first generic parameter of
an impl block
([#&#8203;1025](https://togithub.com/dtolnay/syn/issues/1025), thanks
[@&#8203;taiki-e](https://togithub.com/taiki-e))

### [`v1.0.70`](https://togithub.com/dtolnay/syn/releases/tag/1.0.70)

[Compare
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- Fix precedence of closure body vs ExprRange rhs: `|| .. .method()`
([#&#8203;1019](https://togithub.com/dtolnay/syn/issues/1019))
- Parse inner attributes inside of structs and enums
([#&#8203;1022](https://togithub.com/dtolnay/syn/issues/1022))

### [`v1.0.69`](https://togithub.com/dtolnay/syn/releases/tag/1.0.69)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.68...1.0.69)

- Improve Parse impls of ExprBox, ExprUnary, ExprLet, ExprClosure,
ExprReference, ExprBreak, ExprContinue, ExprReturn, ExprYield to respect
precedence for parsing a subexpression beginning with the respective
keyword/punctuation
([#&#8203;1007](https://togithub.com/dtolnay/syn/issues/1007),
[#&#8203;1008](https://togithub.com/dtolnay/syn/issues/1008),
[#&#8203;1009](https://togithub.com/dtolnay/syn/issues/1009),
[#&#8203;1010](https://togithub.com/dtolnay/syn/issues/1010),
[#&#8203;1011](https://togithub.com/dtolnay/syn/issues/1011),
[#&#8203;1012](https://togithub.com/dtolnay/syn/issues/1012),
[#&#8203;1013](https://togithub.com/dtolnay/syn/issues/1013),
[#&#8203;1014](https://togithub.com/dtolnay/syn/issues/1014),
[#&#8203;1015](https://togithub.com/dtolnay/syn/issues/1015))

### [`v1.0.68`](https://togithub.com/dtolnay/syn/releases/tag/1.0.68)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.67...1.0.68)

- Preserve span of `LexError` on conversion to `syn::Error`
([#&#8203;1006](https://togithub.com/dtolnay/syn/issues/1006))

### [`v1.0.67`](https://togithub.com/dtolnay/syn/releases/tag/1.0.67)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.66...1.0.67)

- Accept outer attributes in the Parse impl of ExprBlock
([#&#8203;1004](https://togithub.com/dtolnay/syn/issues/1004))

### [`v1.0.66`](https://togithub.com/dtolnay/syn/releases/tag/1.0.66)

[Compare
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- Parse equality constraints on generic associated types
([#&#8203;979](https://togithub.com/dtolnay/syn/issues/979))
- Parse default value exprs in const generic declarations
([#&#8203;980](https://togithub.com/dtolnay/syn/issues/980))
- Fix infinite loop parsing malformed type ascription expression in
non-`full` mode
([#&#8203;978](https://togithub.com/dtolnay/syn/issues/978))
- Improve Parse impls of ExprArray, ExprAsync, ExprBlock, ExprMacro,
ExprParen, ExprRepeat, ExprStruct, ExprTryBlock, ExprUnsafe to succeed
parsing even in the presence of trailing binary operators
([#&#8203;991](https://togithub.com/dtolnay/syn/issues/991),
[#&#8203;992](https://togithub.com/dtolnay/syn/issues/992),
[#&#8203;993](https://togithub.com/dtolnay/syn/issues/993),
[#&#8203;994](https://togithub.com/dtolnay/syn/issues/994),
[#&#8203;995](https://togithub.com/dtolnay/syn/issues/995),
[#&#8203;996](https://togithub.com/dtolnay/syn/issues/996),
[#&#8203;997](https://togithub.com/dtolnay/syn/issues/997),
[#&#8203;998](https://togithub.com/dtolnay/syn/issues/998),
[#&#8203;999](https://togithub.com/dtolnay/syn/issues/999))

### [`v1.0.65`](https://togithub.com/dtolnay/syn/releases/tag/1.0.65)

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- Parse visibility on macro_rules
([#&#8203;981](https://togithub.com/dtolnay/syn/issues/981), tracking
issue
[rust-lang/rust#78855](https://togithub.com/rust-lang/rust/issues/78855))
- Parse leading vert in or-patterns
([#&#8203;982](https://togithub.com/dtolnay/syn/issues/982), matching
the Rust grammar change in
[rust-lang/rust#81869](https://togithub.com/rust-lang/rust/issues/81869))
- Parse static with omitted type
([#&#8203;983](https://togithub.com/dtolnay/syn/issues/983))
- Parse placeholder type in type parameter position
([#&#8203;984](https://togithub.com/dtolnay/syn/issues/984))

