Rollup merge of #85050 - FabianWolff:issue-84592, r=jackh726

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:
https://github.com/rust-lang/rust/blob/e5f83d24aee866a14753a7cedbb4e301dfe5bef5/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.

compiler/rustc_errors/src/diagnostic.rs

@@ -282,6 +282,22 @@ impl Diagnostic {
         msg: &str,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
+    ) -> &mut Self {
+        self.multipart_suggestion_with_style(
+            msg,
+            suggestion,
+            applicability,
+            SuggestionStyle::ShowCode,
+        )
+    }
+
+    /// [`Diagnostic::multipart_suggestion()`] but you can set the [`SuggestionStyle`].
+    pub fn multipart_suggestion_with_style(
+        &mut self,
+        msg: &str,
+        suggestion: Vec<(Span, String)>,
+        applicability: Applicability,
+        style: SuggestionStyle,
     ) -> &mut Self {
         assert!(!suggestion.is_empty());
         self.suggestions.push(CodeSuggestion {
@@ -292,7 +308,7 @@ impl Diagnostic {
                     .collect(),
             }],
             msg: msg.to_owned(),
-            style: SuggestionStyle::ShowCode,
+            style,
             applicability,
             tool_metadata: Default::default(),
         });

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -2956,7 +2956,6 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             return;
         }
 
-        let span = lifetime_refs[0].span;
         let mut late_depth = 0;
         let mut scope = self.scope;
         let mut lifetime_names = FxHashSet::default();
@@ -3035,18 +3034,27 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             }
         };
 
-        let mut err = self.report_missing_lifetime_specifiers(span, lifetime_refs.len());
+        let mut spans: Vec<_> = lifetime_refs.iter().map(|lt| lt.span).collect();
+        spans.sort();
+        let mut spans_dedup = spans.clone();
+        spans_dedup.dedup();
+        let spans_with_counts: Vec<_> = spans_dedup
+            .into_iter()
+            .map(|sp| (sp, spans.iter().filter(|nsp| *nsp == &sp).count()))
+            .collect();
+
+        let mut err = self.report_missing_lifetime_specifiers(spans.clone(), lifetime_refs.len());
 
         if let Some(params) = error {
             // If there's no lifetime available, suggest `'static`.
             if self.report_elision_failure(&mut err, params) && lifetime_names.is_empty() {
                 lifetime_names.insert(kw::StaticLifetime);
             }
         }
+
         self.add_missing_lifetime_specifiers_label(
             &mut err,
-            span,
-            lifetime_refs.len(),
+            spans_with_counts,
             &lifetime_names,
             lifetime_spans,
             error.unwrap_or(&[]),

src/test/ui/suggestions/issue-84592.rs

@@ -0,0 +1,17 @@
+/* Checks whether issue #84592 has been resolved. The issue was
+ * that in this example, there are two expected/missing lifetime
+ * parameters with *different spans*, leading to incorrect
+ * suggestions from rustc.
+ */
+
+struct TwoLifetimes<'x, 'y> {
+    x: &'x (),
+    y: &'y (),
+}
+
+fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
+//~^ ERROR missing lifetime specifiers [E0106]
+    TwoLifetimes { x: &(), y: &() }
+}
+
+fn main() {}

src/test/ui/suggestions/issue-84592.stderr

@@ -0,0 +1,17 @@
+error[E0106]: missing lifetime specifiers
+  --> $DIR/issue-84592.rs:12:57
+   |
+LL | 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
+   |
+LL | 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`.

src/test/ui/suggestions/missing-lt-for-hrtb.stderr

@@ -44,6 +44,10 @@ note: these named lifetimes are available to use
    |
 LL | struct V<'a>(&'a dyn for<'b> Fn(&X) -> &X);
    |          ^^              ^^
+help: consider using one of the available lifetimes here
+   |
+LL | struct V<'a>(&'a dyn for<'b> Fn(&X) -> &'lifetime X);
+   |                                        ^^^^^^^^^^
 
 error[E0106]: missing lifetime specifier
   --> $DIR/missing-lt-for-hrtb.rs:5:41

src/test/ui/suggestions/return-elided-lifetime.rs

@@ -0,0 +1,37 @@
+/* Checks all four scenarios possible in report_elision_failure() of
+ * rustc_resolve::late::lifetimes::LifetimeContext related to returning
+ * borrowed values, in various configurations.
+ */
+
+fn f1() -> &i32 { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+fn f1_() -> (&i32, &i32) { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+//~^^ ERROR missing lifetime specifier [E0106]
+
+fn f2(a: i32, b: i32) -> &i32 { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+fn f2_(a: i32, b: i32) -> (&i32, &i32) { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+//~^^ ERROR missing lifetime specifier [E0106]
+
+struct S<'a, 'b> { a: &'a i32, b: &'b i32 }
+fn f3(s: &S) -> &i32 { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+fn f3_(s: &S, t: &S) -> (&i32, &i32) { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+//~^^ ERROR missing lifetime specifier [E0106]
+
+fn f4<'a, 'b>(a: &'a i32, b: &'b i32) -> &i32 { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+//~^^ ERROR missing lifetime specifier [E0106]
+
+fn f5<'a>(a: &'a i32, b: &i32) -> &i32 { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+fn f5_<'a>(a: &'a i32, b: &i32) -> (&i32, &i32) { loop {} }
+//~^ ERROR missing lifetime specifier [E0106]
+//~^^ ERROR missing lifetime specifier [E0106]
+
+fn main() {}

