Rollup merge of #72194 - doctorn:dispatch-from-dyn-ice, r=estebank

Don't ICE on missing `Unsize` impl

Previously code of the form

```rust
#![feature(unsize, dispatch_from_dyn)]

use std::marker::Unsize;
use std::ops::DispatchFromDyn;

pub struct Foo<'a, T: ?Sized> {
    _inner: &'a &'a T,
}

impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Foo<'a, U>> for Foo<'a, T> {}
```

would generate an ICE due to the missing `Unsize` impl being run through the `suggest_change_mut` suggestion. This PR adds an early exit and a pointer to the appropriate docs regarding `Unsize` instead:

```
error[E0277]: the trait bound `&'a T: std::marker::Unsize<&'a U>` is not satisfied
  --> src/test/ui/issues/issue-71036.rs:11:1
   |
11 | impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Foo<'a, U>> for Foo<'a, T> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Unsize<&'a U>` is not implemented for `&'a T`
   |
   = note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information
   = note: required because of the requirements on the impl of `std::ops::DispatchFromDyn<&'a &'a U>` for `&'a &'a T`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
```

r? @estebank

Resolves #71036

0 files changed, 0 insertions, 0 deletions