rework GAT borrowck limitation error #145041

lcnr · 2025-08-07T09:32:21Z

The old one depends on the ConstraintCategory of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization.

This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in #140737 (comment).

r? types cc @amandasystems

rustbot · 2025-08-07T09:32:28Z

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

lcnr · 2025-08-07T09:33:34Z

compiler/rustc_borrowck/src/diagnostics/mod.rs

-                && let ConstraintCategory::Predicate(span) = constraint.category
+                && let NllRegionVariableOrigin::Placeholder(placeholder) = origin.origin
+                && let Some(id) = placeholder.bound.kind.get_id()
+                && let Some(placeholder_id) = id.as_local()


This lint should ideally also work for placeholders from foreign crates by looking at the bound vars and span from the parent predicates_of.

Changing that is annoying and I did not do it in this PR.

This logic will probably help a lot with various order-dependent things I introduce in my PRs, since selection of the correct placeholder is a lot less sensitive now! Thanks!

lcnr · 2025-08-07T09:36:13Z

tests/ui/borrowck/implementation-not-general-enough-ice-133252.stderr

-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/implementation-not-general-enough-ice-133252.rs:16:18
-   |
-LL | fn force_send<T: Send>(_: T) {}


The fact that this note worked and was also applicable here feels somewhat incidental to me. It's caused by incorrectly handling the existential regions in opaque types. This is an instantiation of #110338 for opaque types instead of coroutines

compiler/rustc_borrowck/src/diagnostics/mod.rs

BoxyUwU · 2025-08-13T12:41:44Z

r? BoxyUwU

compiler/rustc_borrowck/src/diagnostics/mod.rs

BoxyUwU · 2025-08-14T15:09:39Z

r=me after comment or not

BoxyUwU · 2025-08-16T14:18:26Z

@bors r+

bors · 2025-08-16T14:18:29Z

📌 Commit 3ebf611 has been approved by BoxyUwU

It is now in the queue for this repository.

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc `@amandasystems`

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ``@amandasystems``

Rollup of 19 pull requests Successful merges: - #140956 (`impl PartialEq<{str,String}> for {Path,PathBuf}`) - #141744 (Stabilize `ip_from`) - #144804 (Don't warn on never to any `as` casts as unreachable) - #144983 (Rehome 37 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`) - #145025 (run spellcheck as a tidy extra check in ci) - #145041 (rework GAT borrowck limitation error) - #145243 (take attr style into account in diagnostics) - #145359 (Fix bug where `rustdoc-js` tester would not pick the right `search.js` file if there is more than one) - #145429 (Couple of codegen_fn_attrs improvements) - #145452 (Do not strip binaries in bootstrap everytime if they are unchanged) - #145486 (Fix `unicode_data.rs` mention message) - #145489 (library: Migrate from `cfg_if` to `cfg_select`) - #145493 (remove `should_render` in `PrintAttribute` derive) - #145500 (Port must_use to the new target checking) - #145505 (Simplify span caches) - #145511 (Rust build fails on OpenBSD after using file_lock feature) - #145516 (Weekly `cargo update`) - #145533 (Reorder `lto` options from most to least optimizing) - #145550 (Avoid using `()` in `derive(From)` output.) r? `@ghost` `@rustbot` modify labels: rollup

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ```@amandasystems```

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc `@amandasystems`

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ``@amandasystems``

Rollup of 19 pull requests Successful merges: - #140956 (`impl PartialEq<{str,String}> for {Path,PathBuf}`) - #141744 (Stabilize `ip_from`) - #142681 (Remove the `#[no_sanitize]` attribute in favor of `#[sanitize(xyz = "on|off")]`) - #142871 (Trivial improve doc for transpose ) - #144252 (Do not copy .rmeta files into the sysroot of the build compiler during check of rustc/std) - #144476 (rustdoc-search: search backend with partitioned suffix tree) - #144567 (Fix RISC-V Test Failures in ./x test for Multiple Codegen Cases) - #144804 (Don't warn on never to any `as` casts as unreachable) - #144960 ([RTE-513] Ignore sleep_until test on SGX) - #145013 (overhaul `&mut` suggestions in borrowck errors) - #145041 (rework GAT borrowck limitation error) - #145243 (take attr style into account in diagnostics) - #145405 (cleanup: use run_in_tmpdir in run-make/rustdoc-scrape-examples-paths) - #145432 (cg_llvm: Small cleanups to `owned_target_machine`) - #145484 (Remove `LlvmArchiveBuilder` and supporting code/bindings) - #145557 (Fix uplifting in `Assemble` step) - #145563 (Remove the `From` derive macro from prelude) - #145565 (Improve context of bootstrap errors in CI) - #145584 (interpret: avoid forcing all integer newtypes into memory during clear_provenance) Failed merges: - #145359 (Fix bug where `rustdoc-js` tester would not pick the right `search.js` file if there is more than one) - #145573 (Add an experimental unsafe(force_target_feature) attribute.) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #145041 - lcnr:borrowck-limitations-error, r=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in #140737 (comment). r? types cc ```@amandasystems```

Zalathar · 2025-08-19T10:10:13Z

Bors hasn't noticed that this was merged.

@bors r- retry

rustbot assigned spastorino Aug 7, 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 7, 2025

lcnr commented Aug 7, 2025

View reviewed changes

lcnr force-pushed the borrowck-limitations-error branch from 5a3ddbc to ba8fa8f Compare August 7, 2025 09:49

amandasystems reviewed Aug 7, 2025

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compiler/rustc_borrowck/src/diagnostics/mod.rs Show resolved Hide resolved

amandasystems mentioned this pull request Aug 8, 2025

Region inference: Use outlives-static constraints in constraint search #140737

Open

lcnr mentioned this pull request Aug 13, 2025

"due to current limitations in the borrow checker" is often not a current limitation #145347

Open

lcnr added 3 commits August 13, 2025 14:03

avoid duplicate error string

4e2d420

also consider HR bounds

d62e857

rework add_placeholder_from_predicate_note

a95a2ac

lcnr force-pushed the borrowck-limitations-error branch from 07a0562 to da17a53 Compare August 13, 2025 12:09

rustbot assigned BoxyUwU and unassigned spastorino Aug 13, 2025

BoxyUwU approved these changes Aug 14, 2025

View reviewed changes

compiler/rustc_borrowck/src/diagnostics/mod.rs Show resolved Hide resolved

BoxyUwU 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

lcnr added 2 commits August 14, 2025 17:43

add comment

5c716bd

it's not a borrow checker limitation :<

3ebf611

lcnr force-pushed the borrowck-limitations-error branch from da17a53 to 3ebf611 Compare August 14, 2025 15:43

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 16, 2025

jhpratt mentioned this pull request Aug 17, 2025

Rollup of 19 pull requests #145546

Closed

Zalathar mentioned this pull request Aug 18, 2025

Rollup of 19 pull requests #145552

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Zalathar mentioned this pull request Aug 18, 2025

Rollup of 18 pull requests #145566

Closed

Zalathar mentioned this pull request Aug 19, 2025

Rollup of 19 pull requests #145589

Merged

bors merged commit f3f1847 into rust-lang:master Aug 19, 2025
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rustbot added this to the 1.91.0 milestone Aug 19, 2025

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rework GAT borrowck limitation error #145041

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