Rollup merge of #105859 - compiler-errors:hr-lifetime-add, r=davidtwco

Point out span where we could introduce higher-ranked lifetime

Somewhat addresses #105422, but not really. We don't have that much useful information here since we're still in resolution :^(

Maybe this suggestion isn't worth it. If the reviewer has an idea how we can get a more succinct binder information for a structured suggestion, it would be appreciated.

compiler/rustc_resolve/src/late.rs

@@ -1515,7 +1515,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             count: 1,
         };
         let elision_candidate = LifetimeElisionCandidate::Missing(missing_lifetime);
-        for rib in self.lifetime_ribs.iter().rev() {
+        for (i, rib) in self.lifetime_ribs.iter().enumerate().rev() {
             debug!(?rib.kind);
             match rib.kind {
                 LifetimeRibKind::AnonymousCreateParameter { binder, .. } => {
@@ -1532,16 +1532,31 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     } else {
                         ("`'_` cannot be used here", "`'_` is a reserved lifetime name")
                     };
-                    rustc_errors::struct_span_err!(
+                    let mut diag = rustc_errors::struct_span_err!(
                         self.r.session,
                         lifetime.ident.span,
                         E0637,
                         "{}",
                         msg,
-                    )
-                    .span_label(lifetime.ident.span, note)
-                    .emit();
-
+                    );
+                    diag.span_label(lifetime.ident.span, note);
+                    if elided {
+                        for rib in self.lifetime_ribs[i..].iter().rev() {
+                            if let LifetimeRibKind::Generics {
+                                span,
+                                kind: LifetimeBinderKind::PolyTrait | LifetimeBinderKind::WhereBound,
+                                ..
+                            } = &rib.kind
+                            {
+                                diag.span_help(
+                                    *span,
+                                    "consider introducing a higher-ranked lifetime here with `for<'a>`",
+                                );
+                                break;
+                            }
+                        }
+                    }
+                    diag.emit();
                     self.record_lifetime_res(lifetime.id, LifetimeRes::Error, elision_candidate);
                     return;
                 }

src/test/ui/error-codes/E0637.stderr

@@ -21,6 +21,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL |     T: Into<&u32>,
    |             ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/E0637.rs:13:8
+   |
+LL |     T: Into<&u32>,
+   |        ^
 
 error: aborting due to 3 previous errors
 

src/test/ui/generics/issue-65285-incorrect-explicit-lifetime-name-needed.stderr

@@ -3,6 +3,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL | fn should_error<T>() where T : Into<&u32> {}
    |                                     ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/issue-65285-incorrect-explicit-lifetime-name-needed.rs:5:32
+   |
+LL | fn should_error<T>() where T : Into<&u32> {}
+   |                                ^
 
 error[E0106]: missing lifetime specifier
   --> $DIR/issue-65285-incorrect-explicit-lifetime-name-needed.rs:9:20

src/test/ui/underscore-lifetime/where-clause-inherent-impl-ampersand.rust2015.stderr

@@ -3,6 +3,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL |     T: WithType<&u32>
    |                 ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/where-clause-inherent-impl-ampersand.rs:13:8
+   |
+LL |     T: WithType<&u32>
+   |        ^
 
 error: aborting due to previous error
 

src/test/ui/underscore-lifetime/where-clause-inherent-impl-ampersand.rust2018.stderr

@@ -3,6 +3,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL |     T: WithType<&u32>
    |                 ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/where-clause-inherent-impl-ampersand.rs:13:8
+   |
+LL |     T: WithType<&u32>
+   |        ^
 
 error: aborting due to previous error
 

src/test/ui/underscore-lifetime/where-clause-trait-impl-region.rust2015.stderr

@@ -3,6 +3,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL |     T: WithType<&u32>
    |                 ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/where-clause-trait-impl-region.rs:11:8
+   |
+LL |     T: WithType<&u32>
+   |        ^
 
 error: aborting due to previous error
 

src/test/ui/underscore-lifetime/where-clause-trait-impl-region.rust2018.stderr

@@ -3,6 +3,12 @@ error[E0637]: `&` without an explicit lifetime name cannot be used here
    |
 LL |     T: WithType<&u32>
    |                 ^ explicit lifetime name needed here
+   |
+help: consider introducing a higher-ranked lifetime here with `for<'a>`
+  --> $DIR/where-clause-trait-impl-region.rs:11:8
+   |
+LL |     T: WithType<&u32>
+   |        ^
 
 error: aborting due to previous error