Stabilize #[track_caller].
#72445
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Stabilize #[track_caller].
#72445
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r? @dtolnay (rust_highfive has picked a reviewer for you, use r? to override) |
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Looks reasonable to me. |
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Thanks! Per #47809 (comment) I think this needs |
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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. |
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Nominating just to call attention to the FCP here |
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Stabilize `#[track_caller]`. # Stabilization Report RFC: [2091] Tracking issue: rust-lang#47809 ## Summary From the [rustc-dev-guide chapter][dev-guide]: > Take this example program: ```rust fn main() { let foo: Option<()> = None; foo.unwrap(); // this should produce a useful panic message! } ``` > Prior to Rust 1.42, panics like this `unwrap()` printed a location in libcore: ``` $ rustc 1.41.0 example.rs; example.exe thread 'main' panicked at 'called `Option::unwrap()` on a `None` value',...core\macros\mod.rs:15:40 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace. ``` > As of 1.42, we get a much more helpful message: ``` $ rustc 1.42.0 example.rs; example.exe thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', example.rs:3:5 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ``` > These error messages are achieved through a combination of changes to `panic!` internals to make use of `core::panic::Location::caller` and a number of `#[track_caller]` annotations in the standard library which propagate caller information. The attribute adds an implicit caller location argument to the ABI of annotated functions, but does not affect the type or MIR of the function. We implement the feature entirely in codegen and in the const evaluator. ## Bottom Line This PR stabilizes the use of `#[track_caller]` everywhere, including traits and extern blocks. It also stabilizes `core::panic::Location::caller`, although the use of that function in a const context remains gated by `#![feature(const_caller_location)]`. The implementation for the feature already changed the output of panic messages for a number of std functions, as described in the [1.42 release announcement]. The attribute's use in `Index` and `IndexMut` traits is visible to users since 1.44. ## Tests All of the tests for this feature live under [src/test/ui/rfc-2091-track-caller][tests] in the repo. Noteworthy cases: * [use of attr in std] * validates user-facing benefit of the feature * [trait attribute inheritance] * covers subtle behavior designed during implementation and not RFC'd * [const/codegen equivalence] * this was the result of a suspected edge case and investigation * [diverging function support] * covers an unresolved question from the RFC * [fn pointers and shims] * covers important potential sources of unsoundness ## Documentation The rustc-dev-guide now has a chapter on [Implicit Caller Location][dev-guide]. I have an [open PR to the reference][attr-reference-pr] documenting the attribute. The intrinsic's [wrapper] includes some examples as well. ## Implementation History * 2019-10-02: [`#[track_caller]` feature gate (RFC 2091 1/N) rust-lang#65037](rust-lang#65037) * Picked up the patch that @ayosec had started on the feature gate. * 2019-10-13: [Add `Instance::resolve_for_fn_ptr` (RFC 2091 rust-lang#2/N) rust-lang#65182](rust-lang#65182) * 2019-10-20: ~~[WIP Add MIR argument for #[track_caller] (RFC 2091 3/N) rust-lang#65258](rust-lang#65258 * Abandoned approach to send location as a MIR argument. * 2019-10-28: [`std::panic::Location` is a lang_item, add `core::intrinsics::caller_location` (RFC 2091 3/N) rust-lang#65664](rust-lang#65664) * 2019-12-07: [Implement #[track_caller] attribute. (RFC 2091 4/N) rust-lang#65881](rust-lang#65881) * 2020-01-04: [libstd uses `core::panic::Location` where possible. rust-lang#67137](rust-lang#67137) * 2020-01-08: [`Option::{expect,unwrap}` and `Result::{expect, expect_err, unwrap, unwrap_err}` have `#[track_caller]` rust-lang#67887](rust-lang#67887) * 2020-01-20: [Fix #[track_caller] and function pointers rust-lang#68302](rust-lang#68302) (fixed rust-lang#68178) * 2020-03-23: [#[track_caller] in traits rust-lang#69251](rust-lang#69251) * 2020-03-24: [#[track_caller] on core::ops::{Index, IndexMut}. rust-lang#70234](rust-lang#70234) * 2020-04-08 [Support `#[track_caller]` on functions in `extern "Rust" { ... }` rust-lang#70916](rust-lang#70916) ## Unresolveds ### From the RFC > Currently the RFC simply prohibit applying #[track_caller] to trait methods as a future-proofing > measure. **Resolved.