#[track_caller] in traits #69251

anp · 2020-02-18T00:37:22Z

Per #47809 (comment), this allows the #[track_caller] attribute on trait methods.

Includes tests for #[track_caller] with:

"regular" trait impls
default trait impls
"blanket-tracked" trait impls, where the annotation is in the trait definition and is inherited by "regular" impls of the trait

rust-highfive · 2020-02-18T00:37:33Z

(rust_highfive has picked a reviewer for you, use r? to override)

anp · 2020-02-18T00:42:53Z

cc @eddyb you mentioned wanting to have some particular tests around ReifyShim and the trait impl. I've added traits to the fn-ptr tests, take a look?

sfackler · 2020-02-18T02:19:43Z

r? @eddyb

eddyb · 2020-02-18T13:48:58Z

LGTM, as long as we want to do this.

anp · 2020-02-18T16:31:43Z

Cool! IIUC the lang team approved this.

eddyb · 2020-02-18T16:35:11Z

r? @nikomatsakis for the rubber-stamp

src/test/ui/rfc-2091-track-caller/warn-fn-prototype.rs

Centril · 2020-02-18T20:24:44Z

Let's just confirm that perf doesn't regress; @bors try @rust-timer queue

rust-timer · 2020-02-18T20:24:45Z

Awaiting bors try build completion

bors · 2020-02-18T20:24:57Z

⌛ Trying commit 1e136ee97249970af0f50820b1e3d0dc9b263a98 with merge cb29a8883f4d99b4554b6c987c8e58194efe3d8e...

bors · 2020-02-18T23:11:41Z

☀️ Try build successful - checks-azure
Build commit: cb29a8883f4d99b4554b6c987c8e58194efe3d8e (cb29a8883f4d99b4554b6c987c8e58194efe3d8e)

rust-timer · 2020-02-18T23:11:43Z

Queued cb29a8883f4d99b4554b6c987c8e58194efe3d8e with parent b0d5813, future comparison URL.

anp · 2020-02-20T23:11:41Z

I'm not sure what typical variances are on perf.rlo these days, but those numbers don't look great :/.

Centril · 2020-02-20T23:14:11Z

I think that's a clear, but not too big regression.

eddyb · 2020-02-20T23:44:20Z

You can ignore syn-opt, there's an issue open about its nondeterminism.

The weird thing here is -check builds being affected by what should be a codegen change only, and especially "clean incremental" runs!

For "baseline incremental" runs the regression is usually 0.3% or less except for:

helloworld-debug at 0.6%
cranelift-codegen-opt at 0.7%
token-stream-stress-debug at 0.4%

Going back to "clean incremental" runs, the only one over 1.0% with a change in the number of queries is ripgrep-opt, all the other ones execute the same number of queries, just slower.

Also, many runs that differ in total time, don't differ as much (or at all?) in total query time, that's confusing.

The commonly regressed queries appear to be incr_comp_load_dep_graph and... param_env?

Hang on, could this entire thing be because of the effect of adding #[track_caller] to all of these impls on the compiler code itself?

cc @Mark-Simulacrum any way we could run perf with a stage1 build, or maybe only enable the #[cfg_attr(not(bootstrap), track_caller)] added in this PR for stage2 libstd?

@anp In the meanwhile, maybe we can land the PR without the changed impls? And then maybe figure out separately which ones are responsible for slowing down rustc?

Mark-Simulacrum · 2020-02-21T00:51:48Z

any way we could run perf with a stage1 build, or maybe only enable the #[cfg_attr(not(bootstrap), track_caller)] added in this PR for stage2 libstd?

Stage 1 build will be quite hard I think. The latter, presuming @anp or whoever can rename the attribute to #[cfg_attr(tracking_caller, track_caller)] or so, fairly easy, you'd just add some code here gated on stage 2 I guess (or whatever logic you go with):

rust/src/bootstrap/builder.rs

Line 772 in 2c462a2

rustflags.arg("--cfg=bootstrap");

bors · 2020-03-23T00:11:33Z

📌 Commit 6c2c559dab1c3a353f9cc57501a4d155baadfdf2 has been approved by eddyb

The codegen implementation already works for this, so we're: * propagating track_caller attr from trait def to impl * relaxing errors * adding tests Approved in a recent lang team meeting: https://github.com/rust-lang/lang-team/blob/master/minutes/2020-01-09.md

We can do this now that opt_associated_item doesn't have any panicking paths.

eddyb · 2020-03-23T00:29:11Z

@bors r+

bors · 2020-03-23T00:29:13Z

📌 Commit 69bd46a has been approved by eddyb

#[track_caller] in traits Per rust-lang#47809 (comment), this allows the `#[track_caller]` attribute on trait methods. Includes tests for `#[track_caller]` with: * "regular" trait impls * default trait impls * "blanket-tracked" trait impls, where the annotation is in the trait definition and is inherited by "regular" impls of the trait

