Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does #140985

zachs18 · 2025-05-14T03:16:53Z

The docs on impl<I: Default> Default for core::iter::Fuse say (as the I: Default bound implies) that Fuse::::default "Creates a Fuse iterator from the default value of I". However, the implementation creates a Fuse with Fuse { iter: Default::default() }, and since the iter field is an Option, this is actually Fuse { iter: None }, not Fuse { iter: Some(I::default()) }, so Fuse::::default() always returns an empty iterator, even if I::default() would not be empty.

This PR changes Fuse's Default implementation to match the documentation. This will be a behavior change for anyone currently using Fuse::::default() where I::default() is not an empty iterator¹, as Fuse::::default() will now also not be an empty iterator.

(Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if I::default() would have yielded items). With this option, the I: Default bound could also be removed to reflect that no I is ever created.)

Current behavior example (maybe an example like this should be added to the docs either way?)

This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP?

r? libs-api

cc #140961

impl<I: Default> Default for Fuse was added in 1.70.0 (#99929), and it's docs and behavior do not appear to have changed since (Fuse's iter field has been an Option since before the impl was added).

IIUC it is a "de facto" guideline for the stdlib that an iterator type's default() should be empty (and for iterators where that would not make sense, they should not implement Default): cc https://github.com/rust-lang/libs-team/issues/77#issuecomment-1194681709 , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom Iterator type I, and they are using Fuse, and they call Fuse::::default(), this may change the behavior of their code. ↩

hanna-kruppe · 2025-05-14T13:00:59Z

Either way (fix the impl or fix the docs) this should probably gain a test since it’s extremely non-obvious from the code alone.

scottmcm · 2025-05-14T20:07:55Z

And maybe change the code to either ~~Option::default()~~ just None or to I::default() to help emphasize, too.

tgross35 · 2025-05-15T10:51:16Z

I doubt this will have much of a noticeable impact but we may as well try to measure it.

@bors try

Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does The [docs on `impl<I: Default> Default for core::iter::Fuse`](https://doc.rust-lang.org/nightly/std/iter/struct.Fuse.html#impl-Default-for-Fuse%3CI%3E) say (as the `I: Default` bound implies) that `Fuse::::default` "Creates a `Fuse` iterator from the default value of `I`". However, the implementation creates a `Fuse` with `Fuse { iter: Default::default() }`, and since the `iter` field is an `Option`, this is actually `Fuse { iter: None }`, not `Fuse { iter: Some(I::default()) }`, so `Fuse::::default()` always returns an empty iterator, even if `I::default()` would not be empty. This PR changes `Fuse`'s `Default` implementation to match the documentation. This will be a behavior change for anyone currently using `Fuse::::default()` where `I::default()` is not an empty iterator[^1], as `Fuse::::default()` will now also not be an empty iterator. (Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if `I::default()` would have yielded items). With this option, the `I: Default` bound could also be removed to reflect that no `I` is ever created.) [Current behavior example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a1e0adc4badca3dc11bfb70a99213249) (maybe an example like this should be added to the docs either way?) This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP? r? libs-api cc rust-lang#140961 `impl<I: Default> Default for Fuse` was added in 1.70.0 (rust-lang#99929), and it's docs and behavior do not appear to have changed since (`Fuse`'s `iter` field has been an `Option` since before the impl was added). [^1]: IIUC it is a "de facto" guideline for the stdlib that an iterator type's `default()` should be empty (and for iterators where that would not make sense, they should not implement `Default`): cc rust-lang/libs-team#77 (comment) , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom `Iterator` type `I`, *and* they are using `Fuse`, *and* they call `Fuse::::default()`, this may change the behavior of their code.

bors · 2025-05-15T10:52:26Z

⌛ Trying commit fe06fc3 with merge 17092ad...