** See the dev-guide documentation and the tests section above. > Diverging functions should be supported. **Resolved.** See the tests section above. > The closure foo::{{closure}} should inherit most attributes applied to the function foo, ... **Resolved.** This unknown was related to specifics of the implementation which were made irrelevant by the final implementation. ### Binary Size I [instrumented track_caller to use custom sections][measure-size] in a local build and discovered relatively minor binary size usage for the feature overall. I'm leaving the issue open to discuss whether we want to upstream custom section support. There's an [open issue to discuss mitigation strategies][mitigate-size]. Some decisions remain about the "right" strategies to reduce size without overly constraining the compiler implementation. I'd be excited to see someone carry that work forward but my opinion is that we shouldn't block stabilization on implementing compiler flags for redaction. ### Specialization There's an [open issue][specialization] on the semantics of the attribute in specialization chains. I'm inclined to move forward with stabilization without an exact resolution here given that specialization is itself unstable, but I also think it should be an easy question to resolve. ### Location only points to the start of a call span rust-lang#69977 was resolved by rust-lang#73182, and the next step should probably be to [extend `Location` with a notion of the end of a call](rust-lang#73554). ### Regression of std's panic messages rust-lang#70963 should be resolved by serializing span hygeine to crate metadata: rust-lang#68686. [2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md [dev-guide]: https://rustc-dev-guide.rust-lang.org/codegen/implicit-caller-location.html [specialization]: rust-lang#70293 [measure-size]: rust-lang#70579 [mitigate-size]: rust-lang#70580 [attr-reference-pr]: rust-lang/reference#742 [wrapper]: https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller [tests]: https://github.com/rust-lang/rust/tree/master/src/test/ui/rfc-2091-track-caller [const/codegen equivalence]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/caller-location-fnptr-rt-ctfe-equiv.rs [diverging function support]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/diverging-caller-location.rs [use of attr in std]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/std-panic-locations.rs [fn pointers and shims]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-fn-ptr-with-arg.rs [trait attribute inheritance]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-trait-impls.rs [1.42 release announcement]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#useful-line-numbers-in-option-and-result-panic-messages
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Does not yet make its constness stable, though. Use of `Location::caller` in const contexts is still gated by `#![feature(const_caller_location)]`.
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Hm, I'm confused actually. The logs point to: But I can't find that in my checkout. I rebased on top of tree and pushed again. |
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@bors r=oli-obk delegate |
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📌 Commit f07d10d has been approved by |
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✌️ @anp can now approve this pull request |
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Stabilize `#[track_caller]`. # Stabilization Report RFC: [2091] Tracking issue: rust-lang#47809 ## Summary From the [rustc-dev-guide chapter][dev-guide]: > Take this example program: ```rust fn main() { let foo: Option<()> = None; foo.unwrap(); // this should produce a useful panic message! } ``` > Prior to Rust 1.42, panics like this `unwrap()` printed a location in libcore: ``` $ rustc 1.41.0 example.rs; example.exe thread 'main' panicked at 'called `Option::unwrap()` on a `None` value',...core\macros\mod.rs:15:40 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace. ``` > As of 1.42, we get a much more helpful message: ``` $ rustc 1.42.0 example.rs; example.exe thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', example.rs:3:5 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ``` > These error messages are achieved through a combination of changes to `panic!