Fix suggestions for missing return type lifetime specifiers

[FabianWolff]

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:

rust/compiler/rustc_resolve/src/late/lifetimes.rs

Line 2959 in e5f83d2

let span = lifetime_refs[0].span;

This is incorrect, though, and leads to incorrect error messages and invalid suggestions. For instance, the example from #84592:

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.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[petrochenkov]

r? @jackh726

[jackh726]

Initial comments

[FabianWolff]

Thanks for your suggestions, @jackh726! I have just pushed a commit addressing your comments (those that I did not reply to above).

[jackh726]

I think last comments :)

[FabianWolff]

@jackh726 thanks again! I've implemented your suggested changes.

[jackh726]

Thanks! @bors r+

@FabianWolff in the future, it's best not to tag someone in the commit message itself, because they get pinged as the commit goes through CI and such :)

[bors]

📌 Commit 2448c76 has been approved by jackh726

[FabianWolff]

@FabianWolff in the future, it's best not to tag someone in the commit message itself, because they get pinged as the commit goes through CI and such :)

Ah, I see. Sorry about that!

[jackh726]

Ah, I see. Sorry about that!

No worries, just wanted to let you know :)