### [`v1.0.64`](https://togithub.com/dtolnay/syn/releases/tag/1.0.64)

[Compare
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- Avoid `clippy::expl_impl_clone_on_copy` pedantic lint in downstream
custom token types
([#&#8203;976](https://togithub.com/dtolnay/syn/issues/976))

### [`v1.0.63`](https://togithub.com/dtolnay/syn/releases/tag/1.0.63)

[Compare
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- Fix parsing associated types with empty trait bound list after colon
([#&#8203;974](https://togithub.com/dtolnay/syn/issues/974))

### [`v1.0.62`](https://togithub.com/dtolnay/syn/releases/tag/1.0.62)

[Compare
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- Republish to work around docs.rs bug
[https://github.com/rust-lang/docs.rs/issues/1300](https://togithub.com/rust-lang/docs.rs/issues/1300)

### [`v1.0.61`](https://togithub.com/dtolnay/syn/releases/tag/1.0.61)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.60...1.0.61)

- Improve assertion failures on invalid use of `Punctuated` API
([#&#8203;970](https://togithub.com/dtolnay/syn/issues/970), thanks
[@&#8203;osa1](https://togithub.com/osa1))
- Add `Lifetime::span` and `Lifetime::set_span` accessors
([#&#8203;971](https://togithub.com/dtolnay/syn/issues/971))

### [`v1.0.60`](https://togithub.com/dtolnay/syn/releases/tag/1.0.60)

[Compare
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- Provide an idiom for testing exhaustiveness of pattern matches on
`Expr`, `Type`, `Pat`, `Item`, `ForeignItem`, `TraitItem`, and
`ImplItem` ([#&#8203;694](https://togithub.com/dtolnay/syn/issues/694))

    ```rust
    match expr {
        Expr::Array(e) => {...}
        Expr::Assign(e) => {...}
        ...
        Expr::Yield(e) => {...}

        #[cfg(test)]
        Expr::__TestExhaustive(_) => unimplemented!(),
        #[cfg(not(test))]
        _ => { /* some sane fallback */ }
    }
    ```

The above is the only supported idiom for exhaustive matching of those
enum. Do not write anything differently as it is not supported.

The conditional compilation on match-arms lets us fail your tests but
not break your library when adding a variant. You will be notified by a
test failure when a variant is added, so that you can add code to handle
it, but your library will continue to compile and work for downstream
users in the interim.

### [`v1.0.59`](https://togithub.com/dtolnay/syn/releases/tag/1.0.59)

[Compare
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- Parse empty supertrait lists consistently with rustc
([#&#8203;952](https://togithub.com/dtolnay/syn/issues/952))
- Fix loss of span information on comma tokens inside of Type::Tuple
during parse
([#&#8203;959](https://togithub.com/dtolnay/syn/issues/959))

### [`v1.0.58`](https://togithub.com/dtolnay/syn/releases/tag/1.0.58)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.57...1.0.58)

- Allow literals to parse as a const generic path argument even without
"full" feature enabled
([#&#8203;951](https://togithub.com/dtolnay/syn/issues/951))

    ```rust
    pub struct Struct {
        array: Array<10>,  // used to require `features = "full"`
    }
    ```

### [`v1.0.57`](https://togithub.com/dtolnay/syn/releases/tag/1.0.57)

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- Make Punctuated::new available as a `const fn`
([#&#8203;949](https://togithub.com/dtolnay/syn/issues/949))

### [`v1.0.56`](https://togithub.com/dtolnay/syn/releases/tag/1.0.56)

[Compare
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- Add
[`Error::into_compile_error`](https://docs.rs/syn/1.0.56/syn/struct.Error.html#method.into_compile_error)

### [`v1.0.55`](https://togithub.com/dtolnay/syn/releases/tag/1.0.55)

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- Preserve None-delimited group around macro metavariable when parsing
`T<$ty>` ([#&#8203;944](https://togithub.com/dtolnay/syn/issues/944),
[#&#8203;945](https://togithub.com/dtolnay/syn/issues/945))

