fix ICE when suggesting ::new

[lukas-code]

fixes #146174

This code suggests to write Foo::new(...) when the user writes Foo(...) or Foo { ... } and the constructor is private, where new is some associated function that returns Self.

When checking that the return type of new is Self, we need to instantiate the parameters of new with infer vars, so we don't end up with a type like Box<$param(0)> in a context that doesn't have any parameters. But then we can't use normalize_erasing_late_bound_regions anymore because that goes though a query that can't deal with infer vars.

Since this is diagnostic-only code that is supposed to check for exactly -> Self, I think it's fine to just skip normalizing here, especially since The Correct Way^TM would involve a probe and make this code even more complicated.

Also, the code here does almost the same thing, and these suggestions can probably be unified in the future:

rust/compiler/rustc_hir_typeck/src/method/suggest.rs

Lines 2123 to 2129 in 4ca8078

	// Only assoc fns that return `Self`, `Option<Self>` or `Result<Self, _>`.
	let ret_ty = self
	.tcx
	.fn_sig(item.def_id)
	.instantiate(self.tcx, self.fresh_args_for_item(DUMMY_SP, item.def_id))
	.output();
	let ret_ty = self.tcx.instantiate_bound_regions_with_erased(ret_ty);

r? @compiler-errors
cc @Qelxiros -- this should unblock #144420

[rustbot]

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[compiler-errors]

Thanks

r=me when green

@bors delegate+

[lukas-code]

@bors r=compiler-errors rollup

[bors]

📌 Commit 4ca8078 has been approved by compiler-errors

It is now in the queue for this repository.