fix: Insert a generic arg for impl Trait when lowering generic args

[ShoyuVanilla]

Fixes #17191

We are not inserting a generic arg when lowering generics like

fn foo<T: B<impl A>(..) { ... }

but when we are lowering predicates we do;

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

Lines 1697 to 1718 in aa00ddc

	pub(crate) fn generic_predicates_query(
	db: &dyn HirDatabase,
	def: GenericDefId,
	) -> GenericPredicates {
	let resolver = def.resolver(db.upcast());
	let (impl_trait_lowering, param_lowering) = match def {
	GenericDefId::FunctionId(_) => {
	(ImplTraitLoweringMode::Variable, ParamLoweringMode::Variable)
	}
	_ => (ImplTraitLoweringMode::Disallowed, ParamLoweringMode::Variable),
	};
	let ctx = TyLoweringContext::new(db, &resolver, def.into())
	.with_impl_trait_mode(impl_trait_lowering)
	.with_type_param_mode(param_lowering);
	let generics = generics(db.upcast(), def);
	let mut predicates = resolver
	.where_predicates_in_scope()
	.flat_map(|(pred, def)| {
	ctx.lower_where_predicate(pred, def, false).map(|p| make_binders(db, &generics, p))
	})
	.collect::<Vec<_>>();

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

Line 310 in aa00ddc

TypeRef::ImplTrait(bounds) => {

and this mismatch causes index out of bound panic while substituting the predicates

[ShoyuVanilla]

This is making a panic like in the original issue

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 8fa454d has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8fa454d with merge c9109f2...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing c9109f2 to master...