Report higher-ranked trait error when higher-ranked projection goal fails in new solver by compiler-errors · Pull Request #139513 · rust-lang/rust

compiler-errors · 2025-04-08T05:23:35Z

~~See HACK comment inline. Not actually sure if it should be marked as a HACK, b/c~~ it's kinda a legitimate case we want to care about unless we're going to make the proof tree visitor smarter about the leak check than the actual trait solver itself.

Encountered this while battling with NiceRegionErrors in the old solver b/c I wondered what this code ended up giving us in the new solver as a comparison:

traitFoo{}impl<T:FnOnce(&())>FooforT{}fnbaz<T:Foo>(){}fnmain(){baz::<fn(&'static())>();}

On master it's pretty bad:

error[E0271]: type mismatch resolving `<fn(&()) as FnOnce<(&(),)>>::Output == ()`
--> <source>:8:11
|
8 | baz::<fn(&'static ())>();
| ^^^^^^^^^^^^^^^ types differ
|
note: required for `fn(&'static ())` to implement `Foo`
--> <source>:3:22
|
3 | impl<T: FnOnce(&())> Foo for T {}
| ----------- ^^^ ^
| |
| unsatisfied trait bound introduced here

After this PR it's much better:

error[E0277]: the trait bound `fn(&'static ()): Foo` is not satisfied
--> /home/mgx/test.rs:8:11
|
8 | baz::<fn(&'static ())>();
| ^^^^^^^^^^^^^^^ the trait `for<'a> FnOnce(&'a ())` is not implemented for `fn(&'static ())`
|
= note: expected a closure with arguments `(&'static (),)`
found a closure with arguments `(&(),)`
note: required for `fn(&'static ())` to implement `Foo`
--> /home/mgx/test.rs:3:22
|
3 | impl<T: FnOnce(&())> Foo for T {}
| ----------- ^^^ ^
| |
| unsatisfied trait bound introduced here
note: required by a bound in `baz`
--> /home/mgx/test.rs:5:11
|
5 | fn baz<T: Foo>() {}
| ^^^ required by this bound in `baz`

r? lcnr

rustbot · 2025-04-08T05:23:41Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

…ails in new solver

compiler-errors · 2025-04-09T17:55:26Z

Pulled this into another detect_* helper method, so it's consistent with the others.

@bors r=lcnr rollup

bors · 2025-04-09T17:55:29Z

📌 Commit f6faaee has been approved by lcnr

It is now in the queue for this repository.

lcnr · 2025-04-09T18:39:58Z

+ /// `NormalizesTo` goal, so we don't fall back to the rigid projection check
+ /// that should catch when a projection goal fails due to an unsatisfied trait
+ /// goal.
+ fn detect_error_in_higher_ranked_projection(


Suggested change
fndetect_error_in_higher_ranked_projection(
fndetect_trait_error_in_higher_ranked_projection(
r=me with this change

Already got rolled up, and I don't think this is a significant enough change to kick it out of the rollup. I'll fold this into #139513 which is based off this one.

Unless this fails to merge in which case I'll amend this. I'll keep an eye on the rollup lol

…iaskrgr Rollup of 10 pull requests Successful merges: - rust-lang#138470 (Test interaction between RFC 2229 migration and use closures) - rust-lang#138628 (Add more ergonomic clone tests) - rust-lang#139164 (std: improve documentation for get_mut() methods regarding forgotten guards) - rust-lang#139488 (Add missing regression GUI test) - rust-lang#139489 (compiletest: Add directive `dont-require-annotations`) - rust-lang#139513 (Report higher-ranked trait error when higher-ranked projection goal fails in new solver) - rust-lang#139521 (triagebot: roll compiler reviewers for rustc/unstable book) - rust-lang#139532 (Update `u8`-to-and-from-`i8` suggestions.) - rust-lang#139551 (report call site of inlined scopes for large assignment lints) - rust-lang#139575 (Remove redundant words) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#139513 - compiler-errors:higher-ranked-proj, r=lcnr Report higher-ranked trait error when higher-ranked projection goal fails in new solver ~~See HACK comment inline. Not actually sure if it should be marked as a *HACK*, b/c~~ it's kinda a legitimate case we want to care about unless we're going to make the proof tree visitor *smarter* about the leak check than the actual trait solver itself. Encountered this while battling with `NiceRegionError`s in the old solver b/c I wondered what this code ended up giving us in the *new* solver as a comparison: ```rust trait Foo {} impl<T: FnOnce(&())> Foo for T {} fn baz<T: Foo>() {} fn main() { baz::<fn(&'static ())>(); } ``` On master it's pretty bad: ``` error[E0271]: type mismatch resolving `<fn(&()) as FnOnce<(&(),)>>::Output == ()` --> <source>:8:11 | 8 | baz::<fn(&'static ())>(); | ^^^^^^^^^^^^^^^ types differ | note: required for `fn(&'static ())` to implement `Foo` --> <source>:3:22 | 3 | impl<T: FnOnce(&())> Foo for T {} | ----------- ^^^ ^ | | | unsatisfied trait bound introduced here ``` After this PR it's much better: ``` error[E0277]: the trait bound `fn(&'static ()): Foo` is not satisfied --> /home/mgx/test.rs:8:11 | 8 | baz::<fn(&'static ())>(); | ^^^^^^^^^^^^^^^ the trait `for<'a> FnOnce(&'a ())` is not implemented for `fn(&'static ())` | = note: expected a closure with arguments `(&'static (),)` found a closure with arguments `(&(),)` note: required for `fn(&'static ())` to implement `Foo` --> /home/mgx/test.rs:3:22 | 3 | impl<T: FnOnce(&())> Foo for T {} | ----------- ^^^ ^ | | | unsatisfied trait bound introduced here note: required by a bound in `baz` --> /home/mgx/test.rs:5:11 | 5 | fn baz<T: Foo>() {} | ^^^ required by this bound in `baz` ``` r? lcnr

Deeply normalize obligations in `BestObligation` folder Built on rust-lang#139513. This establishes a somewhat rough invariant that the `Obligation`'s predicate is always deeply normalized in the folder; when we construct a new obligation we normalize it. Putting this up for discussion since it does affect some goals. r? lcnr

Rollup merge of rust-lang#139564 - compiler-errors:deeply-norm, r=lcnr Deeply normalize obligations in `BestObligation` folder Built on rust-lang#139513. This establishes a somewhat rough invariant that the `Obligation`'s predicate is always deeply normalized in the folder; when we construct a new obligation we normalize it. Putting this up for discussion since it does affect some goals. r? lcnr

…iaskrgr Rollup of 10 pull requests Successful merges: - rust-lang#138470 (Test interaction between RFC 2229 migration and use closures) - rust-lang#138628 (Add more ergonomic clone tests) - rust-lang#139164 (std: improve documentation for get_mut() methods regarding forgotten guards) - rust-lang#139488 (Add missing regression GUI test) - rust-lang#139489 (compiletest: Add directive `dont-require-annotations`) - rust-lang#139513 (Report higher-ranked trait error when higher-ranked projection goal fails in new solver) - rust-lang#139521 (triagebot: roll compiler reviewers for rustc/unstable book) - rust-lang#139532 (Update `u8`-to-and-from-`i8` suggestions.) - rust-lang#139551 (report call site of inlined scopes for large assignment lints) - rust-lang#139575 (Remove redundant words) r? `@ghost` `@rustbot` modify labels: rollup

compiler-errors · 2025-04-08T05:23:35Z