` internals to make use of `core::panic::Location::caller` and a number of `#[track_caller]` annotations in the standard library which propagate caller information. The attribute adds an implicit caller location argument to the ABI of annotated functions, but does not affect the type or MIR of the function. We implement the feature entirely in codegen and in the const evaluator. ## Bottom Line This PR stabilizes the use of `#[track_caller]` everywhere, including traits and extern blocks. It also stabilizes `core::panic::Location::caller`, although the use of that function in a const context remains gated by `#![feature(const_caller_location)]`. The implementation for the feature already changed the output of panic messages for a number of std functions, as described in the [1.42 release announcement]. The attribute's use in `Index` and `IndexMut` traits is visible to users since 1.44. ## Tests All of the tests for this feature live under [src/test/ui/rfc-2091-track-caller][tests] in the repo. Noteworthy cases: * [use of attr in std] * validates user-facing benefit of the feature * [trait attribute inheritance] * covers subtle behavior designed during implementation and not RFC'd * [const/codegen equivalence] * this was the result of a suspected edge case and investigation * [diverging function support] * covers an unresolved question from the RFC * [fn pointers and shims] * covers important potential sources of unsoundness ## Documentation The rustc-dev-guide now has a chapter on [Implicit Caller Location][dev-guide]. I have an [open PR to the reference][attr-reference-pr] documenting the attribute. The intrinsic's [wrapper] includes some examples as well. ## Implementation History * 2019-10-02: [`#[track_caller]` feature gate (RFC 2091 1/N) rust-lang#65037](rust-lang#65037) * Picked up the patch that @ayosec had started on the feature gate. * 2019-10-13: [Add `Instance::resolve_for_fn_ptr` (RFC 2091 rust-lang#2/N) rust-lang#65182](rust-lang#65182) * 2019-10-20: ~~[WIP Add MIR argument for #[track_caller] (RFC 2091 3/N) rust-lang#65258](rust-lang#65258 * Abandoned approach to send location as a MIR argument. * 2019-10-28: [`std::panic::Location` is a lang_item, add `core::intrinsics::caller_location` (RFC 2091 3/N) rust-lang#65664](rust-lang#65664) * 2019-12-07: [Implement #[track_caller] attribute. (RFC 2091 4/N) rust-lang#65881](rust-lang#65881) * 2020-01-04: [libstd uses `core::panic::Location` where possible. rust-lang#67137](rust-lang#67137) * 2020-01-08: [`Option::{expect,unwrap}` and `Result::{expect, expect_err, unwrap, unwrap_err}` have `#[track_caller]` rust-lang#67887](rust-lang#67887) * 2020-01-20: [Fix #[track_caller] and function pointers rust-lang#68302](rust-lang#68302) (fixed rust-lang#68178) * 2020-03-23: [#[track_caller] in traits rust-lang#69251](rust-lang#69251) * 2020-03-24: [#[track_caller] on core::ops::{Index, IndexMut}. rust-lang#70234](rust-lang#70234) * 2020-04-08 [Support `#[track_caller]` on functions in `extern "Rust" { ... }` rust-lang#70916](rust-lang#70916) ## Unresolveds ### From the RFC > Currently the RFC simply prohibit applying #[track_caller] to trait methods as a future-proofing > measure. **Resolved.** See the dev-guide documentation and the tests section above. > Diverging functions should be supported. **Resolved.** See the tests section above. > The closure foo::{{closure}} should inherit most attributes applied to the function foo, ... **Resolved.** This unknown was related to specifics of the implementation which were made irrelevant by the final implementation. ### Binary Size I [instrumented track_caller to use custom sections][measure-size] in a local build and discovered relatively minor binary size usage for the feature overall. I'm leaving the issue open to discuss whether we want to upstream custom section support. There's an [open issue to discuss mitigation strategies][mitigate-size]. Some decisions remain about the "right" strategies to reduce size without overly constraining the compiler implementation. I'd be excited to see someone carry that work forward but my opinion is that we shouldn't block stabilization on implementing compiler flags for redaction. ### Specialization There's an [open issue][specialization] on the semantics of the attribute in specialization chains. I'm inclined to move forward with stabilization without an exact resolution here given that specialization is itself unstable, but I also