### [`v1.0.54`](https://togithub.com/dtolnay/syn/releases/tag/1.0.54)

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- Fix parsing of `impl` items with macro metavariable in the trait path:
`impl $trait for Type {...}`
([#&#8203;942](https://togithub.com/dtolnay/syn/issues/942))

### [`v1.0.53`](https://togithub.com/dtolnay/syn/releases/tag/1.0.53)

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- Parse `impl !Trait {...}` syntax
([#&#8203;936](https://togithub.com/dtolnay/syn/issues/936))

### [`v1.0.52`](https://togithub.com/dtolnay/syn/releases/tag/1.0.52)

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- Parse `unsafe extern` block syntax
([#&#8203;918](https://togithub.com/dtolnay/syn/issues/918))
- Parse `unsafe mod` syntax
([#&#8203;919](https://togithub.com/dtolnay/syn/issues/919))
- Parse `const {...}` block syntax
([#&#8203;921](https://togithub.com/dtolnay/syn/issues/921))
- Parse destructuring assignment syntax
([#&#8203;933](https://togithub.com/dtolnay/syn/issues/933))

### [`v1.0.51`](https://togithub.com/dtolnay/syn/releases/tag/1.0.51)

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- Allow parsing signatures in which const generic params appear in front
of lifetime params
([#&#8203;920](https://togithub.com/dtolnay/syn/issues/920))

### [`v1.0.50`](https://togithub.com/dtolnay/syn/releases/tag/1.0.50)

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- Apply `doc(cfg(...))` on feature gated APIs for docs.rs-rendered
documentation
([#&#8203;925](https://togithub.com/dtolnay/syn/issues/925))

### [`v1.0.48`](https://togithub.com/dtolnay/syn/releases/tag/1.0.48)

[Compare
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-   Rebuild for https://astexplorer.net

### [`v1.0.47`](https://togithub.com/dtolnay/syn/releases/tag/1.0.47)

[Compare
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-   Documentation improvements

### [`v1.0.46`](https://togithub.com/dtolnay/syn/releases/tag/1.0.46)

[Compare
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- Fix parsing structured attributes that contain nested absolute paths,
such as `#[derive(::serde::Serialize)]`
([#&#8203;909](https://togithub.com/dtolnay/syn/issues/909))

### [`v1.0.45`](https://togithub.com/dtolnay/syn/releases/tag/1.0.45)

[Compare
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- Provide more detailed error messages when parsing specific literal
kind ([#&#8203;908](https://togithub.com/dtolnay/syn/issues/908))

### [`v1.0.44`](https://togithub.com/dtolnay/syn/releases/tag/1.0.44)

[Compare
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- Fix some parsing of patterns inside of None-delimited groups
([#&#8203;907](https://togithub.com/dtolnay/syn/issues/907))

### [`v1.0.43`](https://togithub.com/dtolnay/syn/releases/tag/1.0.43)

[Compare
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-   Add Parse impl for syn::Signature

### [`v1.0.42`](https://togithub.com/dtolnay/syn/releases/tag/1.0.42)

[Compare
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- Fix several bugs involving unusual suffixes on integer and floating
point literal tokens
([#&#8203;898](https://togithub.com/dtolnay/syn/issues/898),
[#&#8203;899](https://togithub.com/dtolnay/syn/issues/899),
[#&#8203;900](https://togithub.com/dtolnay/syn/issues/900))

### [`v1.0.41`](https://togithub.com/dtolnay/syn/releases/tag/1.0.41)

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- Fix panic on printing an incomplete (having fewer path segments than
originally parsed with) qualified path (ExprPath, PatPath, TypePath
containing QSelf)
([#&#8203;891](https://togithub.com/dtolnay/syn/issues/891), thanks
[@&#8203;taiki-e](https://togithub.com/taiki-e))
- Fix panic triggered by syntactically invalid overflowing negative
float literal after `.` in a field access position, e.g. `let _ =
obj.-0.9E999999`
([#&#8203;895](https://togithub.com/dtolnay/syn/issues/895), thanks
[@&#8203;sameer](https://togithub.com/sameer))
- Enable using `parse_macro_input!` with a Parser function rather than
type having a Parse impl
([#&#8203;896](https://togithub.com/dtolnay/syn/issues/896), thanks
[@&#8203;sbrocket](https://togithub.com/sbrocket))

