Avoid suggesting closures when captures cross associated item boundaries

compiler/rustc_resolve/src/diagnostics/impls.rs

@@ -1180,8 +1180,11 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
                 err
             }
-            ResolutionError::CannotCaptureDynamicEnvironmentInFnItem => {
-                self.dcx().create_err(diagnostics::CannotCaptureDynamicEnvironmentInFnItem { span })
+            ResolutionError::CannotCaptureDynamicEnvironmentInFnItem { suggest_closure } => {
+                self.dcx().create_err(diagnostics::CannotCaptureDynamicEnvironmentInFnItem {
+                    span,
+                    suggest_closure,
+                })
             }
             ResolutionError::AttemptToUseNonConstantValueInConstant {
                 ident,

compiler/rustc_resolve/src/diagnostics/mod.rs

@@ -268,10 +268,11 @@ pub(crate) struct UnreachableLabelWithSimilarNameExists {
 
 #[derive(Diagnostic)]
 #[diag("can't capture dynamic environment in a fn item", code = E0434)]
-#[help("use the `|| {\"{\"} ... {\"}\"}` closure form instead")]
 pub(crate) struct CannotCaptureDynamicEnvironmentInFnItem {
     #[primary_span]
     pub(crate) span: Span,
+    #[help("use the `|| {\"{\"} ... {\"}\"}` closure form instead")]
+    pub(crate) suggest_closure: bool,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_resolve/src/ident.rs

@@ -1506,6 +1506,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 use ResolutionError::*;
                 let mut res_err = None;
 
+                let suggest_closure =
+                    !ribs.iter().any(|rib| matches!(rib.kind, RibKind::AssocItem));
+
                 for rib in ribs {
                     match rib.kind {
                         RibKind::Normal
@@ -1525,7 +1528,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                 // we want certain other resolution errors (namely those
                                 // emitted for `ConstantItemRibKind` below) to take
                                 // precedence.
-                                res_err = Some((span, CannotCaptureDynamicEnvironmentInFnItem));
+                                res_err = Some((
+                                    span,
+                                    CannotCaptureDynamicEnvironmentInFnItem { suggest_closure },
+                                ));
                             }
                         }
                         RibKind::ConstantItem(_, item, requires_type) => {

compiler/rustc_resolve/src/lib.rs

@@ -275,7 +275,7 @@ enum ResolutionError<'ra> {
         message: String,
     },
     /// Error E0434: can't capture dynamic environment in a fn item.
-    CannotCaptureDynamicEnvironmentInFnItem,
+    CannotCaptureDynamicEnvironmentInFnItem { suggest_closure: bool },
     /// Error E0435: attempt to use a non-constant value in a constant.
     AttemptToUseNonConstantValueInConstant {
         ident: Ident,

tests/ui/resolve/closure-suggestion-in-impl.rs

@@ -0,0 +1,28 @@
+trait T {
+    fn t() -> impl FnOnce() -> ();
+}
+
+fn f(x: String) {
+    struct S;
+    impl T for S {
+        fn t() -> impl FnOnce() -> () {
+            move || {
+                let _ = x; //~ ERROR can't capture dynamic environment in a fn item
+            }
+        }
+    }
+
+    impl S {
+        fn foo() {
+            let bar = 42;
+
+            fn inner() {
+                let _ = bar; //~ ERROR can't capture dynamic environment in a fn item
+
+                let _ = x; //~ ERROR can't capture dynamic environment in a fn item
+            }
+        }
+    }
+}
+
+fn main() {}

tests/ui/resolve/closure-suggestion-in-impl.stderr

@@ -0,0 +1,23 @@
+error[E0434]: can't capture dynamic environment in a fn item
+  --> $DIR/closure-suggestion-in-impl.rs:10:25
+   |
+LL |                 let _ = x;
+   |                         ^
+
+error[E0434]: can't capture dynamic environment in a fn item
+  --> $DIR/closure-suggestion-in-impl.rs:20:25
+   |
+LL |                 let _ = bar;
+   |                         ^^^
+   |
+   = help: use the `|| { ... }` closure form instead
+
+error[E0434]: can't capture dynamic environment in a fn item
+  --> $DIR/closure-suggestion-in-impl.rs:22:25
+   |
+LL |                 let _ = x;
+   |                         ^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0434`.

tests/ui/resolve/impl-method-cant-capture-env.stderr

@@ -3,8 +3,6 @@ error[E0434]: can't capture dynamic environment in a fn item
    |
 LL |            self.v0 = k0 ^ 0x736f6d6570736575;
    |                      ^^
-   |
-   = help: use the `|| { ... }` closure form instead
 
 error: aborting due to 1 previous error
 

tests/ui/resolve/impl-trait-for-type-cant-capture-env.stderr

@@ -3,8 +3,6 @@ error[E0434]: can't capture dynamic environment in a fn item
    |
 LL |            self.v0 = k0 ^ 0x736f6d6570736575;
    |                      ^^
-   |
-   = help: use the `|| { ... }` closure form instead
 
 error: aborting due to 1 previous error
 

tests/ui/resolve/trait-impl-cant-capture-env.stderr

@@ -3,16 +3,12 @@ error[E0434]: can't capture dynamic environment in a fn item
    |
 LL |             self.v0 = k0 ^ 0x736f6d6570736575;
    |                       ^^
-   |
-   = help: use the `|| { ... }` closure form instead
 
 error[E0434]: can't capture dynamic environment in a fn item
   --> $DIR/trait-impl-cant-capture-env.rs:25:23
    |
 LL |             self.v1 = k1 ^ 0x646f72616e646f6d;
    |                       ^^
-   |
-   = help: use the `|| { ... }` closure form instead
 
 error: aborting due to 2 previous errors