Fix tail calls to `#[track_caller]` functions #144865

WaffleLapkin · 2025-08-03T19:04:50Z

We want #[track_caller] to be semver independent, i.e. it should not be a breaking change to add or remove it. Since it changes ABI of a function (adding an additional argument) we have to be careful to preserve this property when adding tail calls.

The only way to achieve this that I can see is:

we forbid tail calls in functions which are marked with #[track_caller] (already implemented)
tail-calling a #[track_caller] marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr)

Ideally the downgrade would be performed by a MIR pass, but that requires post mono MIR opts (cc @saethlin, #131650). For now I've changed code in cg_ssa to accomodate this behaviour (+ added a hack to mono collector so that the shim is actually generated)

Additionally I added a lint, although I don't think it's strictly necessary.

Alternative to #144762 (and thus closes #144762)
Fixes #144755

rustbot · 2025-08-03T19:04:53Z

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

rustbot · 2025-08-03T19:04:56Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

WaffleLapkin · 2025-08-04T08:09:43Z

I do not get it, both tests pass for me locally ?_?

WaffleLapkin · 2025-08-04T09:13:53Z

Update: @jieyouxu figured this out. It looks like the test suite that failed was running with --pass=check, which caused the tests that depend on diagnostic from codegen backend to fail. I've added //@ ignore-pass so it should be good now

fee1-dead · 2025-08-05T05:19:59Z

not too familiar with tail calls but the diff looks good to me

r? compiler

WaffleLapkin · 2025-08-05T08:37:39Z

r? lqd maybe (since you were excited about tail calls ^^')

lqd · 2025-08-06T15:24:04Z

It also LGTM -- though to be clear, I'm not familiar with the #[track_caller] infra. I wonder if people will have questions about the wording "has no special effect", as in "will not be TCEd yet".

Also, do we have to involve t-lang for new lints?

VorpalBlade · 2025-08-07T05:11:36Z

tail-calling a #[track_caller] marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr)

Would this be a silent downgrade? I think that would be a terrible idea, since for me the main usage of become is guaranteed tail calls for threaded interpreters. The tail call turning into a normal call means the code won't work and will eventually overflow the stack. This sort of implicit behaviour is the antithesis of what rust stands for.

WaffleLapkin · 2025-08-07T07:36:59Z

@lqd the wording is certainly not ideal, although I'm not entirely sure what would be the proper way to describe this situation. I don't think T-lang needs to be involved, since this lint is triggered only by an unstable/experimental feature (but they'll certainly have to look into the lint before stabilizing the feature).

@VorpalBlade it will not be silent, as this PR also adds a lint. However, note that functions marked with #[track_caller] can't tail call other functions, and as such can't participate in any tail call cycles, so you don't have any guarantees about stack exhaustion regardless.

lqd · 2025-08-07T11:54:58Z

Ok I wasn't aware of the procedure for new lints.

lqd

👍

lqd · 2025-08-07T11:55:32Z

compiler/rustc_lint_defs/src/builtin.rs

+
+declare_lint! {
+    /// The `tail_call_track_caller` lint detects usage of `become` attempting to tail call
+    /// function marked with `#[track_caller]`.


nit, "a function" or "functions"

compiler-errors · 2025-08-12T19:46:32Z

compiler/rustc_lint_defs/src/builtin.rs

+    /// is no different than a normal call (except for changes in drop order).
+    pub TAIL_CALL_TRACK_CALLER,
+    Warn,
+    "detects tail calls of functions marked with `#[track_caller]`"


Could you gate this behind the tail call feature?

I can't remember the syntax but I think it's something like @feature = ....

I did not know this is something that can be done!

The syntax is apparently @feature_gate = ident;. I've gated the lint.

lqd · 2025-08-14T20:24:03Z

🚀

@bors r+

bors · 2025-08-14T20:24:06Z

📌 Commit fa18b3e has been approved by lqd

It is now in the queue for this repository.

Fix tail calls to `#[track_caller]` functions We want `#[track_caller]` to be semver independent, i.e. it should not be a breaking change to add or remove it. Since it changes ABI of a function (adding an additional argument) we have to be careful to preserve this property when adding tail calls. The only way to achieve this that I can see is: - we forbid tail calls in functions which are marked with `#[track_caller]` (already implemented) - tail-calling a `#[track_caller]` marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr) Ideally the downgrade would be performed by a MIR pass, but that requires post mono MIR opts (cc `@saethlin,` rust-lang#131650). For now I've changed code in cg_ssa to accomodate this behaviour (+ added a hack to mono collector so that the shim is actually generated) Additionally I added a lint, although I don't think it's strictly necessary. Alternative to rust-lang#144762 (and thus closes rust-lang#144762) Fixes rust-lang#144755

Rollup of 22 pull requests Successful merges: - #118087 (Add Ref/RefMut try_map method) - #122661 (Change the desugaring of `assert!` for better error output) - #140740 (Add `-Zindirect-branch-cs-prefix`) - #142640 (Implement autodiff using intrinsics) - #143075 (compiler: Allow `extern "interrupt" fn() -> !`) - #144865 (Fix tail calls to `#[track_caller]` functions) - #144944 (E0793: Clarify that it applies to unions as well) - #144947 (Fix description of unsigned `checked_exact_div`) - #145004 (Couple of minor cleanups) - #145005 (strip prefix of temporary file names when it exceeds filesystem name length limit) - #145012 (Tail call diagnostics to include lifetime info) - #145065 (resolve: Introduce `RibKind::Block`) - #145120 (llvm: Accept new LLVM lifetime format) - #145189 (Weekly `cargo update`) - #145235 (Minor `[const]` tweaks) - #145275 (fix(compiler/rustc_codegen_llvm): apply `target-cpu` attribute) - #145322 (Resolve the prelude import in `build_reduced_graph`) - #145331 (Make std use the edition 2024 prelude) - #145369 (Do not ICE on private type in field of unresolved struct) - #145378 (Add `FnContext` in parser for diagnostic) - #145389 ([rustdoc] Revert "rustdoc search: prefer stable items in search results") - #145392 (coverage: Remove intermediate data structures from mapping creation) r? `@ghost` `@rustbot` modify labels: rollup

