Fix an ICE observed with an explicit tail-call in a default trait method by jakubadamw · Pull Request #145270 · rust-lang/rust

jakubadamw · 2025-08-11T20:32:49Z

tracking issue: #112788

Right now, explicit tail-calls cannot be used in functions that hold the #[track_caller] attribute. This check is performed on a resolved concrete instance of a function, which would be absent for a tail-call performed from the body of a default trait method. This PR fixes the issue by checking for the relevant attribute in default trait methods separately, without expecting a specific resolved instance.

Closes #144985.

rustbot · 2025-08-11T20:32:54Z

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compiler-errors

I don't think this is the right approach. Frankly, using caller_ty seems a bit unnecessary throughout check_tail_calls, though that's preexisting.

I think it's better if we refactor check_tail_calls to no longer rely at all on caller_ty and instead just have a body def id which corresponds to the item that has the THIR. We shouldn't need substs either -- those are always identity.

Then we shouldn't need to use Instance::expect_resolve at all here.

jakubadamw · 2025-08-12T11:28:15Z

I don't think this is the right approach. Frankly, using caller_ty seems a bit unnecessary throughout check_tail_calls, though that's preexisting.

I think it's better if we refactor check_tail_calls to no longer rely at all on caller_ty and instead just have a body def id which corresponds to the item that has the THIR. We shouldn't need substs either -- those are always identity.

Then we shouldn't need to use Instance::expect_resolve at all here.

Thank you for your feedback! I addressed it. There's a test failing in LLVM codegen, but this may be a pre-existing issue.

compiler-errors · 2025-08-12T18:48:42Z

Unfortunately, that's a legitimate bug.

I think the problem with #[track_caller] is deeper than this -- specifically, I think we actually need to disallow calling #[track_caller] functions altogether (which I think we can only check for during monomorphization), or at least we need to be adjusting such calls to a "reify shim" when monomorphizing those tail calls, though I think that ends up leading to us pushing a stack frame which seems not good.

I'm gonna mark this as blocked on #145321 (edit: actually #144755)

@rustbot blocked

As a side-note, please make sure your commits are squashed. It's not a requirement that you have 1 commit per PR, but this PR definitely shouldn't need 4 commits for a single change like this :)

lcnr · 2025-08-13T08:41:38Z

r? compiler-errors

rustbot · 2025-08-13T08:41:42Z

compiler-errors is not on the review rotation at the moment.
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jakubadamw · 2025-08-14T08:25:20Z

I think the problem with #[track_caller] is deeper than this -- specifically, I think we actually need to disallow calling #[track_caller] functions altogether (which I think we can only check for during monomorphization), or at least we need to be adjusting such calls to a "reify shim" when monomorphizing those tail calls, though I think that ends up leading to us pushing a stack frame which seems not good.

I'm gonna mark this as blocked on #145321 (edit: actually #144755)

@compiler-errors, thanks for elaborating on this!

So, as I gather from #145321 and #144755, if this is a pre-existing issue, would it be okay for us to move forward with this PR if we change the relevant test to be check-pass rather than run-pass, and let the codegen issue be independently covered by #145321?

As a side-note, please make sure your commits are squashed. It's not a requirement that you have 1 commit per PR, but this PR definitely shouldn't need 4 commits for a single change like this :)

Sure. I tend to make a commit per a logical chunk of work, and then in the course of PR’s review, only add new commits to make it clear what new changes have been made. But, of course, I’ll adjust and squash this, if you can confirm that converting the test to check-pass would be a sensible change.

bors · 2025-08-15T13:49:17Z

☔ The latest upstream changes (presumably #145423) made this pull request unmergeable. Please resolve the merge conflicts.

Dylan-DPC · 2025-10-14T07:47:19Z

unblocking as #144755 is resolved

WaffleLapkin · 2025-10-25T21:42:37Z

@jakubadamw could you rebase this PR?

folkertdev · 2026-04-13T20:04:28Z

@jakubadamw looks like you've opened some PRs here again, any chance you can rebase this? We'd like to move forward here, so if you're not able to that's also fine and we'll take it from here.

I tried to rebase locally but keep running into some weird linting errors, but maybe I'm missing something.

folkertdev · 2026-04-13T20:36:36Z

Turns out the troublesome lint was not properly registered. I've got things working on this branch main...folkertdev:rust:tail-call-trait-method, feel free to take things from there, otherwise I'll force push at some point.

jakubadamw · 2026-04-13T20:55:27Z

@WaffleLapkin, oops, my apologies, this one has slipped my attention/notifications. 🙂
@folkertdev, thank you for picking this up, I pushed the tree of your branch to this one. 🙂
@rustbot ready

WaffleLapkin

r=me with nit

View changes since this review

WaffleLapkin · 2026-04-16T10:44:12Z

+    }
+
+    fn foo(&self) -> usize {
+        #[allow(tail_call_track_caller)]


Suggested change

#[allow(tail_call_track_caller)]

#[expect(tail_call_track_caller)]

Turns out this does not work

error: 2 diagnostics reported in JSON output but not expected in test file tests/ui/explicit-tail-calls/default-trait-method.rs:17:18: WARN: this lint expectation is unfulfilled [unfulfilled_lint_expectations] tests/ui/explicit-tail-calls/default-trait-method.rs:17:18: WARN: this lint expectation is unfulfilled [unfulfilled_lint_expectations]

I think that is because there is some inconsistency between which HIR id is used, I actually suspect that we can/should emit this lint earlier so that we have more HIR stuff available.