### [`v1.0.40`](https://togithub.com/dtolnay/syn/releases/tag/1.0.40)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.39...1.0.40)

- Fix panic on parsing float literals having both an exponent and a
suffix beginning with 'e' or 'E', such as `9e99e999`
([#&#8203;893](https://togithub.com/dtolnay/syn/issues/893))

### [`v1.0.39`](https://togithub.com/dtolnay/syn/releases/tag/1.0.39)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.38...1.0.39)

- Improve compile time by pre-expanding derives
([#&#8203;885](https://togithub.com/dtolnay/syn/issues/885))
- Parse const generic parameters in any order relative to type
parameters ([#&#8203;886](https://togithub.com/dtolnay/syn/issues/886))

### [`v1.0.38`](https://togithub.com/dtolnay/syn/releases/tag/1.0.38)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.37...1.0.38)

- Accept traits with parenthesized path arguments in impls
([#&#8203;880](https://togithub.com/dtolnay/syn/issues/880), thanks
[@&#8203;alecmocatta](https://togithub.com/alecmocatta))

### [`v1.0.37`](https://togithub.com/dtolnay/syn/releases/tag/1.0.37)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.36...1.0.37)

- Handle shebang in a way that matches rustc 1.46+
([#&#8203;876](https://togithub.com/dtolnay/syn/issues/876),
[https://github.com/rust-lang/rust/pull/71487](https://togithub.com/rust-lang/rust/pull/71487),
[https://github.com/rust-lang/rust/pull/73596](https://togithub.com/rust-lang/rust/pull/73596))

    ```rust
    #!//am/i/a/comment

    fn main() {} // ^ shebang
    ```

    ```rust
    #!//am/i/a/comment

    [allow(dead_code)] // ^ not a shebang
    fn main() {}
    ```

- Accept <code>tuple.0.  0</code> as a tuple indexing expression
([#&#8203;877](https://togithub.com/dtolnay/syn/issues/877))

### [`v1.0.36`](https://togithub.com/dtolnay/syn/releases/tag/1.0.36)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.35...1.0.36)

- Add Lit::span, Lit::set_span
([#&#8203;874](https://togithub.com/dtolnay/syn/issues/874))

### [`v1.0.35`](https://togithub.com/dtolnay/syn/releases/tag/1.0.35)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.34...1.0.35)

- Fix parsing of Expr::Field in non-full mode
([#&#8203;870](https://togithub.com/dtolnay/syn/issues/870))

### [`v1.0.34`](https://togithub.com/dtolnay/syn/releases/tag/1.0.34)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.33...1.0.34)

-   Documentation improvements

### [`v1.0.33`](https://togithub.com/dtolnay/syn/releases/tag/1.0.33)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.32...1.0.33)

- Parse `tuple.0.0` as an indexing expression
([https://github.com/rust-lang/rust/pull/71322](https://togithub.com/rust-lang/rust/pull/71322))
- Add `Parse` impls for optional of proc-macro2 types:
`Option<TokenTree>`, `Option<Punct>`, `Option<Literal>`, `Option<Group>`

### [`v1.0.32`](https://togithub.com/dtolnay/syn/releases/tag/1.0.32)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.31...1.0.32)

- Fix parsing $:item macro_rules metavariables containing outer
attributes ([#&#8203;852](https://togithub.com/dtolnay/syn/issues/852))

### [`v1.0.31`](https://togithub.com/dtolnay/syn/releases/tag/1.0.31)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.30...1.0.31)

- Add
[`Expr::parse_without_eager_brace`](https://docs.rs/syn/1.0.31/syn/enum.Expr.html#method.parse_without_eager_brace)
to parse expressions in ambiguous syntactic position.

Rust grammar has an ambiguity where braces sometimes turn a path
expression into a struct initialization and sometimes do not. In the
following code, the expression `S {}` is one expression. Presumably
there is an empty struct `struct S {}` defined somewhere which it is
instantiating.

    ```rust
    let _ = *S {};

    // parsed by rustc as: `*(S {})`
    ```

We would want to parse the above using `Expr::parse` after the `=`
token.

    But in the following, `S {}` is *not* a struct init expression.