Fix tail calls to `#[track_caller]` functions We want `#[track_caller]` to be semver independent, i.e. it should not be a breaking change to add or remove it. Since it changes ABI of a function (adding an additional argument) we have to be careful to preserve this property when adding tail calls. The only way to achieve this that I can see is: - we forbid tail calls in functions which are marked with `#[track_caller]` (already implemented) - tail-calling a `#[track_caller]` marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr) Ideally the downgrade would be performed by a MIR pass, but that requires post mono MIR opts (cc ``@saethlin,`` rust-lang#131650). For now I've changed code in cg_ssa to accomodate this behaviour (+ added a hack to mono collector so that the shim is actually generated) Additionally I added a lint, although I don't think it's strictly necessary. Alternative to rust-lang#144762 (and thus closes rust-lang#144762) Fixes rust-lang#144755

Fix tail calls to `#[track_caller]` functions We want `#[track_caller]` to be semver independent, i.e. it should not be a breaking change to add or remove it. Since it changes ABI of a function (adding an additional argument) we have to be careful to preserve this property when adding tail calls. The only way to achieve this that I can see is: - we forbid tail calls in functions which are marked with `#[track_caller]` (already implemented) - tail-calling a `#[track_caller]` marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr) Ideally the downgrade would be performed by a MIR pass, but that requires post mono MIR opts (cc ```@saethlin,``` rust-lang#131650). For now I've changed code in cg_ssa to accomodate this behaviour (+ added a hack to mono collector so that the shim is actually generated) Additionally I added a lint, although I don't think it's strictly necessary. Alternative to rust-lang#144762 (and thus closes rust-lang#144762) Fixes rust-lang#144755

WaffleLapkin added A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. F-track_caller `#![feature(track_caller)]` F-explicit_tail_calls `#![feature(explicit_tail_calls)]` labels Aug 3, 2025

rustbot assigned fee1-dead Aug 3, 2025

rustbot added 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 3, 2025

This was referenced Aug 3, 2025

Reject tail calls of #[track_caller] functions #144762

Closed

Tracking Issue for Explicit Tail Calls #112788

Open

This comment has been minimized.

Sign in to view

WaffleLapkin force-pushed the track-tail branch from e747cdd to a487936 Compare August 3, 2025 19:43

This comment has been minimized.

Sign in to view

WaffleLapkin force-pushed the track-tail branch from a487936 to 6fff1d6 Compare August 4, 2025 07:05

This comment has been minimized.

Sign in to view

WaffleLapkin force-pushed the track-tail branch from 6fff1d6 to 15219a0 Compare August 4, 2025 08:56

rustbot assigned petrochenkov and unassigned fee1-dead Aug 5, 2025

rustbot assigned lqd and unassigned petrochenkov Aug 5, 2025

lqd approved these changes Aug 7, 2025

View reviewed changes

lqd mentioned this pull request Aug 12, 2025

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

Closed

compiler-errors reviewed Aug 12, 2025

View reviewed changes

fix tail calls to #[track_caller] functions

85d1c89

WaffleLapkin force-pushed the track-tail branch from 15219a0 to 85d1c89 Compare August 13, 2025 00:27

drive-by: fix typo

fa18b3e

WaffleLapkin mentioned this pull request Aug 14, 2025

Implement support for become and explicit tail call codegen for the LLVM backend #144232

Merged

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 14, 2025

Zalathar mentioned this pull request Aug 15, 2025

Rollup of 22 pull requests #145418

Closed

Zalathar mentioned this pull request Aug 15, 2025

Rollup of 21 pull requests #145423

Open

Fix tail calls to #[track_caller] functions #144865

Are you sure you want to change the base?

Fix tail calls to #[track_caller] functions #144865

Conversation

WaffleLapkin commented Aug 3, 2025 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Aug 3, 2025

Uh oh!

rustbot commented Aug 3, 2025

Uh oh!

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

WaffleLapkin commented Aug 4, 2025

Uh oh!

WaffleLapkin commented Aug 4, 2025

Uh oh!

fee1-dead commented Aug 5, 2025

Uh oh!

WaffleLapkin commented Aug 5, 2025

Uh oh!

lqd commented Aug 6, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

VorpalBlade commented Aug 7, 2025

Uh oh!

WaffleLapkin commented Aug 7, 2025

Uh oh!

lqd commented Aug 7, 2025

Uh oh!

lqd left a comment

Choose a reason for hiding this comment

Uh oh!

lqd Aug 7, 2025

Choose a reason for hiding this comment

Uh oh!

WaffleLapkin Aug 13, 2025

Choose a reason for hiding this comment

Uh oh!

compiler-errors Aug 12, 2025

Choose a reason for hiding this comment

Uh oh!

WaffleLapkin Aug 13, 2025

Choose a reason for hiding this comment

Uh oh!

lqd commented Aug 14, 2025

Uh oh!

bors commented Aug 14, 2025

Uh oh!

Uh oh!

Fix tail calls to `#[track_caller]` functions #144865

Fix tail calls to `#[track_caller]` functions #144865

WaffleLapkin commented Aug 3, 2025 •

edited by rustbot

Loading

lqd commented Aug 6, 2025 •

edited

Loading