clippy gives bad suggestions when Deref is involved

[elichai]

Hi, If I use a closure to pass something to a function using the Deref trait clippy tries to suggest to remove the closure but that doesn't compile.

Suggestion:

warning: redundant closure found
 --> src/lib.rs:5:22
  |
5 |     addrs.iter().map(|a| a.to_string()).collect()
  |                      ^^^^^^^^^^^^^^^^^ help: remove closure as shown: `ToString::to_string`
  |
  = note: #[warn(clippy::redundant_closure)] on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

Code before: https://play.rust-lang.org/?gist=67e025045af6f7989d554b2084f57d13
Code after applying the suggestion(doesn't compile): https://play.rust-lang.org/?gist=83ed70c09362c87cfe584e4744513a11

[Manishearth]

#3942