Don't hint to add a lifetime parameter for trait impls

[mrmonday]

Spotted on a reddit thread (https://www.reddit.com/r/rust/comments/57virt/hey_rustaceans_got_an_easy_question_ask_here/d941p6y/?context=3).

This code (playpen):

use std::str::FromStr;

pub struct Foo<'a> {
    field: &'a str,
}

impl<'a> FromStr for Foo<'a> {
    type Err = ();
    fn from_str(path: &str) -> Result<Self, ()> {
        Ok(Foo { field: path })
    }
}

Gives the following hint:

help: consider using an explicit lifetime parameter as shown: fn from_str(path: &'a str) -> Result<Self, ()>
  --> <anon>:9:5
   |
9  |     fn from_str(path: &str) -> Result<Self, ()> {
   |     ^

Which is never correct, since then there will be a lifetime mismatch between the impl and the trait.