Fix Assist "replace named generic type with impl trait"

[justahero]

This is a follow-up PR to fix the assist "replace named generic type with impl trait" described in #14626 to filter invalid param types. It integrates the feedback given in PR #14816 .

The change updates the logic to determine when a function parameter is safe to replace a type param with its trait implementation. Some parameter definitions are invalid & should not be replaced by their traits, therefore skipping the assist completely.

First, all usages of the generic type under the cursor are determined. These usage references are checked to see if they occur outside the function parameter list. If an outside reference is found, e.g. in body, return type or where clause, the assist is skipped. All remaining usages need to appear only in the function param list. For each usage the param type is further inspected to see if it's valid. The logic to determine if a function parameter is valid, follows a heuristic and may not cover all possible parameter definitions.

With this change the following param types (as given in this comment) are not replaced & therefore skip the assist.

fn foo<P: Trait>(
    _: <P as Trait>::Assoc,          // within path type qualifier
    _: <() as OtherTrait<P>>::Assoc, // same as above
    _: P::Assoc,                     // associated type shorthand
    _: impl OtherTrait<P>            // generic arg in impl trait (note that associated type bindings are fine)
    _: &dyn Fn(P)                    // param type and/or return type for Fn* traits
) {}

[Veykril]

@bors r+

[bors]

📌 Commit c0e9b57 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit c0e9b57 with merge 7738ff4...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 7738ff4 to master...