fix: Wrong BoundVar index when lowering impl trait parameter of parent generics

[ShoyuVanilla]

Fixes #17711

From the following test code;

//- minicore: deref
use core::ops::Deref;

struct Struct<'a, T>(&'a T);

trait Trait {}

impl<'a, T: Deref<Target = impl Trait>> Struct<'a, T> {
    fn foo(&self) -> &Self { self }

    fn bar(&self) {
        let _ = self.foo();
    }

}

when we call register_obligations_for_call for let _ = self.foo();,

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

Lines 1939 to 1952 in 0765978

	fn register_obligations_for_call(&mut self, callable_ty: &Ty) {
	let callable_ty = self.resolve_ty_shallow(callable_ty);
	if let TyKind::FnDef(fn_def, parameters) = callable_ty.kind(Interner) {
	let def: CallableDefId = from_chalk(self.db, *fn_def);
	let generic_predicates =
	self.db.generic_predicates(GenericDefId::from_callable(self.db.upcast(), def));
	for predicate in generic_predicates.iter() {
	let (predicate, binders) = predicate
	.clone()
	.substitute(Interner, parameters)
	.into_value_and_skipped_binders();
	always!(binders.len(Interner) == 0); // quantified where clauses not yet handled
	self.push_obligation(predicate.cast(Interner));
	}

we are querying generic_predicates and it has T: Deref<Target = impl Trait> predicate from the parent impl Struct;

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

Lines 375 to 399 in 0765978

	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)

but as we can see above, lowering TypeRef = impl Trait doesn't take into account the parent generic parameters, so the BoundVar index here is 0, as fn foo has no generic args other than parent's,

But this BoundVar is pointing at 'a in <'a, T: Deref<Target = impl Trait>>.
So, in the first code reference register_obligations_for_call's L:1948 - .substitute(Interner, parameters), we are substituting 'a with Ty, not Lifetime and this makes panic inside the chalk.

This PR fixes this wrong BoundVar index in such cases

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit d7431ec has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit d7431ec with merge 469b062...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 469b062 to master...