Defer tail call ret ty equality to check_tail_calls #144915

compiler-errors · 2025-08-04T16:14:21Z

Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons.

Firstly, we were using subtyping instead of equality in the HIR typeck code:

rust/compiler/rustc_hir_typeck/src/expr.rs

Line 1096 in e1b9081

self.demand_suptype(expr.span, ret_ty, call_expr_ty);

We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions.

That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying.

All of this seems like a lot of work, and we already are manually checking argument equality. Let's just extend the check_tail_calls to account for mismatches in return types anyways.

r? @WaffleLapkin

rustbot · 2025-08-04T16:14:26Z

WaffleLapkin is not on the review rotation at the moment.
They may take a while to respond.

compiler/rustc_borrowck/src/type_check/mod.rs

compiler/rustc_mir_build/src/check_tail_calls.rs

lcnr · 2025-08-05T09:30:21Z

r? lcnr

r=me after nits

WaffleLapkin · 2025-08-05T09:49:22Z

That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying.

That doesn't seem to track for me 🤔

The only reason we care about signature equality is that we need ABIs to match exactly, but ABIs are completely unaffected by lifetimes, so for the purpose of lifetimes we can do the same as normal call+ret.

compiler/rustc_mir_build/src/check_tail_calls.rs

tests/ui/explicit-tail-calls/ret-ty-hr-mismatch.stderr

compiler-errors · 2025-08-05T11:54:43Z

That doesn't seem to track for me 🤔

The only reason we care about signature equality is that we need ABIs to match exactly, but ABIs are completely unaffected by lifetimes, so for the purpose of lifetimes we can do the same as normal call+ret.

They are affected by higher-ranked lifetimes, though, and higher-ranked subtyping is different than higher-ranked equality. So what I'm saying is that checking this the way that normal return types are checked is (theoretically) insufficient since they don't enforce return type equality, but just a subtyping relationship between the thing being returned by the callee and the thing being returned by the caller.

That being said, do we even have a case where a non-invariant arg causes a struct or something to have a different layout so that a subtyping operation would change its layout?

theemathas · 2025-08-05T11:58:05Z

A type and its subtype always has the same layout and ABI, since fn() -> Subtype can be coerced to a fn() -> Supertype without changing the machine code.

WaffleLapkin

this looks like a reasonable step. r=us with nits

compiler/rustc_mir_build/src/check_tail_calls.rs

bors · 2025-08-07T02:34:06Z

☔ The latest upstream changes (presumably #145020) made this pull request unmergeable. Please resolve the merge conflicts.

WaffleLapkin · 2025-08-14T19:17:02Z

@rustbot author

xacrimon

lgtm

rustbot · 2025-08-19T20:16:29Z

This PR was rebased onto a different master 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.

WaffleLapkin · 2025-08-20T12:15:43Z

@bors r+

bors · 2025-08-20T12:15:47Z

📌 Commit 6a088fd has been approved by WaffleLapkin

It is now in the queue for this repository.

WaffleLapkin · 2025-08-20T12:16:16Z

@bors r-
@bors r=WaffleLapkin,lcnr

bors · 2025-08-20T12:16:20Z

📌 Commit 6a088fd has been approved by WaffleLapkin,lcnr

It is now in the queue for this repository.

…quality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes rust-lang#144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? `@WaffleLapkin`

Rollup of 5 pull requests Successful merges: - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145415 (std_detect: RISC-V: implement implication to "C") - #145647 (miri subtree update) - #145650 (Fix JS search scripts path) r? `@ghost` `@rustbot` modify labels: rollup

…quality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes rust-lang#144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? ``@WaffleLapkin``

Rollup of 12 pull requests Successful merges: - #143383 (stabilize `const_array_each_ref`) - #144443 (Make target pointer width in target json an integer) - #144758 ([Doc] Add links to the various collections) - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145415 (std_detect: RISC-V: implement implication to "C") - #145573 (Add an experimental unsafe(force_target_feature) attribute.) - #145642 (Do not use effective_visibilities query for Adt types of a local trait while proving a where-clause) - #145650 (Fix JS search scripts path) - #145654 (Download CI GCC into the correct directory) - #145662 (Enforce correct number of arguments for `"x86-interrupt"` functions) - #145674 (Enable triagebot `[review-changes-since]` feature) Failed merges: - #145647 (miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

