E0382 diagnostic could suggest a mutable borrow for Read/Write/Sync type bounds

[iliana]

I helped someone out with an example like this, which doesn't compile:

use std::io::{Result, Write};

fn write_stuff<W: Write>(mut writer: W) -> Result<()> {
    writeln!(writer, "stuff")
}

fn main() {
    let mut stdout = std::io::stdout();
    writeln!(stdout, "first line").unwrap();
    write_stuff(stdout);
    writeln!(stdout, "second line").unwrap();
}

   Compiling playground v0.0.1 (/playground)
error[E0382]: borrow of moved value: `stdout`
  --> src/main.rs:11:14
   |
8  |     let mut stdout = std::io::stdout();
   |         ---------- move occurs because `stdout` has type `Stdout`, which does not implement the `Copy` trait
9  |     writeln!(stdout, "first line").unwrap();
10 |     write_stuff(stdout);
   |                 ------ value moved here
11 |     writeln!(stdout, "second line").unwrap();
   |              ^^^^^^ value borrowed here after move
   |
note: consider changing this parameter type in function `write_stuff` to borrow instead if owning the value isn't necessary
  --> src/main.rs:3:38
   |
3  | fn write_stuff<W: Write>(mut writer: W) -> Result<()> {
   |    ----------- in this function      ^ this parameter takes ownership of the value

For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to 1 previous error

The compiler suggests changing the parameter of write_stuff to take &mut W, but suppose you do not have control over that function. Because Read/Write/Sync are implemented over mutable references of readers/writers/syncers(?), I suggested changing the code to:

    write_stuff(&mut stdout);

Could the compiler detect that the type bound here is some combination of Read/Write/Sync and suggest a mutable borrow like this?

[ChayimFriedman2]

Maybe even more generally suggest adding a reference when the trait is implemented for it.

@rustbot label +A-diagnostics

[dianne]

@rustbot claim