    ```rust
    if *S {} {}

    // parsed by rustc as:
    //
    //    if (*S) {
    //        /* empty block */
    //    }
    //    {
    //        /* another empty block */
    //    }
    ```

For that reason we would want to parse if-conditions using
`Expr::parse_without_eager_brace` after the `if` token. Same for similar
syntactic positions such as the condition expr after a `while` token or
the expr at the top of a `match`.

The Rust grammar's choices around which way this ambiguity is resolved
at various syntactic positions is fairly arbitrary. Really either parse
behavior could work in most positions, and language designers just
decide each case based on which is more likely to be what the programmer
had in mind most of the time.

    ```rust
    if return S {} {}

    // parsed by rustc as:
    //
    //    if (return (S {})) {
    //    }
    //
    // but could equally well have been this other arbitrary choice:
    //
    //    if (return S) {
    //    }
    //    {}
    ```

Note the grammar ambiguity on trailing braces is distinct from
precedence and is not captured by assigning a precedence level to the
braced struct init expr in relation to other operators. This can be
illustrated by `return 0..S {}` vs `match 0..S {}`. The former parses as
`return (0..(S {}))` implying tighter precedence for struct init than
`..`, while the latter parses as `match (0..S) {}` implying tighter
precedence for `..` than struct init, a contradiction.

### [`v1.0.30`](https://togithub.com/dtolnay/syn/releases/tag/1.0.30)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.29...1.0.30)

- Parse struct init expressions where the type name is an interpolated
macro_rules metavariable, such as `$struct {}` where $struct:ident
([#&#8203;842](https://togithub.com/dtolnay/syn/issues/842))
- Handle nesting of None-delimited groups
([#&#8203;843](https://togithub.com/dtolnay/syn/issues/843), thanks
[@&#8203;Aaron1011](https://togithub.com/Aaron1011))

### [`v1.0.29`](https://togithub.com/dtolnay/syn/releases/tag/1.0.29)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.28...1.0.29)

- Parse macro call exprs where the macro name is an interpolated
macro_rules metavariable, such as `$macro!()`
([#&#8203;838](https://togithub.com/dtolnay/syn/issues/838))
- Parse paths containing generic parameters where the first path segment
is an interpolated macro_rules metavariable, such as `$seg<'a>`
([#&#8203;839](https://togithub.com/dtolnay/syn/issues/839))

### [`v1.0.28`](https://togithub.com/dtolnay/syn/releases/tag/1.0.28)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.27...1.0.28)

- Recognize empty None-delimited group produced by interpolating a $:vis
macro metavariable when parsing a Visibility
([#&#8203;836](https://togithub.com/dtolnay/syn/issues/836))

### [`v1.0.27`](https://togithub.com/dtolnay/syn/releases/tag/1.0.27)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.26...1.0.27)

- Parse function calls in which the callee is an interpolated macro
variable `$fn(...)`
([#&#8203;833](https://togithub.com/dtolnay/syn/issues/833))

### [`v1.0.26`](https://togithub.com/dtolnay/syn/releases/tag/1.0.26)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.25...1.0.26)

- Parse paths containing an interpolated first component, such as
`$first::rest`
([https://github.com/rust-lang/rust/issues/72608](https://togithub.com/rust-lang/rust/issues/72608),
[#&#8203;832](https://togithub.com/dtolnay/syn/issues/832))

### [`v1.0.25`](https://togithub.com/dtolnay/syn/releases/tag/1.0.25)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.24...1.0.25)

- Parse opt-out `?const` trait bounds
([#&#8203;767](https://togithub.com/dtolnay/syn/issues/767))
- Parse const generics in method generic arguments
([#&#8203;816](https://togithub.com/dtolnay/syn/issues/816), thanks
[@&#8203;yodaldevoid](https://togithub.com/yodaldevoid))
- Parse trait bounds on type alias items
([#&#8203;821](https://togithub.com/dtolnay/syn/issues/821))
- Parse const generics on impl blocks
([#&#8203;822](https://togithub.com/dtolnay/syn/issues/822))
- Fix precedence of attributes on binary expressions to match rustc
([#&#8203;823](https://togithub.com/dtolnay/syn/issues/823))
- Remove parsing of `extern::` paths which were removed from nightly in
January 2019
([#&#8203;825](https://togithub.com/dtolnay/syn/issues/825),
[https://github.com/rust-lang/rust/pull/57572](https://togithub.com/rust-lang/rust/pull/57572))
- Add `Punctuated::clear`, analogous to Vec::clear
([#&#8203;828](https://togithub.com/dtolnay/syn/issues/828))