…quality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes rust-lang#144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? ```@WaffleLapkin```

Rollup of 14 pull requests Successful merges: - #143383 (stabilize `const_array_each_ref`) - #144443 (Make target pointer width in target json an integer) - #144758 ([Doc] Add links to the various collections) - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145137 (Consolidate panicking functions in `slice/index.rs`) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145297 (fix(debuginfo): handle false positives in overflow check) - #145415 (std_detect: RISC-V: implement implication to "C") - #145642 (Do not use effective_visibilities query for Adt types of a local trait while proving a where-clause) - #145650 (Fix JS search scripts path) - #145654 (Download CI GCC into the correct directory) - #145662 (Enforce correct number of arguments for `"x86-interrupt"` functions) - #145674 (Enable triagebot `[review-changes-since]` feature) - #145678 (Fix typo in docstring) r? `@ghost` `@rustbot` modify labels: rollup

…quality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes rust-lang#144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? ````@WaffleLapkin````

Rollup of 15 pull requests Successful merges: - #143383 (stabilize `const_array_each_ref`) - #144758 ([Doc] Add links to the various collections) - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145137 (Consolidate panicking functions in `slice/index.rs`) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145297 (fix(debuginfo): handle false positives in overflow check) - #145415 (std_detect: RISC-V: implement implication to "C") - #145590 (Prevent impossible combinations in `ast::ModKind`.) - #145621 (Fix some doc typos) - #145642 (Do not use effective_visibilities query for Adt types of a local trait while proving a where-clause) - #145650 (Fix JS search scripts path) - #145654 (Download CI GCC into the correct directory) - #145662 (Enforce correct number of arguments for `"x86-interrupt"` functions) - #145674 (Enable triagebot `[review-changes-since]` feature) - #145678 (Fix typo in docstring) r? `@ghost` `@rustbot` modify labels: rollup

…quality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes rust-lang#144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? `````@WaffleLapkin`````

Rollup of 19 pull requests Successful merges: - #143383 (stabilize `const_array_each_ref`) - #144758 ([Doc] Add links to the various collections) - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145297 (fix(debuginfo): handle false positives in overflow check) - #145390 (Shorten some dependency chains in the compiler) - #145415 (std_detect: RISC-V: implement implication to "C") - #145525 (stdlib: Replace typedef -> type alias in doc comment) - #145590 (Prevent impossible combinations in `ast::ModKind`.) - #145593 (UnsafePinned::raw_get: sync signature with get) - #145621 (Fix some doc typos) - #145627 (Unconditionally-const supertraits are considered not dyn compatible) - #145642 (Do not use effective_visibilities query for Adt types of a local trait while proving a where-clause) - #145650 (Fix JS search scripts path) - #145654 (Download CI GCC into the correct directory) - #145662 (Enforce correct number of arguments for `"x86-interrupt"` functions) - #145673 (Add flock support for cygwin) - #145674 (Enable triagebot `[review-changes-since]` feature) - #145678 (Fix typo in docstring) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144915 - compiler-errors:tail-call-ret-ty-equality, r=WaffleLapkin,lcnr Defer tail call ret ty equality to check_tail_calls Fixes #144892. Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons. Firstly, we were using subtyping instead of equality in the HIR typeck code: https://github.com/rust-lang/rust/blob/e1b9081e699065badfc1a9419ec9566e5c8615c4/compiler/rustc_hir_typeck/src/expr.rs#L1096 We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions. That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances, since tail call terminators need to relate their arguments invariantly to account for the "signature must be equal" rule. This seems annoying. All of this seems like a lot of work, and we already are *manually* checking argument equality. Let's just extend the `check_tail_calls` to account for mismatches in return types anyways. r? ``````@WaffleLapkin``````

rustbot assigned WaffleLapkin Aug 4, 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 4, 2025

This comment was marked as off-topic.