I suspect that's blocked on some of the other work moving tail call logic to earlier stages, and in any case doesn't seem that relevant to this PR.

Actually, at least part of the issue is that the instance on Struct does not use track_caller, so the expect fails on that monomorphization anyway.

folkertdev · 2026-04-16T10:58:02Z

Also you can after making that fix just squash everything together into one commit

WaffleLapkin · 2026-04-19T01:57:00Z

@rustbot author

rustbot · 2026-04-19T01:57:04Z

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

folkertdev

I'll just squash this and then we can get it merged

View changes since this review

folkertdev · 2026-04-20T09:41:20Z

+    }
+
+    fn foo(&self) -> usize {
+        #[allow(tail_call_track_caller)]


Turns out this does not work

error: 2 diagnostics reported in JSON output but not expected in test file tests/ui/explicit-tail-calls/default-trait-method.rs:17:18: WARN: this lint expectation is unfulfilled [unfulfilled_lint_expectations] tests/ui/explicit-tail-calls/default-trait-method.rs:17:18: WARN: this lint expectation is unfulfilled [unfulfilled_lint_expectations]

I think that is because there is some inconsistency between which HIR id is used, I actually suspect that we can/should emit this lint earlier so that we have more HIR stuff available.

I suspect that's blocked on some of the other work moving tail call logic to earlier stages, and in any case doesn't seem that relevant to this PR.

The logic determining whether the relevant function is marked as `#[track_caller]` only worked with functions that could be resolved to a concrete instance, which default trait methods cannot. We need to check the codegen attribute on the default method itself. Co-authored-by: Folkert de Vries <folkert@folkertdev.nl>

rustbot · 2026-04-20T09:42:40Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

jakubadamw · 2026-04-20T10:34:34Z

@folkertdev, hey, apologies for not having been responsive, it was a very hectic week, and I didn't find the time. Thank you for doing the rebase for me!

folkertdev · 2026-04-20T11:29:02Z

Thanks for your work here!

@bors r=WaffleLapkin

rust-bors · 2026-04-20T11:29:05Z

📌 Commit ac5734a has been approved by WaffleLapkin

It is now in the queue for this repository.

…uwer Rollup of 11 pull requests Successful merges: - #154654 (Move `std::io::ErrorKind` to `core::io`) - #145270 (Fix an ICE observed with an explicit tail-call in a default trait method) - #154895 (borrowck: Apply `user_arg_index` nomenclature more broadly) - #155213 (resolve: Make sure visibilities of import declarations make sense) - #155346 (`single_use_lifetimes`: respect `anonymous_lifetime_in_impl_trait`) - #155517 (Add a test for Mach-O `#[link_section]` API inherited from LLVM) - #155549 (Remove some unnecessary lifetimes.) - #154248 (resolve : mark repr_simd as internal) - #154772 (slightly optimize the `non-camel-case-types` lint) - #155541 (Add `#[rust_analyzer::prefer_underscore_import]` to the traits in `rustc_type_ir::inherent`) - #155544 (bootstrap: Make "detected modifications" for download-rustc less verbose)

Rollup merge of #145270 - jakubadamw:issue-144985, r=WaffleLapkin Fix an ICE observed with an explicit tail-call in a default trait method Right now, explicit tail-calls cannot be used in functions that hold the `#[track_caller]` attribute. This check is performed on a resolved concrete instance of a function, which would be absent for a tail-call performed from the body of a default trait method. This PR fixes the issue by checking for the relevant attribute in default trait methods separately, without expecting a specific resolved instance. Closes #144985.

…uwer Rollup of 11 pull requests Successful merges: - rust-lang/rust#154654 (Move `std::io::ErrorKind` to `core::io`) - rust-lang/rust#145270 (Fix an ICE observed with an explicit tail-call in a default trait method) - rust-lang/rust#154895 (borrowck: Apply `user_arg_index` nomenclature more broadly) - rust-lang/rust#155213 (resolve: Make sure visibilities of import declarations make sense) - rust-lang/rust#155346 (`single_use_lifetimes`: respect `anonymous_lifetime_in_impl_trait`) - rust-lang/rust#155517 (Add a test for Mach-O `#[link_section]` API inherited from LLVM) - rust-lang/rust#155549 (Remove some unnecessary lifetimes.) - rust-lang/rust#154248 (resolve : mark repr_simd as internal) - rust-lang/rust#154772 (slightly optimize the `non-camel-case-types` lint) - rust-lang/rust#155541 (Add `#[rust_analyzer::prefer_underscore_import]` to the traits in `rustc_type_ir::inherent`) - rust-lang/rust#155544 (bootstrap: Make "detected modifications" for download-rustc less verbose)

rustbot assigned lcnr Aug 11, 2025

rustbot added F-explicit_tail_calls `#![feature(explicit_tail_calls)]` S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 11, 2025

jakubadamw changed the title ~~Fix an ICE observed with an explicit tail call in a default trait method~~ Fix an ICE observed with an explicit tail-call in a default trait method Aug 11, 2025

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compiler-errors mentioned this pull request Aug 12, 2025

We shouldn't be able to tail-call #[track_caller] functions #145321

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Tracking Issue for Explicit Tail Calls #112788

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Rollup of 11 pull requests #155552

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rustbot added this to the 1.97.0 milestone Apr 20, 2026

	#[allow(tail_call_track_caller)]
	#[expect(tail_call_track_caller)]

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