fix: Panic when a TAIT exists in a RPIT

[ShoyuVanilla]

Fixes #17921

When there is a TAIT inside of a RPIT like;

trait Foo {}
type Bar = impl Foo;
fn foo<A>() -> impl Future<Output = Bar> { .. }

while inferencing fn foo, insert_inference_vars_for_impl_trait tries to substitute impl trait bounds of Bar, i.e. Implemented(Foo) with RPITs placeholders, and this causes panic

rust-analyzer/crates/hir-ty/src/infer.rs

Lines 903 to 905 in fa00326

	let var_predicate = self
	.insert_inference_vars_for_impl_trait(var_predicate, placeholders.clone());
	self.push_obligation(var_predicate.cast(Interner));

[bors]

☔ The latest upstream changes (presumably #17925) made this pull request unmergeable. Please resolve the merge conflicts.

[Veykril]

Suggested change

	tait_candidates.insert(ty);
	if ty.data(Interner).flags.intersects(TypeFlags::HAS_TY_OPAQUE) {
	tait_candidates.insert(ty);
	}

(just noticed the flag exists)

[ShoyuVanilla]

It's strange that this makes the tests fail 🤔

failures:

---- tests::diagnostics::no_mismatches_on_atpit stdout ----
thread 'tests::diagnostics::no_mismatches_on_atpit' panicked at crates/hir-ty/src/tests/diagnostics.rs:102:5:
Unexpected type mismatches:
257..259: expected impl Sized, got ()

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- tests::type_alias_impl_traits::associated_type_impl_trait stdout ----
thread 'tests::type_alias_impl_traits::associated_type_impl_trait' panicked at crates/hir-ty/src/tests/type_alias_impl_traits.rs:7:5:
Unexpected type mismatches:
203..205: expected impl Foo + ?Sized, got S1


---- tests::type_alias_impl_traits::associated_type_impl_traits_complex stdout ----
thread 'tests::type_alias_impl_traits::associated_type_impl_traits_complex' panicked at crates/hir-ty/src/tests/type_alias_impl_traits.rs:35:5:
Unexpected type mismatches:
293..295: expected impl Foo + ?Sized, got S1
585..587: expected impl Foo + ?Sized, got S1
595..597: expected impl Bar + ?Sized, got S2



failures:
    tests::diagnostics::no_mismatches_on_atpit
    tests::type_alias_impl_traits::associated_type_impl_trait
    tests::type_alias_impl_traits::associated_type_impl_traits_complex

test result: FAILED. 783 passed; 3 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.67s

[ShoyuVanilla]

Oh, I see. It seems that the types we are inserting have some inference vars inside and

rust-analyzer/crates/hir-ty/src/infer.rs

Line 949 in c9955bf

let ty = self.table.resolve_ty_shallow(ty);

this line is adding opaque types when the top type doesn't have the flag HAS_TY_OPAQUE

[Veykril]

I guess we can check for either HAS_TY_OPAQUE or HAS_TY_INFER then? If not feel free to merge as is

[ShoyuVanilla]

Reasonable alternative 👍

[Veykril]

Suggested change

	tait_candidates.insert(self.return_ty.clone());
	if self.return_ty.data(Interner).flags.intersects(TypeFlags::HAS_TY_OPAQUE) {
	tait_candidates.insert(self.return_ty.clone());
	}

[Veykril]

Thanks!
@bors delegate+

[bors]

✌️ @ShoyuVanilla, you can now approve this pull request!

If @Veykril told you to "r=me" after making some further change, please make that change, then do @bors r=@Veykril

[ShoyuVanilla]

@bors r=@Veykril

[bors]

📌 Commit 722f0d3 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 722f0d3 with merge 6446173...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 6446173 to master...