fix: let-and-return suggests invalid cast (#16135)

[ErinvanderVeen]

Simple PR that handles a special case where a raw pointer was invalidly cast via as _.

First PR, would appreciate feedback.

changelog: [let_and_return]: fix lint suggestion for invalid cast to raw pointer.

[rustbot]

r? @Jarcho

rustbot has assigned @Jarcho.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Jarcho]

The test you have still works with the extra cast, it's just unnecessary. The type cast is needed to work around deficiencies in type inference:

struct S;
trait T {}
impl T for S {}

fn f(x: *const S) -> *const dyn T {
    Clone::clone(&x) as _
}

[Jarcho]

You aren't including a test for the actual issue. Just copy the function from the issue since using as _ in that case messes up type inference.

View changes since this review

[Jarcho]

This doesn't test the lint. The function body should be:

let x = Clone::clone(&x);
x

[ErinvanderVeen]

You're absolutely right. I'll try to get to this asap.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.