2018 idioms: duplicate suggestion for anonymous lifetime

[alexcrichton]

First reported here this code:

#![warn(rust_2018_idioms)]

macro_rules! foo {
    ($name:ident) => {
        pub struct $name;
        impl ::std::fmt::Debug for $name {
            fn fmt(&self, _fmt: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
                Ok(())
            }
        }
    }
}

foo!(One);
foo!(Two);

fn main() {}

when compiled yields two warnings to insert anonymous lifetimes. While we as humans can figure this out pretty well, Cargo as a merciless program can't figure this out and cargo fix attempts to apply the fix twice, creating invalid <'_><'_> syntax.

It's not clear whether it's best to fix this in cargo fix or fix this in the compiler itself. I figure it's easiest to track all idiom-lint related issues in this repository though, so I've opted to move it over here.

[zackmdavis]

I think we could use the session's one-time diagnostics functionality to de-duplicate just the suggestion? (I assume this would require stripping expansion info from the spans.) If this is a good idea, it might also generalize to other lints ...