Fix Debug implementations that hardcode the representation of a struct

[dtolnay]

The Debug implementation for Mutex looks like this:

write!(f, "Mutex {{ data: {:?} }}", &*guard)

This is bad because it does not honor the # pretty-print modifier.

println!("{:#?}", std::sync::Mutex::new(5));

It prints

Mutex { data: 5 }

instead of

Mutex {
    data: 5
}

A correct implementation would use debug_struct.

f.debug_struct("Mutex").field("data", &*guard).finish()

There seem to be lots of instances of this in the standard library. The following search is a good place to start.

grep -r ' {{ ' libcore libstd

[MaloJaffre]

I'll try to tackle and fix this issue.