### [`v1.0.24`](https://togithub.com/dtolnay/syn/releases/tag/1.0.24)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.23...1.0.24)

- Parse `mut self` receiver in function pointer type
([#&#8203;812](https://togithub.com/dtolnay/syn/issues/812),
[#&#8203;814](https://togithub.com/dtolnay/syn/issues/814))
- Parse const trait impls
([#&#8203;813](https://togithub.com/dtolnay/syn/issues/813))
- Improve error reporting inside struct expressions and struct patterns
([#&#8203;818](https://togithub.com/dtolnay/syn/issues/818))

### [`v1.0.23`](https://togithub.com/dtolnay/syn/releases/tag/1.0.23)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.22...1.0.23)

- Parse inner attributes in traits
([#&#8203;803](https://togithub.com/dtolnay/syn/issues/803))
- Parse const underscore in traits and impls
([#&#8203;804](https://togithub.com/dtolnay/syn/issues/804))
- Implement Extend<Error> for Error
([#&#8203;805](https://togithub.com/dtolnay/syn/issues/805))
- Parse Or patterns
([#&#8203;806](https://togithub.com/dtolnay/syn/issues/806))
- Parse outer attributes on Expr\* structs
([#&#8203;807](https://togithub.com/dtolnay/syn/issues/807))
- Parse top level const/static without value
([#&#8203;808](https://togithub.com/dtolnay/syn/issues/808))
- Parse syntactically accepted functions
([#&#8203;809](https://togithub.com/dtolnay/syn/issues/809))
- Parse extern static with value
([#&#8203;810](https://togithub.com/dtolnay/syn/issues/810))

Thanks [@&#8203;taiki-e](https://togithub.com/taiki-e) for all of these.

### [`v1.0.22`](https://togithub.com/dtolnay/syn/releases/tag/1.0.22)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.21...1.0.22)

- Parse literal suffix on byte string, byte, and char literal tokens:
`br#"..."#suffix`, `b'?'suffix`, `'?'suffix`
([#&#8203;799](https://togithub.com/dtolnay/syn/issues/799),
[#&#8203;800](https://togithub.com/dtolnay/syn/issues/800))

### [`v1.0.21`](https://togithub.com/dtolnay/syn/releases/tag/1.0.21)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.20...1.0.21)

-   Documentation improvements

### [`v1.0.20`](https://togithub.com/dtolnay/syn/releases/tag/1.0.20)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.19...1.0.20)

- Improve span of error message when an error during
`syn::Macro::parse_body` is triggered past the last token of the macro
body ([#&#8203;791](https://togithub.com/dtolnay/syn/issues/791))

### [`v1.0.19`](https://togithub.com/dtolnay/syn/releases/tag/1.0.19)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.18...1.0.19)

- Parse a more lenient extern type syntax inside extern blocks
([#&#8203;763](https://togithub.com/dtolnay/syn/issues/763))

### [`v1.0.18`](https://togithub.com/dtolnay/syn/releases/tag/1.0.18)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.17...1.0.18)

- Ignore unparsed empty None-delimited groups at the end of a macro
input ([#&#8203;783](https://togithub.com/dtolnay/syn/issues/783))

### [`v1.0.17`](https://togithub.com/dtolnay/syn/releases/tag/1.0.17)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.16...1.0.17)

-   Expose `syn::Lit` in `default-features = false` mode

### [`v1.0.16`](https://togithub.com/dtolnay/syn/releases/tag/1.0.16)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.15...1.0.16)

- Fix parsing of `&raw` raw reference operator
([https://github.com/rust-lang/rust/issues/64490](https://togithub.com/rust-lang/rust/issues/64490))
to require explicitly specified constness, `&raw mut` or `&raw const`

### [`v1.0.15`](https://togithub.com/dtolnay/syn/releases/tag/1.0.15)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.14...1.0.15)

