Fix panic with impl trait associated types in where clause

[Jesse-Bakker]

Not sure if this is the correct fix, but the tests are green :')

Fixes #16823

[ShoyuVanilla]

The root cause of this is my wrong assumption in #16769 😢

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

Lines 1125 to 1141 in 5ecace4

	if matches!(
	self.impl_trait_mode,
	ImplTraitLoweringState::Param(_)
	| ImplTraitLoweringState::Variable(_)
	) {
	// Find the generic index for the target of our `bound`
	let target_param_idx = self
	.resolver
	.where_predicates_in_scope()
	.find_map(|p| match p {
	WherePredicate::TypeBound {
	target: WherePredicateTypeTarget::TypeOrConstParam(idx),
	bound: b,
	} if b == bound => Some(idx),
	_ => None,
	});
	if let Some(target_param_idx) = target_param_idx {

In the above L1136, I thought that the predicate target T is always WherePredicateTypeTarget::TypeOrConstParam in the predicates like T: Outer<Item = impl Inner>>, but it actually is WherePredicateTypeTarget::TypeRef in this case.

My intention was to handle case when the binding.type_ref - the bound of associated type binding - is not ImplTrait by itself, but contains ImplTrait inside generic parameters, at here

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

Lines 1109 to 1112 in 5ecace4

	if let Some(type_ref) = &binding.type_ref {
	if let (TypeRef::ImplTrait(bounds), ImplTraitLoweringState::Disallowed) =
	(type_ref, &self.impl_trait_mode)
	{

like the case at here (it's Path - R<impl Y, T>>, not ImplTrait - impl Y)

rust-analyzer/crates/hir-ty/src/tests/traits.rs

Line 1255 in 5ecace4

fn foo<T>(x: impl X<Item = R<impl Y, T>>) -> T { loop {} }

and I think that the direct ImplTrait cases can be handled with preivious "redirecting lowered bounds" stretagy in these lines (and the recusive impl trait bounds inside that I intended to fix can still be handled properly as it triggers assoc_type_bindings_from_type_bound again in lower_type_bound)

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

Lines 1113 to 1121 in 5ecace4

	for bound in bounds {
	predicates.extend(
	self.lower_type_bound(
	bound,
	TyKind::Alias(AliasTy::Projection(projection_ty.clone()))
	.intern(Interner),
	false,
	),
	);

So, I think that your PR is proper and effective 👍

[Veykril]

Thanks!
@bors r=ShoyuVanilla

[bors]

📌 Commit 9582885 has been approved by ShoyuVanilla

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9582885 with merge a71a032...

[bors]

☀️ Test successful - checks-actions
Approved by: ShoyuVanilla
Pushing a71a032 to master...