No fix suggestion for extra_unused_lifetimes

[jsgf]

Resolving extra_unused_lifetimes seems pretty straightforward - just remove the lifetimes, but the diagnostic clippy emits doesn't have any fix suggestions. Is there any reason for it not to?

For example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1af0f2b1989bfb76f491af4a115e67a0

pub fn look_at_my_unused_lifetimes<'a>(msg: &str) {
    println!("print a msg: {}", msg);
}

produces the diagnostic:

    Checking playground v0.0.1 (/playground)
warning: this lifetime isn't used in the function definition
 --> src/lib.rs:1:36
  |
1 | pub fn look_at_my_unused_lifetimes<'a>(msg: &str) {
  |                                    ^^
  |
  = note: `#[warn(clippy::extra_unused_lifetimes)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes

    Finished dev [unoptimized + debuginfo] target(s) in 0.55s

which points out the problem clearly, but no fix suggestion for auto-replacement.

[flip1995]

I think building a suggestion is harder, then one might think. E.g. compare the two cases <'a, 'b> (where 'b is used) and <'a>. In the first case, only 'a, has to be removed. In the other case <'a> has to be removed completely.

Now there is also the case, where both <'a, 'b> is unused, then <'a, 'b> also has to be removed completely.

And another case, in a situation like this:

pub fn look_at_my_unused_lifetimes<'a, 'b: 'a>(msg: &'a str) {
    println!("print a msg: {}", msg);
}

The , 'b: 'a has to be removed, but not the 'a and not only the 'b.

Building a suggestion for this, might not be hard, but there are many edge cases, that have to be taken into account, so it will require excessive testing. (And might be a tedious task)

[jsgf]

Sure there's a number of cases to test, but I don't think there's that many. And once there's something we can iterate on it.