Suggest Struct::<'_> instead of Self when lifetime does not match

[QuineDot]

Heads-up: PR 100976 probably fixes this but I still felt its own issue was warranted (and didn't find an existing one).

---

Given the following code:

pub struct Path<'a> {
    string: &'a mut String,
}

impl<'a> Path<'a> {
    pub fn sub(&mut self) -> Path/*<'_>*/ {
        Self {
            string: self.string,
        }
    }
}

The current output is:

error: lifetime may not live long enough
 --> src/lib.rs:8:21
  |
5 | impl<'a> Path<'a> {
  |      -- lifetime `'a` defined here
6 |     pub fn sub(&mut self) -> Path/*<'_>*/ {
  |                - let's call the lifetime of this reference `'1`
7 |         Self {
8 |             string: self.string,
  |                     ^^^^^^^^^^^ this usage requires that `'1` must outlive `'a`

Ideally the output should look like:

7 |         Self {
   |         ^^^^ help: Consider replacing `Self` with `Path`

Programmers not-infrequently expect Self to act as a type constructor and not fully aliased type (including lifetimes). Using Path in this context infers the lifetime, whereas using Self make the lifetime exactly 'a.

Though I haven't included it in my example, consider also the case when Self is used as the return type.

Another recent URLO example which inspired this report.

The same error appears on stable, nightly, and beta. But see PR 100976 which may resolve this issue.

@rustbot label +A-lifetimes +A-suggestion-diagnostics +D-newcomer-roadblock

[MariaSolOs]

Bumping this! I recently encountered this error and it took me way too long to fix it.