think it should be an easy question to resolve. ### Location only points to the start of a call span rust-lang#69977 was resolved by rust-lang#73182, and the next step should probably be to [extend `Location` with a notion of the end of a call](rust-lang#73554). ### Regression of std's panic messages rust-lang#70963 should be resolved by serializing span hygeine to crate metadata: rust-lang#68686. [2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md [dev-guide]: https://rustc-dev-guide.rust-lang.org/codegen/implicit-caller-location.html [specialization]: rust-lang#70293 [measure-size]: rust-lang#70579 [mitigate-size]: rust-lang#70580 [attr-reference-pr]: rust-lang/reference#742 [wrapper]: https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller [tests]: https://github.com/rust-lang/rust/tree/master/src/test/ui/rfc-2091-track-caller [const/codegen equivalence]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/caller-location-fnptr-rt-ctfe-equiv.rs [diverging function support]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/diverging-caller-location.rs [use of attr in std]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/std-panic-locations.rs [fn pointers and shims]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-fn-ptr-with-arg.rs [trait attribute inheritance]: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2091-track-caller/tracked-trait-impls.rs [1.42 release announcement]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#useful-line-numbers-in-option-and-result-panic-messages
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…arth Rollup of 17 pull requests Successful merges: - rust-lang#72071 (Added detailed error code explanation for issue E0687 in Rust compiler.) - rust-lang#72369 (Bring net/parser.rs up to modern up to date with modern rust patterns) - rust-lang#72445 (Stabilize `#[track_caller]`.) - rust-lang#73466 (impl From<char> for String) - rust-lang#73548 (remove rustdoc warnings) - rust-lang#73649 (Fix sentence structure) - rust-lang#73678 (Update Box::from_raw example to generalize better) - rust-lang#73705 (stop taking references in Relate) - rust-lang#73716 (Document the static keyword) - rust-lang#73752 (Remap Windows ERROR_INVALID_PARAMETER to ErrorKind::InvalidInput from Other) - rust-lang#73776 (Move terminator to new module) - rust-lang#73778 (Make `likely` and `unlikely` const, gated by feature `const_unlikely`) - rust-lang#73805 (Document the type keyword) - rust-lang#73806 (Use an 'approximate' universal upper bound when reporting region errors) - rust-lang#73828 (Fix wording for anonymous parameter name help) - rust-lang#73846 (Fix comma in debug_assert! docs) - rust-lang#73847 (Edit cursor.prev() method docs in lexer) Failed merges: r? @ghost
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according to various people on tech-pkg@, there are no problems with
the Firefox build
Version 1.46.0 (2020-08-27)
==========================
Language
--------
- [`if`, `match`, and `loop` expressions can now be used in const functions.][72437]
- [Additionally you are now also able to coerce and cast to slices (`&[T]`) in
const functions.][73862]
- [The `#[track_caller]` attribute can now be added to functions to use the
function's caller's location information for panic messages.][72445]
- [Recursively indexing into tuples no longer needs parentheses.][71322] E.g.
`x.0.0` over `(x.0).0`.
- [`mem::transmute` can now be used in static and constants.][72920] **Note**
You currently can't use `mem::transmute` in constant functions.
Compiler
--------
- [You can now use the `cdylib` target on Apple iOS and tvOS platforms.][73516]
- [Enabled static "Position Independent Executables" by default
for `x86_64-unknown-linux-musl`.][70740]
Libraries
---------
- [`mem::forget` is now a `const fn`.][73887]
- [`String` now implements `From<char>`.][73466]
- [The `leading_ones`, and `trailing_ones` methods have been stabilised for all
integer types.][73032]
- [`vec::IntoIter<T>` now implements `AsRef<[T]>`.][72583]
- [All non-zero integer types (`NonZeroU8`) now implement `TryFrom` for their
zero-able equivalent (e.g. `TryFrom<u8>`).][72717]
- [`&[T]` and `&mut [T]` now implement `PartialEq<Vec<T>>`.][71660]
- [`(String, u16)` now implements `ToSocketAddrs`.][73007]
- [`vec::Drain<'_, T>` now implements `AsRef<[T]>`.][72584]
Stabilized APIs
---------------
- [`Option::zip`]
- [`vec::Drain::as_slice`]
Cargo
-----
Added a number of new environment variables that are now available when
compiling your crate.