- Add
[`Punctuated::first_mut`](https://docs.rs/syn/1.0.15/syn/punctuated/struct.Punctuated.html#method.first_mut)
to return a mut reference to the first sequence element

### [`v1.0.14`](https://togithub.com/dtolnay/syn/releases/tag/1.0.14)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.13...1.0.14)

- Produce more helpful error messages from
[Attribute::parse_args](https://docs.rs/syn/1.0/syn/struct.Attribute.html#method.parse_args)

### [`v1.0.13`](https://togithub.com/dtolnay/syn/releases/tag/1.0.13)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.12...1.0.13)

- Allow parse_quote! to parse Vec\<Stmt>, with the same behavior as
Block::parse_within
([#&#8203;741](https://togithub.com/dtolnay/syn/issues/741))

### [`v1.0.12`](https://togithub.com/dtolnay/syn/releases/tag/1.0.12)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.11...1.0.12)

- Reject function signatures with an incorrectly placed receiver
parameter, like `fn f(x: u8, &self)`
- Produce correctly spanned error when parsing punct beyond the end of a
delimited group
([#&#8203;739](https://togithub.com/dtolnay/syn/issues/739))

### [`v1.0.11`](https://togithub.com/dtolnay/syn/releases/tag/1.0.11)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.10...1.0.11)

- Implement quote::IdentFragment for syn::Member and syn::Index so that
spans are preserved when using these types in quote's `format_ident!`
macro

    ```rust
    use quote::format_ident;
    use syn::Index;

    let index: Index = /* ... */;
    let ident = format_ident!("__{}", index); // produces __0, __1, etc
    ```

### [`v1.0.10`](https://togithub.com/dtolnay/syn/releases/tag/1.0.10)

[Compare
Source](https://togithub.com/dtolnay/syn/compare/1.0.9...1.0.10)

- Provide `Hash` and `Eq` impls for syn::Member even without
"extra-traits" feature enabled, as this type is commonly useful in a
hashset

### [`v1.0.9`](https://togithub.com/dtolnay/syn/releases/tag/1.0.9)

[Compare Source](https://togithub.com/dtolnay/syn/compare/1.0.8...1.0.9)

- Fix failure to parse tuple struct fields of tuple type starting with
`crate` ([#&#8203;720](https://togithub.com/dtolnay/syn/issues/720),
[#&#8203;723](https://togithub.com/dtolnay/syn/issues/723), thanks
[@&#8203;mystor](https://togithub.com/mystor))
- Fix unexpected tokens being ignored when using Speculative::advance_to
([#&#8203;721](https://togithub.com/dtolnay/syn/issues/721),
[#&#8203;723](https://togithub.com/dtolnay/syn/issues/723), thank

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@fmease fmease added B-RFC-approved Blocker: Approved by a merged RFC but not yet implemented. B-unstable Blocker: Implemented in the nightly compiler and unstable. labels Mar 1, 2024
@zirconium-n
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Is this feature blocked by any ongoing process? The implementation seems completed for a while, maybe we can push this to stabilization?

@WaffleLapkin WaffleLapkin removed their assignment Mar 7, 2024
@opsnull
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opsnull commented Apr 1, 2024

Is this feature blocked by any ongoing process? The implementation seems completed for a while, maybe we can push this to stabilization?

same question here.

@runiq
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runiq commented Apr 2, 2024

A question out of interest: Is this in any way a prerequisite for #68923?

@safinaskar
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Unfortunately, for<'a> |x: &'a ()| -> &'a () { x } and |x: &()| -> &() { x } currently are not equivalent. Here is minimal example:

#![feature(closure_lifetime_binder)]
#![feature(unboxed_closures)]

fn foo<F>(f: F)
where
    F: for<'a> FnOnce<(&'a (),)>,
{}

fn main() {
    foo(for<'a> |x: &'a ()| -> &'a () { x }); // COMPILES
    foo(|x: &()| -> &() { x }); // DOESN'T COMPILE
}

Here is playground:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=47b1d8af93240be0cc1326a892dc01fd

This was discovered because of #34162 . Here is (larger) code, which tries to fix sort_by_key (by Jules-Bertholet):

#![feature(closure_lifetime_binder)]
#![feature(unboxed_closures)]

fn sort_by_key<T, F>(s: &mut [T], mut f: F)
where
    F: for<'a> FnMut<(&'a T,)>,
    // instead of `B: Ord`
    for<'a> <F as FnOnce<(&'a T,)>>::Output: Ord,
{
    s.sort_by(|a, b| f(a).cmp(&f(b)))
}