bors · 2025-05-15T13:01:22Z

☀️ Try build successful - checks-actions
Build commit: 17092ad (17092ad00ed8fe1e7f81a7e38238ff70779034b1)

tgross35 · 2025-05-15T21:02:55Z

@craterbot run mode=build-and-test

craterbot · 2025-05-15T21:03:09Z

👌 Experiment pr-140985 created and queued.
🤖 Automatically detected try build 17092ad
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-05-15T21:03:57Z

🚧 Experiment pr-140985 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-05-17T04:40:58Z

🎉 Experiment pr-140985 is completed!
📊 208 regressed and 149 fixed (631768 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

zachs18 · 2025-05-28T23:47:54Z

I've looked at the crates.io regressions:

Definitely spurious

Some form of "no space left on device":

aeiou-macros
april
argsyn
art_dice
async-peek
badmf
mu_pi
puzz-http
rstats
rusty_audio
safe-regex
simple-cache-rs
standard_card
timely-master
wallpaper_rs

Some form of "failed to create temporary directory":

ambiq-apollo3p-pac
async-pidfd-next
banka
simpl_cache
storage-path-generator

"Error building SSL dependencies": artcode

Other OS error ("Resource temporarily unavailable" when opening a file?): cannyls

I think are spurious:

due to measuring runtimes or similar: async-retry, dust_dds, ratelimit, shared-resource-pool-builder, simple-clock, slow_function_warning, stringsort, tokio-context
due to using env vars from multiple tests (and thus multiple threads): fuel-streams-nats, should-color
due to relying on hashmap iteration order: axsys-noun, serde-command-opts
due to relying on non-hashmap randomness: rs-tool

Otherwise unreliable/incorrect tests: rayoff, samus, sigalign-core (see linked PRs/issue)

I haven't figured out yet: gouth, rs-store, rust-netrc, sigio, sitrep, there. None seem to use Fuse directly at least (rg -iw fuse (case-insensitive, word-based search) on their sources yields no results).

tgross35 · 2025-05-28T23:50:16Z

Thanks for the analysis. I think we can hand this off to libs-api.

@rustbot label +I-libs-api-nominated +T-libs-api -T-libs

joshtriplett · 2025-06-03T15:07:32Z

This makes sense, and it sounds like it doesn't have any issues in crater, so:

@rfcbot merge

rfcbot · 2025-06-03T15:07:35Z

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rfcbot · 2025-06-13T15:22:33Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

tgross35 · 2025-06-16T04:29:13Z

@rustbot author, to address @camsteffen's comments

rustbot · 2025-06-16T04:29:17Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

…oes. Add a doctest with a non-empty-by-default iterator.

zachs18 · 2025-06-18T21:37:17Z

@rustbot ready

Assuming #140985 (comment) is good

tgross35 · 2025-06-23T16:57:16Z

@bors r+ rollup

bors · 2025-06-23T16:57:19Z

📌 Commit f3d4278 has been approved by tgross35

It is now in the queue for this repository.

Rollup of 9 pull requests Successful merges: - #140985 (Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does) - #141324 (std: sys: random: uefi: Provide rdrand based fallback) - #142134 (Reject unsupported `extern "{abi}"`s consistently in all positions) - #142784 (Add codegen timing section) - #142827 (Move error code explanation removal check into tidy) - #142873 (Don't suggest changing a method inside a expansion) - #142908 (Fix install-template.sh for Solaris tr) - #142922 (Fix comment on NoMangle) - #142923 (fix `-Zmin-function-alignment` on functions without attributes) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #140985 - zachs18:fuse-default-some, r=tgross35 Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does The [docs on `impl<I: Default> Default for core::iter::Fuse`](https://doc.rust-lang.org/nightly/std/iter/struct.Fuse.html#impl-Default-for-Fuse%3CI%3E) say (as the `I: Default` bound implies) that `Fuse::::default` "Creates a `Fuse` iterator from the default value of `I`". However, the implementation creates a `Fuse` with `Fuse { iter: Default::default() }`, and since the `iter` field is an `Option`, this is actually `Fuse { iter: None }`, not `Fuse { iter: Some(I::default()) }`, so `Fuse::::default()` always returns an empty iterator, even if `I::default()` would not be empty. This PR changes `Fuse`'s `Default` implementation to match the documentation. This will be a behavior change for anyone currently using `Fuse::::default()` where `I::default()` is not an empty iterator[^1], as `Fuse::::default()` will now also not be an empty iterator. (Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if `I::default()` would have yielded items). With this option, the `I: Default` bound could also be removed to reflect that no `I` is ever created.) [Current behavior example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a1e0adc4badca3dc11bfb70a99213249) (maybe an example like this should be added to the docs either way?) This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP? r? libs-api cc #140961 `impl<I: Default> Default for Fuse` was added in 1.70.0 (#99929), and it's docs and behavior do not appear to have changed since (`Fuse`'s `iter` field has been an `Option` since before the impl was added). [^1]: IIUC it is a "de facto" guideline for the stdlib that an iterator type's `default()` should be empty (and for iterators where that would not make sense, they should not implement `Default`): cc rust-lang/libs-team#77 (comment) , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom `Iterator` type `I`, *and* they are using `Fuse`, *and* they call `Fuse::::default()`, this may change the behavior of their code.

Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does The [docs on `impl<I: Default> Default for core::iter::Fuse`](https://doc.rust-lang.org/nightly/std/iter/struct.Fuse.html#impl-Default-for-Fuse%3CI%3E) say (as the `I: Default` bound implies) that `Fuse::::default` "Creates a `Fuse` iterator from the default value of `I`". However, the implementation creates a `Fuse` with `Fuse { iter: Default::default() }`, and since the `iter` field is an `Option`, this is actually `Fuse { iter: None }`, not `Fuse { iter: Some(I::default()) }`, so `Fuse::::default()` always returns an empty iterator, even if `I::default()` would not be empty. This PR changes `Fuse`'s `Default` implementation to match the documentation. This will be a behavior change for anyone currently using `Fuse::::default()` where `I::default()` is not an empty iterator[^1], as `Fuse::::default()` will now also not be an empty iterator. (Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if `I::default()` would have yielded items). With this option, the `I: Default` bound could also be removed to reflect that no `I` is ever created.) [Current behavior example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a1e0adc4badca3dc11bfb70a99213249) (maybe an example like this should be added to the docs either way?) This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP? r? libs-api cc rust-lang#140961 `impl<I: Default> Default for Fuse` was added in 1.70.0 (rust-lang#99929), and it's docs and behavior do not appear to have changed since (`Fuse`'s `iter` field has been an `Option` since before the impl was added). [^1]: IIUC it is a "de facto" guideline for the stdlib that an iterator type's `default()` should be empty (and for iterators where that would not make sense, they should not implement `Default`): cc rust-lang/libs-team#77 (comment) , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom `Iterator` type `I`, *and* they are using `Fuse`, *and* they call `Fuse::::default()`, this may change the behavior of their code.