Sign in to view

WaffleLapkin added the F-explicit_tail_calls `#![feature(explicit_tail_calls)]` label Aug 5, 2025

WaffleLapkin mentioned this pull request Aug 5, 2025

Tracking Issue for Explicit Tail Calls #112788

Open

26 tasks

lcnr reviewed Aug 5, 2025

View reviewed changes

compiler/rustc_borrowck/src/type_check/mod.rs Outdated Show resolved Hide resolved

lcnr reviewed Aug 5, 2025

View reviewed changes

compiler/rustc_mir_build/src/check_tail_calls.rs Show resolved Hide resolved

rustbot assigned lcnr and unassigned WaffleLapkin Aug 5, 2025

WaffleLapkin reviewed Aug 5, 2025

View reviewed changes

compiler/rustc_mir_build/src/check_tail_calls.rs Show resolved Hide resolved

WaffleLapkin reviewed Aug 5, 2025

View reviewed changes

tests/ui/explicit-tail-calls/ret-ty-hr-mismatch.stderr Outdated Show resolved Hide resolved

WaffleLapkin approved these changes Aug 5, 2025

View reviewed changes

compiler/rustc_mir_build/src/check_tail_calls.rs Outdated Show resolved Hide resolved

This comment was marked as off-topic.

Sign in to view

This comment has been minimized.

Sign in to view

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 14, 2025

xacrimon approved these changes Aug 19, 2025

View reviewed changes

Defer tail call ret ty equality to check_tail_calls

6a088fd

compiler-errors force-pushed the tail-call-ret-ty-equality branch from e42f3ae to 6a088fd Compare August 19, 2025 20:16

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 20, 2025

bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 20, 2025

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 20, 2025

Zalathar mentioned this pull request Aug 20, 2025

Rollup of 5 pull requests #145658

Closed

jhpratt mentioned this pull request Aug 20, 2025

Rollup of 12 pull requests #145680

Closed

jhpratt mentioned this pull request Aug 20, 2025

Rollup of 14 pull requests #145681

Closed

jhpratt mentioned this pull request Aug 21, 2025

Rollup of 15 pull requests #145691

Closed

This was referenced Aug 21, 2025

Rollup of 20 pull requests #145697

Closed

Rollup of 19 pull requests #145701

Merged

bors merged commit f1a7294 into rust-lang:master Aug 21, 2025
10 checks passed

rustbot added this to the 1.91.0 milestone Aug 21, 2025

Defer tail call ret ty equality to check_tail_calls #144915

Defer tail call ret ty equality to check_tail_calls #144915

Conversation

compiler-errors commented Aug 4, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Aug 4, 2025

Uh oh!

This comment was marked as off-topic.

This comment was marked as off-topic.

Uh oh!

Uh oh!

lcnr commented Aug 5, 2025

Uh oh!

WaffleLapkin commented Aug 5, 2025

Uh oh!

Uh oh!

Uh oh!

compiler-errors commented Aug 5, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

theemathas commented Aug 5, 2025

Uh oh!

WaffleLapkin left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

bors commented Aug 7, 2025

Uh oh!

This comment was marked as off-topic.

This comment has been minimized.

This comment has been minimized.

WaffleLapkin commented Aug 14, 2025

Uh oh!

xacrimon left a comment

Choose a reason for hiding this comment

Uh oh!

rustbot commented Aug 19, 2025

Uh oh!

WaffleLapkin commented Aug 20, 2025

Uh oh!

bors commented Aug 20, 2025

Uh oh!

WaffleLapkin commented Aug 20, 2025

Uh oh!

bors commented Aug 20, 2025

Uh oh!

Uh oh!

Uh oh!

compiler-errors commented Aug 4, 2025 •

edited

Loading

compiler-errors commented Aug 5, 2025 •

edited

Loading