#[derive(Debug)]
struct Client(String);

impl Client {
    fn key(&self) -> &str {
        &self.0
    }
}

fn main() {
    let mut test = vec![
        Client("c".to_string()),
        Client("a".to_string()),
        Client("b".to_string()),
    ];
    sort_by_key(&mut test, for<'a> |c: &'a Client| -> &'a str { c.key() }); // compiles
    //sort_by_key(&mut test, |c: &Client| -> &str { c.key() }); // doesn't compile
    dbg!(test);
}

The code above compiles on nightly without any additional flags (assuming you choose for<'a> version)

Also, here is another problematic code, which doesn't require unboxed_closures:

#![feature(closure_lifetime_binder)]

trait X<'a> {
}

impl<'a, T, K> X<'a> for T
where
    T: FnOnce(&'a ()) -> K
{}

fn foo<F>(f: F)
where
    F: for<'a> X<'a>,
{}

fn main() {
    foo(for<'a> |x: &'a ()| -> &'a () { x }); // COMPILES
    foo(|x: &()| -> &() { x }); // DOESN'T COMPILE
}

playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bfcdede24f2e7b27380e2990df1af328

@workingjubilee
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So you're saying we need the syntactic ability to specifically annotate all closure lifetimes in order to be able to write the programs we want to write? Some kind of for<'lt> syntax, maybe?

@safinaskar
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@workingjubilee , no. We already have such syntax, it is available with #![feature(closure_lifetime_binder)]. I'm just saying for<'a> |x: &'a ()| -> &'a () { x } should work exactly like |x: &()| -> &() { x }, but it is currently not. And this causes problems as I showed above, which prevent us from having proper ergonomic sort_by_key

@Jules-Bertholet
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Ideally, the compiler would infer the most permissive signature possible, and so the for<'a> syntax would not be necessary (except as documentation).

@workingjubilee
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workingjubilee commented Jun 21, 2024

I'm just saying for<'a> |x: &'a ()| -> &'a () { x } should work exactly like |x: &()| -> &() { x }, but it is currently not.

It is my understanding that the binding of a specific lifetime and a for<'lt> (which, as I understand it, is effectively the ∀ lifetime) are not interchangeable in all cases, for the precise reason that ∀ and ∃ are different quantifiers.

@WanderLanz
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WanderLanz commented Jun 21, 2024

@safinaskar As far as I understand compiler internals, a closure is inherently not invariant in regards to the lifetime of the arguments, and you need strong hints in order to make a closure such similar to a normal fn. For example, the higher ranked closure macros in the lending_iterator crate.

In other words, the compiler is very conservative when closures could possibly have something like a for<'all> where Closure: 'all bound and instead only inherently satisfies FnOnce(&'1 ()) -> &'2 ().

#![feature(closure_lifetime_binder)]

trait X<'a> {
}

impl<'a, T, K> X<'a> for T
where
    T: FnOnce(&'a ()) -> K
{}

fn foo<F>(f: F)
where
    F: for<'a> X<'a>,
{}

fn main() {
    foo(for<'a> |x: &'a ()| -> &'a () { x }); // COMPILES
    // foo(|x: &()| -> &() { x }); // DOESN'T COMPILE
    
    fn hrc_hint<F>(f: F) -> F
    where
        F: for<'all> FnOnce(&'all ()) -> &'all ()
    {
        f
    }
    foo(hrc_hint(|x: &()| -> &() { x })); // NOW COMPILES
}

Finally, to avoid this, as far as I can tell, either: the compiler becomes less conservative on closures (possibly E.2024?), we get bounded HRTBs (for<'all> where *: 'all) (very unlikely), or closure_lifetime_binder becomes popular enough to be stabilized.

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B-RFC-approved Blocker: Approved by a merged RFC but not yet implemented. B-unstable Blocker: Implemented in the nightly compiler and unstable. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. F-closure_lifetime_binder `#![feature(closure_lifetime_binder)]` S-tracking-needs-summary Status: It's hard to tell what's been done and what hasn't! Someone should do some investigation. T-lang Relevant to the language team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.
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