Suggest reborrowing unsized value when borrowed trait object wanted

[ijackson]

Given the following code:

use std::ffi::{OsStr, OsString};
use std::path::Path;

fn check(p: &dyn AsRef<Path>) {
    let m = std::fs::metadata(&p);
    println!("{:?}", &m);
}

fn main() {
    let s: OsString = ".".into();
    let s: &OsStr = &s;
    check(s);
}

The current output is:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
  --> src/main.rs:12:11
   |
12 |     check(s);
   |     ----- ^ doesn't have a size known at compile-time
   |     |
   |     required by a bound introduced by this call
   |
   = help: within `OsStr`, the trait `Sized` is not implemented for `[u8]`
   = note: required because it appears within the type `OsStr`
   = note: required for the cast to the object type `dyn AsRef<Path>`

Ideally the output would suggest borrowing, since this:

    check(&s);

compiles and works just fine.

The problem is that you can't make an object which is a wide pointer directly into a trait object refernce, because it would have to be a triple-sized pointer. But often, you can just reborrow instead.

[ijackson]

@rustbot modify labels +A-suggestion-diagnostics +A-traits

[ijackson]

@rustbot modify labels +C-enhancement