"use of moved value" diagnostic could suggest reborrow when passing &mut to &mut

[pnkfelix]

Spawned off of #45142

Consider the following code (play):

struct Events<R>(R);

struct Other;

pub trait Trait<T> { 
    fn handle(value: T) -> Self;
}

// Blanket impl. (If you comment this out, compiler figures out that it
// is passing an `&mut` to a method that must be expecting an `&mut`,
// and injects an auto-reborrow.)
impl<T, U> Trait<U> for T where T: From<U> {
    fn handle(_: U) -> Self { unimplemented!() }
}

impl<'a, R> Trait<&'a mut Events<R>> for Other {
    fn handle(_: &'a mut Events<R>) -> Self { unimplemented!() }
}

fn this_compiles<'a, R>(value: &'a mut Events<R>) {
    for _ in 0..3 {
        Other::handle(&mut *value);
    }
}

fn this_does_not<'a, R>(value: &'a mut Events<R>) {
    for _ in 0..3 {
        Other::handle(      value);
    }
}

fn main() {}

Today, this yields the diagnostic:

error[E0382]: use of moved value: `value`
  --> src/main.rs:30:29
   |
28 | fn this_does_not<'a, R>(value: &'a mut Events<R>) {
   |                         ----- move occurs because `value` has type `&mut Events<R>`, which does not implement the `Copy` trait
29 |     for _ in 0..3 {
30 |         Other::handle(      value);
   |                             ^^^^^ value moved here, in previous iteration of loop

We could do better. For example, the diagnostic could additionally say:

consider writing `Other::handle(&mut *value)` to create a fresh reborrow of `value`.

---

Background:

The compiler tries hard to auto-inject &mut-reborrows when it makes sense; as a way to create a fresh &mut reference to pass into the method invocation, and avoid moving the pre-existing &mut reference.

But there are times, often involving generic code, when it cannot automatically inject a reborrow. (After all, there are cases where the intent is to move the borrow into some place whose dynamic extent outlives that of the method invocation itself; and the static analysis cannot tell if this was the case at the point where it is deciding whether to insert an &mut-reborrow or not.)

This can lead to a diagnostic for the user telling them that they have a used of a moved value.

But new-comers to Rust do not always know about the option of putting in a &mut-reborrow themselves.

Our diagnostics could help them here. Even though we choose to not attempt to auto-inject the auto-reborrow in the compiler itself, our diagnostics have enough information about the method signature and the invocation to suggest to the user that they write out the &mut *value themselves.