- [`CARGO_BIN_NAME` and `CARGO_CRATE_NAME`][cargo/8270] Providing the name of
the specific binary being compiled and the name of the crate.
- [`CARGO_PKG_LICENSE`][cargo/8325] The license from the manifest of the package.
- [`CARGO_PKG_LICENSE_FILE`][cargo/8387] The path to the license file.
Compatibility Notes
-------------------
- [The target configuration option `abi_blacklist` has been renamed
to `unsupported_abis`.][74150] The old name will still continue to work.
- [Rustc will now warn if you cast a C-like enum that implements `Drop`.][72331]
This was previously accepted but will become a hard error in a future release.
- [Rustc will fail to compile if you have a struct with
`#[repr(i128)]` or `#[repr(u128)]`.][74109] This representation is currently only
allowed on `enum`s.
- [Tokens passed to `macro_rules!` are now always captured.][73293] This helps
ensure that spans have the correct information, and may cause breakage if you
were relying on receiving spans with dummy information.
- [The InnoSetup installer for Windows is no longer available.][72569] This was
a legacy installer that was replaced by a MSI installer a few years ago but
was still being built.
- [`{f32, f64}::asinh` now returns the correct values for negative numbers.][72486]
- [Rustc will no longer accept overlapping trait implementations that only
differ in how the lifetime was bound.][72493]
- [Rustc now correctly relates the lifetime of an existential associated
type.][71896] This fixes some edge cases where `rustc` would erroneously allow
you to pass a shorter lifetime than expected.
- [Rustc now dynamically links to `libz` (also called `zlib`) on Linux.][74420]
The library will need to be installed for `rustc` to work, even though we
expect it to be already available on most systems.
- [Tests annotated with `#[should_panic]` are broken on ARMv7 while running
under QEMU.][74820]
- [Pretty printing of some tokens in procedural macros changed.][75453] The
exact output returned by rustc's pretty printing is an unstable
implementation detail: we recommend any macro relying on it to switch to a
more robust parsing system.
[75453]: rust-lang/rust#75453
[74820]: rust-lang/rust#74820
[74420]: rust-lang/rust#74420
[74109]: rust-lang/rust#74109
[74150]: rust-lang/rust#74150
[73862]: rust-lang/rust#73862
[73887]: rust-lang/rust#73887
[73466]: rust-lang/rust#73466
[73516]: rust-lang/rust#73516
[73293]: rust-lang/rust#73293
[73007]: rust-lang/rust#73007
[73032]: rust-lang/rust#73032
[72920]: rust-lang/rust#72920
[72569]: rust-lang/rust#72569
[72583]: rust-lang/rust#72583
[72584]: rust-lang/rust#72584
[72717]: rust-lang/rust#72717
[72437]: rust-lang/rust#72437
[72445]: rust-lang/rust#72445
[72486]: rust-lang/rust#72486
[72493]: rust-lang/rust#72493
[72331]: rust-lang/rust#72331
[71896]: rust-lang/rust#71896
[71660]: rust-lang/rust#71660
[71322]: rust-lang/rust#71322
[70740]: rust-lang/rust#70740
[cargo/8270]: rust-lang/cargo#8270
[cargo/8325]: rust-lang/cargo#8325
[cargo/8387]: rust-lang/cargo#8387
[`Option::zip`]: https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.zip
[`vec::Drain::as_slice`]: https://doc.rust-lang.org/stable/std/vec/struct.Drain.html#method.as_slice
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Pkgsrc changes:
* Portability patches for Illumos have been intregrated upstream,
so are no longer needed in pkgsrc.
