rustdoc: Fn trait sugar output types are broken for inlined signatures

[tomjakubowski]

For example, see: http://doc.rust-lang.org/std/collections/struct.Bitv.html#method.from_fn

fn from_fn<F>(len: usize, f: F) -> Bitv 
where F: FnMut(usize), <F as FnMut(usize)>::Output == bool

Should look like:

fn from_fn<F>(len: usize, f: F) -> Bitv 
where F: FnMut(usize) -> bool

This is because of the change to use associated types for the output.

[tomjakubowski]

Specifically, this:

ac94ae5#diff-6ad072c1ebc79b7620832915f2323d55R550

Like all "resugaring" of projection predicates, rustdoc will need to merge the F: Fn(usize) and <F as FnMut(usize)>::Output == bool bounds into one, which will likely mean an extra step when cleaning ty::Generics, after cleaning the individual ty::Predicates in the predicates field.

[tomjakubowski]

I can't nominate issues but I think this is crucial for 1.0 because of the pervasive use of closures in the standard library. Rendering Fn trait bounds this way all over the standard library documentation is not going to do Rust any PR favors.

[pnkfelix]

1.0 polish