@ghost

Rollup of 9 pull requests Successful merges: - rust-lang#69251 (#[track_caller] in traits) - rust-lang#69880 (miri engine: turn error sanity checks into assertions) - rust-lang#70207 (Use getentropy(2) on macos) - rust-lang#70227 (Only display definition when suggesting a typo) - rust-lang#70236 (resolve: Avoid "self-confirming" import resolutions in one more case) - rust-lang#70248 (parser: simplify & remove unused field) - rust-lang#70249 (handle ConstKind::Unresolved after monomorphizing) - rust-lang#70269 (remove redundant closures (clippy::redundant_closure)) - rust-lang#70270 (Clean up E0449 explanation) Failed merges: r? @ghost

#[track_caller] on core::ops::{Index, IndexMut}. Applies the attribute to `core::ops::Index(Mut)` and enough std internals to cover the [functions listed in "tier 1" in the original RFC](https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#survey-of-panicking-standard-functions). Split out from rust-lang#69251 to allow separate assessment of perf impact. To my knowledge, this is the last piece of implementing RFC 2091. Tracking issue: rust-lang#47809

@eddyb

Support `#[track_caller]` on functions in `extern "Rust" { ... }` Fixes rust-lang#70830 which is the follow-up to @eddyb's suggestion in rust-lang#69251 (comment) to allow `#[track_caller]` on `fn`s in FFI imports, that is, on functions in `extern "Rust" { ... }` blocks. This requires that the other side, the FFI export, also have the `#[track_caller]` attribute. Otherwise, undefined behavior is triggered and the blame lies, as usual, with the `unsafe { ... }` block which called the FFI imported function. After this PR, all forms of `fn` items with the right ABI (`"Rust"`) support `#[track_caller]`. As a drive-by, the PR also hardens the check rejecting `#[naked] #[track_caller]` such that methods and other forms of `fn` items are also considered. r? @eddyb cc @rust-lang/lang

@ayosec

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

@ayosec

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

@ayosec

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

@ayosec

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

@ayosec

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

Since PR rust-lang#69251, the `#[track_caller]` attribute has been supported on traits. However, it only has an effect on direct (monomorphized) method calls. Calling a `#[track_caller]` method on a trait object will *not* propagate caller location information - instead, `Location::caller()` will return the location of the method definition. This PR forwards caller location information when `#[track_caller]` is present on the method definition in the trait. This is possible because `#[track_caller]` in this position is 'inherited' by any impls of that trait, so all implementations will have the same ABI. This PR does *not* change the behavior in the case where `#[track_caller]` is present only on the impl of a trait. While all implementations of the method might have an explicit `#[track_caller]`, we cannot know this at codegen time, since other crates may have impls of the trait. Therefore, we keep the current behavior of not forwarding the caller location, ensuring that all implementations of the trait will have the correct ABI. See the modified test for examples of how this works

Support forwarding caller location through trait object method call Since PR rust-lang#69251, the `#[track_caller]` attribute has been supported on traits. However, it only has an effect on direct (monomorphized) method calls. Calling a `#[track_caller]` method on a trait object will *not* propagate caller location information - instead, `Location::caller()` will return the location of the method definition. This PR forwards caller location information when `#[track_caller]` is present on the method definition in the trait. This is possible because `#[track_caller]` in this position is 'inherited' by any impls of that trait, so all implementations will have the same ABI. This PR does *not* change the behavior in the case where `#[track_caller]` is present only on the impl of a trait. While all implementations of the method might have an explicit `#[track_caller]`, we cannot know this at codegen time, since other crates may have impls of the trait. Therefore, we keep the current behavior of not forwarding the caller location, ensuring that all implementations of the trait will have the correct ABI. See the modified test for examples of how this works

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Tracking issue for RFC 2091: Implicit caller location #47809

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rust-highfive assigned eddyb and unassigned sfackler Feb 18, 2020

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Centril added the F-track_caller `#![feature(track_caller)]` label Feb 18, 2020

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anp and others added 2 commits March 22, 2020 17:21

Remove special-casing from TyCtxt::impl_of_method.

69bd46a

We can do this now that opt_associated_item doesn't have any panicking paths.

anp force-pushed the track-caller-in-traits branch from 6c2c559 to 69bd46a Compare March 23, 2020 00:21

anp mentioned this pull request Mar 23, 2020

#[track_caller] should inherit through specialization ancestors, too? #70293

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Rollup of 9 pull requests #70295

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Rollup of 9 pull requests #70296

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bors merged commit 5ed9d7e into rust-lang:master Mar 23, 2020

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#[track_caller] on fns in extern "Rust" #70830

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Support #[track_caller] on functions in extern "Rust" { ... } #70916

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anp mentioned this pull request May 21, 2020

Stabilize #[track_caller]. #72445

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Aaron1011 mentioned this pull request Jan 24, 2021

Support forwarding caller location through trait object method call #81360

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#[track_caller] in traits #69251

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