src/test/ui/suggestions/return-elided-lifetime.stderr

@@ -0,0 +1,198 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:6:12
+   |
+LL | fn f1() -> &i32 { loop {} }
+   |            ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime
+   |
+LL | fn f1() -> &'static i32 { loop {} }
+   |            ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:8:14
+   |
+LL | fn f1_() -> (&i32, &i32) { loop {} }
+   |              ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime
+   |
+LL | fn f1_() -> (&'static i32, &i32) { loop {} }
+   |              ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:8:20
+   |
+LL | fn f1_() -> (&i32, &i32) { loop {} }
+   |                    ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime
+   |
+LL | fn f1_() -> (&i32, &'static i32) { loop {} }
+   |                    ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:12:26
+   |
+LL | fn f2(a: i32, b: i32) -> &i32 { loop {} }
+   |                          ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | fn f2(a: i32, b: i32) -> &'static i32 { loop {} }
+   |                          ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:14:28
+   |
+LL | fn f2_(a: i32, b: i32) -> (&i32, &i32) { loop {} }
+   |                            ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | fn f2_(a: i32, b: i32) -> (&'static i32, &i32) { loop {} }
+   |                            ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:14:34
+   |
+LL | fn f2_(a: i32, b: i32) -> (&i32, &i32) { loop {} }
+   |                                  ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | fn f2_(a: i32, b: i32) -> (&i32, &'static i32) { loop {} }
+   |                                  ^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:19:17
+   |
+LL | fn f3(s: &S) -> &i32 { loop {} }
+   |          --     ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say which one of `s`'s 3 lifetimes it is borrowed from
+help: consider introducing a named lifetime parameter
+   |
+LL | fn f3<'a>(s: &'a S) -> &'a i32 { loop {} }
+   |      ^^^^    ^^^^^     ^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:21:26
+   |
+LL | fn f3_(s: &S, t: &S) -> (&i32, &i32) { loop {} }
+   |           --     --      ^ 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 one of `s`'s 3 lifetimes or one of `t`'s 3 lifetimes
+help: consider introducing a named lifetime parameter
+   |
+LL | fn f3_<'a>(s: &'a S, t: &'a S) -> (&'a i32, &i32) { loop {} }
+   |       ^^^^    ^^^^^     ^^^^^      ^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:21:32
+   |
+LL | fn f3_(s: &S, t: &S) -> (&i32, &i32) { loop {} }
+   |           --     --            ^ 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 one of `s`'s 3 lifetimes or one of `t`'s 3 lifetimes
+help: consider introducing a named lifetime parameter
+   |
+LL | fn f3_<'a>(s: &'a S, t: &'a S) -> (&i32, &'a i32) { loop {} }
+   |       ^^^^    ^^^^^     ^^^^^            ^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:25:42
+   |
+LL | fn f4<'a, 'b>(a: &'a i32, b: &'b i32) -> &i32 { loop {} }
+   |                  -------     -------     ^ 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`
+note: these named lifetimes are available to use
+  --> $DIR/return-elided-lifetime.rs:25:7
+   |
+LL | fn f4<'a, 'b>(a: &'a i32, b: &'b i32) -> &i32 { loop {} }
+   |       ^^  ^^
+help: consider using one of the available lifetimes here
+   |
+LL | fn f4<'a, 'b>(a: &'a i32, b: &'b i32) -> &'lifetime i32 { loop {} }
+   |                                          ^^^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:27:44
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) { loop {} }
+   |                   -------     -------      ^ 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`
+note: these named lifetimes are available to use
+  --> $DIR/return-elided-lifetime.rs:27:8
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) { loop {} }
+   |        ^^  ^^
+help: consider using one of the available lifetimes here
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&'lifetime i32, &i32) { loop {} }
+   |                                            ^^^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:27:50
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) { loop {} }
+   |                   -------     -------            ^ 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`
+note: these named lifetimes are available to use
+  --> $DIR/return-elided-lifetime.rs:27:8
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) { loop {} }
+   |        ^^  ^^
+help: consider using one of the available lifetimes here
+   |
+LL | fn f4_<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &'lifetime i32) { loop {} }
+   |                                                  ^^^^^^^^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:31:35
+   |
+LL | fn f5<'a>(a: &'a i32, b: &i32) -> &i32 { loop {} }
+   |              -------     ----     ^ 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 using the `'a` lifetime
+   |
+LL | fn f5<'a>(a: &'a i32, b: &i32) -> &'a i32 { loop {} }
+   |                                   ^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:33:37
+   |
+LL | fn f5_<'a>(a: &'a i32, b: &i32) -> (&i32, &i32) { loop {} }
+   |               -------     ----      ^ 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 using the `'a` lifetime
+   |
+LL | fn f5_<'a>(a: &'a i32, b: &i32) -> (&'a i32, &i32) { loop {} }
+   |                                     ^^^
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/return-elided-lifetime.rs:33:43
+   |
+LL | fn f5_<'a>(a: &'a i32, b: &i32) -> (&i32, &i32) { loop {} }
+   |               -------     ----            ^ 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 using the `'a` lifetime
+   |
+LL | fn f5_<'a>(a: &'a i32, b: &i32) -> (&i32, &'a i32) { loop {} }
+   |                                           ^^^
+
+error: aborting due to 15 previous errors
+
+For more information about this error, try `rustc --explain E0106`.