Suggest Explicit Lifetime for Associated Type Bindings

compiler/rustc_resolve/src/late.rs

@@ -1634,7 +1634,6 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     #[instrument(level = "debug", skip(self))]
     fn resolve_anonymous_lifetime(&mut self, lifetime: &Lifetime, elided: bool) {
         debug_assert_eq!(lifetime.ident.name, kw::UnderscoreLifetime);
-
         let kind =
             if elided { MissingLifetimeKind::Ampersand } else { MissingLifetimeKind::Underscore };
         let missing_lifetime =
@@ -1685,14 +1684,53 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                                 ..
                             } = &rib.kind
                             {
-                                diag.multipart_suggestion(
-                                    "consider introducing a higher-ranked lifetime here",
-                                    vec![
-                                        (span.shrink_to_lo(), "for<'a> ".into()),
-                                        (lifetime.ident.span.shrink_to_hi(), "'a ".into()),
-                                    ],
-                                    Applicability::MachineApplicable,
-                                );
+                                // hacky way to check if lifetime suggestion is for
+                                // an associated type binding
+                                let is_associated_type_binding = lifetime.ident.span
+                                    != self.r.tcx.sess.source_map().span_extend_to_prev_char(
+                                        lifetime.ident.span,
+                                        '=',
+                                        false,
+                                    );
+                                if is_associated_type_binding {
+                                    for upper_rib in self.lifetime_ribs[0..i].iter().rev() {
+                                        if let LifetimeRibKind::Generics {
+                                            kind: LifetimeBinderKind::Function,
+                                            span: function_span,
+                                            ..
+                                        } = upper_rib.kind
+                                        {
+                                            let type_parameter_span = self
+                                                .r
+                                                .tcx
+                                                .sess
+                                                .source_map()
+                                                .span_through_char(function_span, '<')
+                                                .shrink_to_hi();
+                                            diag.multipart_suggestion(
+                                                "consider adding an explicit lifetime here",
+                                                vec![
+                                                    (type_parameter_span, "'a, ".into()),
+                                                    (
+                                                        lifetime.ident.span.shrink_to_hi(),
+                                                        "'a ".into(),
+                                                    ),
+                                                ],
+                                                Applicability::MaybeIncorrect,
+                                            );
+                                            break;
+                                        }
+                                    }
+                                } else {
+                                    diag.multipart_suggestion(
+                                        "consider introducing a higher-ranked lifetime here",
+                                        vec![
+                                            (span.shrink_to_lo(), "for<'a> ".into()),
+                                            (lifetime.ident.span.shrink_to_hi(), "'a ".into()),
+                                        ],
+                                        Applicability::MachineApplicable,
+                                    );
+                                }
                                 break;
                             }
                         }

tests/ui/lifetimes/suggest-explicit-lifetime-for-associated-types-issue-122025.rs

@@ -0,0 +1,28 @@
+struct MyType;
+
+fn foo<F>()
+where
+    F: Iterator<Item = &MyType>, //~ `&` without an explicit lifetime name cannot be used here
+{
+}
+
+fn bar() {
+    fn baz<F>()
+    where
+        F: Iterator<Item = &MyType>, //~ `&` without an explicit lifetime name cannot be used here
+    {
+    }
+}
+
+fn function<T>()
+where
+    T: Iterator<Item = &MyType>, //~ `&` without an explicit lifetime name cannot be used here
+{
+    fn lambda<A>()
+    where
+        A: Iterator<Item = &u8>, //~ `&` without an explicit lifetime name cannot be used here
+    {
+    }
+}
+
+fn main() {}

tests/ui/lifetimes/suggest-explicit-lifetime-for-associated-types-issue-122025.stderr

@@ -0,0 +1,55 @@
+error[E0637]: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/suggest-explicit-lifetime-for-associated-types-issue-122025.rs:5:24
+   |
+LL |     F: Iterator<Item = &MyType>,
+   |                        ^ explicit lifetime name needed here
+   |
+help: consider adding an explicit lifetime here
+   |
+LL ~ fn foo<'a, F>()
+LL | where
+LL ~     F: Iterator<Item = &'a MyType>,
+   |
+
+error[E0637]: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/suggest-explicit-lifetime-for-associated-types-issue-122025.rs:12:28
+   |
+LL |         F: Iterator<Item = &MyType>,
+   |                            ^ explicit lifetime name needed here
+   |
+help: consider adding an explicit lifetime here
+   |
+LL ~     fn baz<'a, F>()
+LL |     where
+LL ~         F: Iterator<Item = &'a MyType>,
+   |
+
+error[E0637]: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/suggest-explicit-lifetime-for-associated-types-issue-122025.rs:19:24
+   |
+LL |     T: Iterator<Item = &MyType>,
+   |                        ^ explicit lifetime name needed here
+   |
+help: consider adding an explicit lifetime here
+   |
+LL ~ fn function<'a, T>()
+LL | where
+LL ~     T: Iterator<Item = &'a MyType>,
+   |
+
+error[E0637]: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/suggest-explicit-lifetime-for-associated-types-issue-122025.rs:23:28
+   |
+LL |         A: Iterator<Item = &u8>,
+   |                            ^ explicit lifetime name needed here
+   |
+help: consider adding an explicit lifetime here
+   |
+LL ~     fn lambda<'a, A>()
+LL |     where
+LL ~         A: Iterator<Item = &'a u8>,
+   |
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0637`.