suggest {var:?} when finding {:?var} in inline format strings

[matthiaskrgr]

    eprintln!("Timings:\n{:?timer}");

The current suggestion is:

error: invalid format string: expected `'}'`, found `'t'`
   --> src/main.rs:696:29
    |
696 |     eprintln!("Timings:\n{:?timer}");
    |                          -  ^ expected `}` in format string
    |                          |
    |                          because of this opening brace
    |
    = note: if you intended to print `{`, you can escape it using `{{`

error: could not compile `icemaker` due to previous error

We could perhaps detect the formatting parameter is at the beginning of the fmt string and that it should go at the end, in order to suggest {timer:?}

[madsravn]

@rustbot claim