Pkgsrc changes: * Adjust patches to adapt to upstream changes and new versions. * assosicated checksums Upstream changes relative to 1.89.0: Version 1.90 (2025-09-18) ========================== Language -------- - [Split up the `unknown_or_malformed_diagnostic_attributes` lint] (rust-lang/rust#140717). This lint has been split up into four finer-grained lints, with `unknown_or_malformed_diagnostic_attributes` now being the lint group that contains these lints: 1. `unknown_diagnostic_attributes`: unknown to the current compiler 2. `misplaced_diagnostic_attributes`: placed on the wrong item 3. `malformed_diagnostic_attributes`: malformed attribute syntax or options 4. `malformed_diagnostic_format_literals`: malformed format string literal - [Allow constants whose final value has references to mutable/external memory, but reject such constants as patterns] (rust-lang/rust#140942) - [Allow volatile access to non-Rust memory, including address 0] (rust-lang/rust#141260) Compiler -------- - [Use `lld` by default on `x86_64-unknown-linux-gnu`] (rust-lang/rust#140525). - [Tier 3 `musl` targets now link dynamically by default] (rust-lang/rust#144410). Affected targets: - `mips64-unknown-linux-muslabi64` - `powerpc64-unknown-linux-musl` - `powerpc-unknown-linux-musl` - `powerpc-unknown-linux-muslspe` - `riscv32gc-unknown-linux-musl` - `s390x-unknown-linux-musl` - `thumbv7neon-unknown-linux-musleabihf` Platform Support ---------------- - [Demote `x86_64-apple-darwin` to Tier 2 with host tools] (rust-lang/rust#145252) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. [platform-support-doc]: https://doc.rust-lang.org/rustc/platform-support.html Libraries --------- - [Stabilize `u*::{checked,overflowing,saturating,wrapping}_sub_signed`] (rust-lang/rust#126043) - [Allow comparisons between `CStr`, `CString`, and `Cow<CStr>`] (rust-lang/rust#137268) - [Remove some unsized tuple impls since unsized tuples can't be constructed] (rust-lang/rust#138340) - [Set `MSG_NOSIGNAL` for `UnixStream`] (rust-lang/rust#140005) - [`proc_macro::Ident::new` now supports `$crate`.] (rust-lang/rust#141996) - [Guarantee the pointer returned from `Thread::into_raw` has at least 8 bytes of alignment] (rust-lang/rust#143859) Stabilized APIs --------------- - [`u{n}::checked_sub_signed`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.checked_sub_signed) - [`u{n}::overflowing_sub_signed`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.overflowing_sub_signed) - [`u{n}::saturating_sub_signed`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.saturating_sub_signed) - [`u{n}::wrapping_sub_signed`] (https://doc.rust-lang.org/stable/std/primitive.usize.html#method.wrapping_sub_signed) - [`impl Copy for IntErrorKind`] (https://doc.rust-lang.org/stable/std/num/enum.IntErrorKind.html#impl-Copy-for-IntErrorKind) - [`impl Hash for IntErrorKind`] (https://doc.rust-lang.org/stable/std/num/enum.IntErrorKind.html#impl-Hash-for-IntErrorKind) - [`impl PartialEq<&CStr> for CStr`] (https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#impl-PartialEq%3C%26CStr%3E-for-CStr) - [`impl PartialEq<CString> for CStr`] (https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#impl-PartialEq%3CCString%3E-for-CStr) - [`impl PartialEq<Cow<CStr>> for CStr`] (https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#impl-PartialEq%3CCow%3C'_,+CStr%3E%3E-for-CStr) - [`impl PartialEq<&CStr> for CString`] (https://doc.rust-lang.org/stable/std/ffi/struct.CString.html#impl-PartialEq%3C%26CStr%3E-for-CString) - [`impl PartialEq<CStr> for CString`] (https://doc.rust-lang.org/stable/std/ffi/struct.CString.html#impl-PartialEq%3CCStr%3E-for-CString) - [`impl PartialEq<Cow<CStr>> for CString`] (https://doc.rust-lang.org/stable/std/ffi/struct.CString.html#impl-PartialEq%3CCow%3C'_,+CStr%3E%3E-for-CString) - [`impl PartialEq<&CStr> for Cow<CStr>`] (https://doc.rust-lang.org/stable/std/borrow/enum.Cow.html#impl-PartialEq%3C%26CStr%3E-for-Cow%3C'_,+CStr%3E) - [`impl PartialEq<CStr> for Cow<CStr>`] (https://doc.rust-lang.org/stable/std/borrow/enum.Cow.html#impl-PartialEq%3CCStr%3E-for-Cow%3C'_,+CStr%3E) - [`impl PartialEq<CString> for Cow<CStr>`] (https://doc.rust-lang.org/stable/std/borrow/enum.Cow.html#impl-PartialEq%3CCString%3E-for-Cow%3C'_,+CStr%3E) These previously stable APIs are now stable in const contexts: - [`<[T]>::reverse`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.reverse) - [`f32::floor`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.floor) - [`f32::ceil`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.ceil) - [`f32::trunc`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.trunc) - [`f32::fract`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.fract) - [`f32::round`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.round) - [`f32::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.round_ties_even) - [`f64::floor`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.floor) - [`f64::ceil`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.ceil) - [`f64::trunc`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.trunc) - [`f64::fract`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.fract) - [`f64::round`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.round) - [`f64::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.round_ties_even) Cargo ----- - [Add `http.proxy-cainfo` config for proxy certs] (rust-lang/cargo#15374) - [Use `gix` for `cargo package`] (rust-lang/cargo#15534) - [feat(publish): Stabilize multi-package publishing] (rust-lang/cargo#15636) Rustdoc ----- - [Add ways to collapse all impl blocks] (rust-lang/rust#141663). Previously the "Summary" button and "-" keyboard shortcut would never collapse `impl` blocks, now they do when shift is held - [Display unsafe attributes with `unsafe()` wrappers] (rust-lang/rust#143662) Compatibility Notes ------------------- - [Use `lld` by default on `x86_64-unknown-linux-gnu`] (rust-lang/rust#140525). See also <https://blog.rust-lang.org/2025/09/01/rust-lld-on-1.90.0-stable/>. - [Make `core::iter::Fuse`'s `Default` impl construct `I::default()` internally as promised in the docs instead of always being empty] (rust-lang/rust#140985) - [Set `MSG_NOSIGNAL` for `UnixStream`] (rust-lang/rust#140005) This may change program behavior but results in the same behavior as other primitives (e.g., stdout, network sockets). Programs relying on signals to terminate them should update handling of sockets to handle errors on write by exiting. - [On Unix `std::env::home_dir` will use the fallback if the `HOME` environment variable is empty] (rust-lang/rust#141840) - We now [reject unsupported `extern "{abi}"`s consistently in all positions] (rust-lang/rust#142134). This primarily affects the use of implementing traits on an `extern "{abi}"` function pointer, like `extern "stdcall" fn()`, on a platform that doesn't support that, like aarch64-unknown-linux-gnu. Direct usage of these unsupported ABI strings by declaring or defining functions was already rejected, so this is only a change for consistency. - [const-eval: error when initializing a static writes to that static] (rust-lang/rust#143084) - [Check that the `proc_macro_derive` macro has correct arguments when applied to the crate root] (rust-lang/rust#143607)