* Adjust one other patch, and update vendor/libc cargo checksum.
Upstream changes:
Version 1.46.0 (2020-08-27)
==========================
Language
--------
- [`if`, `match`, and `loop` expressions can now be used in const functions.]
[72437]
- [Additionally you are now also able to coerce and cast to slices (`&[T]`) in
const functions.][73862]
- [The `#[track_caller]` attribute can now be added to functions to use the
function's caller's location information for panic messages.][72445]
- [Recursively indexing into tuples no longer needs parentheses.][71322] E.g.
`x.0.0` over `(x.0).0`.
- [`mem::transmute` can now be used in static and constants.][72920] **Note**
You currently can't use `mem::transmute` in constant functions.
Compiler
--------
- [You can now use the `cdylib` target on Apple iOS and tvOS platforms.][73516]
- [Enabled static "Position Independent Executables" by default
for `x86_64-unknown-linux-musl`.][70740]
Libraries
---------
- [`mem::forget` is now a `const fn`.][73887]
- [`String` now implements `From<char>`.][73466]
- [The `leading_ones`, and `trailing_ones` methods have been stabilised for all
integer types.][73032]
- [`vec::IntoIter<T>` now implements `AsRef<[T]>`.][72583]
- [All non-zero integer types (`NonZeroU8`) now implement `TryFrom` for their
zero-able equivalent (e.g. `TryFrom<u8>`).][72717]
- [`&[T]` and `&mut [T]` now implement `PartialEq<Vec<T>>`.][71660]
- [`(String, u16)` now implements `ToSocketAddrs`.][73007]
- [`vec::Drain<'_, T>` now implements `AsRef<[T]>`.][72584]
Stabilized APIs
---------------
- [`Option::zip`]
- [`vec::Drain::as_slice`]
Cargo
-----
Added a number of new environment variables that are now available when
compiling your crate.
- [`CARGO_BIN_NAME` and `CARGO_CRATE_NAME`][cargo/8270] Providing the name of
the specific binary being compiled and the name of the crate.
- [`CARGO_PKG_LICENSE`][cargo/8325] The license from the manifest of the
package.
- [`CARGO_PKG_LICENSE_FILE`][cargo/8387] The path to the license file.
Compatibility Notes
-------------------
- [The target configuration option `abi_blacklist` has been renamed
to `unsupported_abis`.][74150] The old name will still continue to work.
- [Rustc will now warn if you have a C-like enum that implements `Drop`.][72331]
This was previously accepted but will become a hard error in a future release.
- [Rustc will fail to compile if you have a struct with
`#[repr(i128)]` or `#[repr(u128)]`.][74109] This representation is currently
only allowed on `enum`s.
- [Tokens passed to `macro_rules!` are now always captured.][73293] This helps
ensure that spans have the correct information, and may cause breakage if you
were relying on receiving spans with dummy information.
- [The InnoSetup installer for Windows is no longer available.][72569] This was
a legacy installer that was replaced by a MSI installer a few years ago but
was still being built.
- [`{f32, f64}::asinh` now returns the correct values for negative numbers.]
[72486]
- [Rustc will no longer accept overlapping trait implementations that only
differ in how the lifetime was bound.][72493]
- [Rustc now correctly relates the lifetime of an existential associated
type.][71896] This fixes some edge cases where `rustc` would erroneously
allow you to pass a shorter lifetime than expected.
- [Rustc now dynamically links to `libz` (also called `zlib`) on Linux.][74420]
The library will need to be installed for `rustc` to work, even though we
expect it to be already available on most systems.
- [Tests annotated with `#[should_panic]` are broken on ARMv7 while running
under QEMU.][74820]
- [Pretty printing of some tokens in procedural macros changed.][75453] The
exact output returned by rustc's pretty printing is an unstable
implementation detail: we recommend any macro relying on it to switch to a
more robust parsing system.
