Suggest as_mut to reborrow Pin when calling method

[Nemo157]

I have seen a lot of newcomers to futures stumble with Pin<&mut Self> methods consuming the pin, if possible this seems like a good candidate for a custom diagnostic.

As a self-contained example (playground):

use std::pin::Pin;

struct Foo;

impl Foo {
    fn foo(self: Pin<&mut Self>) {}
}

fn main() {
    let mut foo = Foo;
    let foo = Pin::new(&mut foo);
    foo.foo();
    foo.foo();
}

currently errors with

   Compiling playground v0.0.1 (/playground)
error[E0382]: use of moved value: `foo`
  --> src/main.rs:13:5
   |
11 |     let foo = Pin::new(&mut foo);
   |         --- move occurs because `foo` has type `std::pin::Pin<&mut Foo>`, which does not implement the `Copy` trait
12 |     foo.foo();
   |     --- value moved here
13 |     foo.foo();
   |     ^^^ value used here after move

error: aborting due to previous error

For more information about this error, try `rustc --explain E0382`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

it would be useful if this error included something like

hint: use `foo.as_mut().foo()` to reborrow the `Pin` instead of moving it

[Centril]

cc @nikomatsakis @matthewjasper @estebank

[nikomatsakis]

Seems reasonable to me. Should we tag this A-async-await? Seems to fall under the "async foundations" purview, anyway. (Slight mismatch between those labels, I guess)

[Centril]

@nikomatsakis Yes, let's. Any thoughts re. implementation? ;)

[gilescope]

Happy to give this one a go - the hint would certainly help me if I stumbled into this.
@rustbot claim

[gilescope]

Rather than being specific to Pin, can we expand the scope and add a hint to the general error message:

      hint: consider [mutably] borrowing '&[mut ]foo' [or foo.clone()] here?
                                                        ^ only suggest if T:Clone

and if possible specialise the error message if the type supports as_ref()/as_mut()?

As a user, that's the kind of error message I'd be greatful to receive - a hint as to whether the type is clonable or not and if there's a as_mut() call I could be using rather than trying to &mut it.

[Nemo157]

For normal &self/&mut self methods the compiler will implicitly reborrow. If there’s a generalisation here that could help suggest cloning that sounds good to me (small playground to check the current error message).

[dtolnay]

@rustbot release-assignment

[estebank]

Addressed the Pin<&mut T> case in #106095.

For the general case of suggesting .clone() can't be accomplished at this time because by the time borrowck comes around I can no longer use the inference context to evaluate T: Clone for an accurate suggestion.

Edit: Figured out a hack to suggest clone. That PR will close this ticket.