Make iter::Empty<T> Send and Sync for any T #68348
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@bors r+ This makes perfect sense and has no future compat concerns, so an obvious r+. FWIW this PR could also be extended to other auto-traits such as |
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Make iter::Empty<T> Send and Sync for any T Continuing from rust-lang#57682 It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`. Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed ` struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
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Rollup of 5 pull requests Successful merges: - #67682 ([const-prop] Remove useless typedef) - #68133 (Slimmer syntax) - #68154 (Add more BTreeMap/BTreeSet benchmarks regarding iteration) - #68247 (Clean up err codes) - #68348 (Make iter::Empty<T> Send and Sync for any T) Failed merges: - #68353 (Remove `rustc_error_codes` deps except in `rustc_driver`) r? @ghost
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Make iter::Empty<T> Send and Sync for any T Continuing from rust-lang#57682 It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`. Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed ` struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
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Version 1.42.0 (2020-03-12)
==========================
Language
--------
- [You can now use the slice pattern syntax with subslices.][67712] e.g.
```rust
fn foo(words: &[&str]) {
match words {
["Hello", "World", "!", ..] => println!("Hello World!"),
["Foo", "Bar", ..] => println!("Baz"),
rest => println!("{:?}", rest),
}
}
```
- [You can now use `#[repr(transparent)]` on univariant `enum`s.][68122] Meaning
that you can create an enum that has the exact layout and ABI of the type
it contains.
- [There are some *syntax-only* changes:][67131]
- `default` is syntactically allowed before items in `trait` definitions.
- Items in `impl`s (i.e. `const`s, `type`s, and `fn`s) may syntactically
leave out their bodies in favor of `;`.
- Bounds on associated types in `impl`s are now syntactically allowed
(e.g. `type Foo: Ord;`).
- `...` (the C-variadic type) may occur syntactically directly as the type of
any function parameter.
These are still rejected *semantically*, so you will likely receive an error
but these changes can be seen and parsed by procedural macros and
conditional compilation.
Compiler
--------
- [Added tier 2* support for `armv7a-none-eabi`.][68253]
- [Added tier 2 support for `riscv64gc-unknown-linux-gnu`.][68339]
- [`Option::{expect,unwrap}` and
`Result::{expect, expect_err, unwrap, unwrap_err}` now produce panic messages
pointing to the location where they were called, rather than
`core`'s internals. ][67887]
* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.
Libraries
---------
- [`iter::Empty<T>` now implements `Send` and `Sync` for any `T`.][68348]
- [`Pin::{map_unchecked, map_unchecked_mut}` no longer require the return type
to implement `Sized`.][67935]
- [`io::Cursor` now derives `PartialEq` and `Eq`.][67233]
- [`Layout::new` is now `const`.][66254]
- [Added Standard Library support for `riscv64gc-unknown-linux-gnu`.][66899]
Stabilized APIs
---------------
- [`CondVar::wait_while`]
- [`CondVar::wait_timeout_while`]
- [`DebugMap::key`]
- [`DebugMap::value`]
- [`ManuallyDrop::take`]
- [`matches!`]
- [`ptr::slice_from_raw_parts_mut`]
- [`ptr::slice_from_raw_parts`]
Cargo
-----
- [You no longer need to include `extern crate proc_macro;` to be able to
`use proc_macro;` in the `2018` edition.][cargo/7700]
Compatibility Notes
-------------------
- [`Error::description` has been deprecated, and its use will now produce a
warning.][66919] It's recommended to use `Display`/`to_string` instead.
- [`use $crate;` inside macros is now a hard error.][37390] The compiler
emitted forward compatibility warnings since Rust 1.14.0.
- [As previously announced, this release reduces the level of support for
32-bit Apple targets to tier 3.][apple-32bit-drop]. This means that the
source code is still available to build, but the targets are no longer tested
and no release binary is distributed by the Rust project. Please refer to the
linked blog post for more information.
[37390]: rust-lang/rust#37390
[68253]: rust-lang/rust#68253
[68348]: rust-lang/rust#68348
[67935]: rust-lang/rust#67935
[68339]: rust-lang/rust#68339
[68122]: rust-lang/rust#68122
[67712]: rust-lang/rust#67712
[67887]: rust-lang/rust#67887
[67131]: rust-lang/rust#67131
[67233]: rust-lang/rust#67233
[66899]: rust-lang/rust#66899
[66919]: rust-lang/rust#66919
[66254]: rust-lang/rust#66254
[cargo/7700]: rust-lang/cargo#7700
[`DebugMap::key`]: https://doc.rust-lang.org/stable/std/fmt/struct.DebugMap.html#method.key
[`DebugMap::value`]: https://doc.rust-lang.org/stable/std/fmt/struct.DebugMap.html#method.value
[`ManuallyDrop::take`]: https://doc.rust-lang.org/stable/std/mem/struct.ManuallyDrop.html#method.take
[`matches!`]: https://doc.rust-lang.org/stable/std/macro.matches.html
[`ptr::slice_from_raw_parts_mut`]: https://doc.rust-lang.org/stable/std/ptr/fn.slice_from_raw_parts_mut.html
[`ptr::slice_from_raw_parts`]: https://doc.rust-lang.org/stable/std/ptr/fn.slice_from_raw_parts.html
[`CondVar::wait_while`]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html#method.wait_while
[`CondVar::wait_timeout_while`]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html#method.wait_timeout_while
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Continuing from #57682
It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of
iter::empty", but then a year later I got a compilation error because it wasn'tSendandSync.Unfortunately,
PhantomData<fn() -> T>still errors out. Oh well. I proposedstruct PhantomFnWorkaround<T>(fn() -> T);, but dtolnay did not like it, so using explicit implementations.