Add beginner friendly lifetime elision hint to E0623

[arifd]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4e10570825edd99bc5f0f03fe82fa34f

fn foo(slice_a: &mut [u8], slice_b: &mut[u8]) {
    core::mem::swap(&mut slice_a, &mut slice_b);
}

fn main() {
    let a = [1u8,2,3];
    let b = [4u8,5,6];
    foo(&mut a, &mut b);
}

The current output is:

error[E0623]: lifetime mismatch
 --> src/main.rs:2:35
  |
1 | fn foo(slice_a: &mut [u8], slice_b: &mut[u8]) {
  |                 ---------           -------- these two types are declared with different lifetimes...
2 |     core::mem::swap(&mut slice_a, &mut slice_b);
  |                                   ^^^^^^^^^^^^ ...but data from `slice_b` flows into `slice_a` here

error[E0623]: lifetime mismatch
 --> src/main.rs:2:35
  |
1 | fn foo(slice_a: &mut [u8], slice_b: &mut[u8]) {
  |                 ---------           --------
  |                 |
  |                 these two types are declared with different lifetimes...
2 |     core::mem::swap(&mut slice_a, &mut slice_b);
  |                                   ^^^^^^^^^^^^ ...but data from `slice_a` flows into `slice_b` here

For more information about this error, try `rustc --explain E0623`.
error: could not compile `playground` due to 2 previous errors

Ideally the output should look (something) like:

Hint: Each elided lifetime in input position becomes a distinct lifetime. To avoid this, explicitly declare a lifetime and assign it to both

Beginners likely aren't aware of lifetime elision rules, they may be thinking "but there are NO lifetimes here"...

[Nilstrieb]

@rustbot claim

[Nilstrieb]

We could maybe even add a suggestion for adding the explicit generic lifetime

[Nilstrieb]

I've implemented it and it has been merged, I hope you like it! (this can be closed)

[arifd]

Great stuff. Congratulations and what an impressively fast turnaround!