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 14, 2025

zachs18 changed the title ~~Change core::iter::Fuse's Default impl to do what it's docs say it does~~ Change core::iter::Fuse's Default impl to do what its docs say it does May 14, 2025

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This was referenced May 28, 2025

Fix incorrect doctest nhwn/rayoff#1

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Fix test stbarrientos/samus#1

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BackTraceMarker should be repr(u16), and Component should not be Pod baku4/sigalign#16

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Tracking issue for release notes of #140985: Change core::iter::Fuse's Default impl to do what its docs say it does #142463

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Fix core::iter::Fuse's Default impl to do what it's docs say it d…

f3d4278

…oes. Add a doctest with a non-empty-by-default iterator.

zachs18 force-pushed the fuse-default-some branch from 9e8af12 to f3d4278 Compare June 18, 2025 21:28

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Rollup of 9 pull requests #142929

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Default implementation on std::iter::Fuse should not requires Default on the inner iterator #140961

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BoxyUwU mentioned this pull request Aug 21, 2025

Draft release notes for 1.90 #145730

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hanna-kruppe mentioned this pull request Sep 18, 2025

implement Default for more iterators rust-lang/libs-team#656

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Change core::iter::Fuse's Default impl to do what its docs say it does #140985

Change core::iter::Fuse's Default impl to do what its docs say it does #140985

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zachs18 commented Jun 18, 2025

Uh oh!

tgross35 commented Jun 23, 2025

Uh oh!

bors commented Jun 23, 2025

Uh oh!

Uh oh!

Uh oh!

Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does #140985

Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does #140985

zachs18 commented May 28, 2025 •

edited

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rfcbot commented Jun 3, 2025 •

edited by dtolnay

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