Use statement: self stays inside braces

[Oughie]

Consider the following example:

use std::io::{self};

rustfmt does not modify this.
Instead, it should automatically turn into:

use std::io;

[ytmimi]

@Oughie To the best of my knowledge use std::io::{self}; and use std::io; are semantically different. See #3750 as an example. That's why rustfmt doesn't modify the code.