fix: Goto definition for index_mut

[ShoyuVanilla]

Mostly same with #16696.

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

Lines 103 to 133 in 0ac05c0

	&Expr::Index { base, index, is_assignee_expr: _ } => {
	if mutability == Mutability::Mut {
	if let Some((f, _)) = self.result.method_resolutions.get_mut(&tgt_expr) {
	if let Some(index_trait) = self
	.db
	.lang_item(self.table.trait_env.krate, LangItem::IndexMut)
	.and_then(|l| l.as_trait())
	{
	if let Some(index_fn) =
	self.db.trait_data(index_trait).method_by_name(&name![index_mut])
	{
	*f = index_fn;
	let base_adjustments = self
	.result
	.expr_adjustments
	.get_mut(&base)
	.and_then(|it| it.last_mut());
	if let Some(Adjustment {
	kind: Adjust::Borrow(AutoBorrow::Ref(mutability)),
	..
	}) = base_adjustments
	{
	*mutability = Mutability::Mut;
	}
	}
	}
	}
	}
	self.infer_mut_expr(base, mutability);
	self.infer_mut_expr(index, Mutability::Not);
	}

Thankfully, we are doing similar method resolutions so we can use them like the mentioned PR.
As there are only three LangItems having Mut in there names; FnMut, DerefMut and IndexMut, I think that this is the last one 😄

[Veykril]

Let's see if bors works again
@bors r+

[bors]

📌 Commit bf23dee has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit bf23dee with merge 0ec6015...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 0ec6015 to master...