Disallow nested impl traits

[ShoyuVanilla]

Fixes #17498

The above issue is due to formatting self referencing, recursive bound like Implemented(^0.0: TraitId(0)<[?0 := ^0.0]>) on the codes like;

trait Foo<T> {}

trait Bar {}

fn test(foo: impl Foo<impl Bar>) { ... }

When lowering predicate impl Foo<impl Bar> in trait_environment_query, the outer impl Foo<...> is treated as predicates, so the first TypeRef that passes the following code is impl Bar;

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

Lines 376 to 400 in cae997e

	ImplTraitLoweringState::Variable(counter) => {
	let idx = counter.get();
	// Count the number of `impl Trait` things that appear within our bounds.
	// Since t hose have been emitted as implicit type args already.
	counter.set(idx + count_impl_traits(type_ref) as u16);
	let (
	_parent_params,
	self_param,
	type_params,
	const_params,
	_impl_trait_params,
	lifetime_params,
	) = self
	.generics()
	.expect("variable impl trait lowering must be in a generic def")
	.provenance_split();
	TyKind::BoundVar(BoundVar::new(
	self.in_binders,
	idx as usize
	+ self_param as usize
	+ type_params
	+ const_params
	+ lifetime_params,
	))
	.intern(Interner)

and thus the idx is 0 in the above context.

But the following code sets self_ty as the BoundVar with idx = 0 because the target param id for predicate impl Foo<...> is 0 since impl Foo is the first generic-like parameter of fn test;

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

Lines 998 to 1025 in cae997e

	pub(crate) fn lower_where_predicate<'b>(
	&'b self,
	where_predicate: &'b WherePredicate,
	&def: &GenericDefId,
	ignore_bindings: bool,
	) -> impl Iterator<Item = QuantifiedWhereClause> + 'b {
	match where_predicate {
	WherePredicate::ForLifetime { target, bound, .. }
	| WherePredicate::TypeBound { target, bound } => {
	let self_ty = match target {
	WherePredicateTypeTarget::TypeRef(type_ref) => self.lower_ty(type_ref),
	&WherePredicateTypeTarget::TypeOrConstParam(local_id) => {
	let param_id = hir_def::TypeOrConstParamId { parent: def, local_id };
	match self.type_param_mode {
	ParamLoweringMode::Placeholder => {
	TyKind::Placeholder(to_placeholder_idx(self.db, param_id))
	}
	ParamLoweringMode::Variable => {
	let idx = generics(self.db.upcast(), def)
	.type_or_const_param_idx(param_id)
	.expect("matching generics");
	TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, idx))
	}
	}
	.intern(Interner)
	}
	};
	Either::Left(self.lower_type_bound(bound, self_ty, ignore_bindings))

For the codes like;

trait Foo {
    type Assoc;
}

trait Bar {}

fn test(foo: impl Foo<Assoc = impl Bar>) { ... }

similar recursive bound doesn't happen because the following codes "count the number of impl Trait things that appear before the target of our bound."

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

Lines 1168 to 1199 in cae997e

	let ty = if let Some(target_param_idx) = target_param_idx {
	let mut counter = 0;
	let generics = self.generics().expect("generics in scope");
	for (idx, data) in generics.iter_self_type_or_consts() {
	// Count the number of `impl Trait` things that appear before
	// the target of our `bound`.
	// Our counter within `impl_trait_mode` should be that number
	// to properly lower each types within `type_ref`
	if data.type_param().is_some_and(|p| {
	p.provenance == TypeParamProvenance::ArgumentImplTrait
	}) {
	counter += 1;
	}
	if idx == *target_param_idx {
	break;
	}
	}
	let mut ext = TyLoweringContext::new_maybe_unowned(
	self.db,
	self.resolver,
	self.owner,
	)
	.with_type_param_mode(self.type_param_mode);
	match &self.impl_trait_mode {
	ImplTraitLoweringState::Param(_) => {
	ext.impl_trait_mode =
	ImplTraitLoweringState::Param(Cell::new(counter));
	}
	ImplTraitLoweringState::Variable(_) => {
	ext.impl_trait_mode =
	ImplTraitLoweringState::Variable(Cell::new(counter));
	}

Instead of doing similar thing like nested impl Foo<impl Bar> thing, this PR lowers such nested impl traits into error types in the similar manner as the rustc does in the following lines (and of course, allows lowering and inferencing nested impl traits for the cases that rustc permits);

• https://github.com/rust-lang/rust/blob/e2cf31a6148725bde4ea48acf1e4fe72675257a2/compiler/rustc_ast_passes/src/ast_validation.rs#L802-L813
• https://github.com/rust-lang/rust/blob/7b21c18fe4de32a7d2faa468e6ef69abff013f85/compiler/rustc_ast_passes/src/ast_validation.rs#L1299-L1314

(Though rustc emits E0666😈, I skipped diagnostics since gathering diagnostics in hir-def has no conventions so far 😅)

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 4bb623d has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4bb623d with merge f2afcb8...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing f2afcb8 to master...