Fix suggestions for missing return type lifetime specifiers
This pull request aims to fix#84592. The issue is that the current code seems to assume that there is only a single relevant span pointing to the missing lifetime, and only looks at the first one:
e5f83d24ae/compiler/rustc_resolve/src/late/lifetimes.rs (L2959)
This is incorrect, though, and leads to incorrect error messages and invalid suggestions. For instance, the example from #84592:
```rust
struct TwoLifetimes<'x, 'y> {
x: &'x (),
y: &'y (),
}
fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
TwoLifetimes { x: &(), y: &() }
}
```
currently leads to:
```
error[E0106]: missing lifetime specifiers
--> src/main.rs:6:57
|
6 | fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
| --- --- ^^ expected 2 lifetime parameters
|
= help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `a` or `b`
help: consider introducing a named lifetime parameter
|
6 | fn two_lifetimes_needed<'a>(a: &'a (), b: &'a ()) -> TwoLifetimes<'_<'a, 'a>, '_> {
| ^^^^ ^^^^^^ ^^^^^^ ^^^^^^^^^^
```
There are two problems:
- The error message is wrong. There is only _one_ lifetime parameter expected at the location pointed to by the error message (and another one at a separate location).
- The suggestion is incorrect and will not lead to correct code.
With the changes in this PR, I get the following output:
```
error[E0106]: missing lifetime specifiers
--> p.rs:6:57
|
6 | fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
| --- --- ^^ ^^ expected named lifetime parameter
| |
| expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `a` or `b`
help: consider introducing a named lifetime parameter
|
6 | fn two_lifetimes_needed<'a>(a: &'a (), b: &'a ()) -> TwoLifetimes<'a, 'a> {
| ^^^^ ^^^^^^ ^^^^^^ ^^ ^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0106`.
```
Mainly, I changed `add_missing_lifetime_specifiers_label()` to receive a _vector_ of spans (and counts) instead of just one, and adjusted its body accordingly.
0740015d59