[75453]: rust-lang/rust#75453
[74820]: rust-lang/rust#74820
[74420]: rust-lang/rust#74420
[74109]: rust-lang/rust#74109
[74150]: rust-lang/rust#74150
[73862]: rust-lang/rust#73862
[73887]: rust-lang/rust#73887
[73466]: rust-lang/rust#73466
[73516]: rust-lang/rust#73516
[73293]: rust-lang/rust#73293
[73007]: rust-lang/rust#73007
[73032]: rust-lang/rust#73032
[72920]: rust-lang/rust#72920
[72569]: rust-lang/rust#72569
[72583]: rust-lang/rust#72583
[72584]: rust-lang/rust#72584
[72717]: rust-lang/rust#72717
[72437]: rust-lang/rust#72437
[72445]: rust-lang/rust#72445
[72486]: rust-lang/rust#72486
[72493]: rust-lang/rust#72493
[72331]: rust-lang/rust#72331
[71896]: rust-lang/rust#71896
[71660]: rust-lang/rust#71660
[71322]: rust-lang/rust#71322
[70740]: rust-lang/rust#70740
[cargo/8270]: rust-lang/cargo#8270
[cargo/8325]: rust-lang/cargo#8325
[cargo/8387]: rust-lang/cargo#8387
[`Option::zip`]: https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.zip
[`vec::Drain::as_slice`]: https://doc.rust-lang.org/stable/std/vec/struct.Drain.html#method.as_slice
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Stabilization Report
RFC: 2091
Tracking issue: #47809
Summary
From the rustc-dev-guide chapter:
The attribute adds an implicit caller location argument to the ABI of annotated functions, but does not affect the type or MIR of the function. We implement the feature entirely in codegen and in the const evaluator.
Bottom Line
This PR stabilizes the use of
#[track_caller]everywhere, including traits and extern blocks. It also stabilizescore::panic::Location::caller, although the use of that function in a const context remains gated by#![feature(const_caller_location)].The implementation for the feature already changed the output of panic messages for a number of std functions, as described in the 1.42 release announcement. The attribute's use in
IndexandIndexMuttraits is visible to users since 1.44.Tests
All of the tests for this feature live under src/test/ui/rfc-2091-track-caller in the repo.
Noteworthy cases:
Documentation
The rustc-dev-guide now has a chapter on Implicit Caller Location.
I have an open PR to the reference documenting the attribute.
The intrinsic's wrapper includes some examples as well.
Implementation History
#[track_caller]feature gate (RFC 2091 1/N) #65037Instance::resolve_for_fn_ptr(RFC 2091 #2/N) #65182WIP Add MIR argument for #[track_caller] (RFC 2091 3/N) #65258std::panic::Locationis a lang_item, addcore::intrinsics::caller_location(RFC 2091 3/N) #65664core::panic::Locationwhere possible. #67137Option::{expect,unwrap}andResult::{expect, expect_err, unwrap, unwrap_err}have#[track_caller]#67887#[track_caller]on functions inextern "Rust" { ... }#70916Unresolveds
From the RFC
Resolved. See the dev-guide documentation and the tests section above.
Resolved. See the tests section above.
Resolved. This unknown was related to specifics of the implementation which were made irrelevant by the final implementation.
Binary Size
I instrumented track_caller to use custom sections in a local build and discovered relatively minor binary size usage for the feature overall. I'm leaving the issue open to discuss whether we want to upstream custom section support.
There's an open issue to discuss mitigation strategies. Some decisions remain about the "right" strategies to reduce size without overly constraining the compiler implementation. I'd be excited to see someone carry that work forward but my opinion is that we shouldn't block stabilization on implementing compiler flags for redaction.
Specialization
There's an open issue on the semantics of the attribute in specialization chains. I'm inclined to move forward with stabilization without an exact resolution here given that specialization is itself unstable, but I also think it should be an easy question to resolve.
Location only points to the start of a call span
#69977 was resolved by #73182, and the next step should probably be to extend
Locationwith a notion of the end of a call.Regression of std's panic messages
#70963 should be resolved by serializing span hygeine to crate metadata: #68686.