Fixing bad suggestion for _ in const type when a function #81885

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2327,6 +2327,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 &generics.params[..],
                 visitor.0,
                 true,
+                true,
             );
         }
 

compiler/rustc_typeck/src/collect.rs

@@ -141,6 +141,7 @@ crate fn placeholder_type_error(
     generics: &[hir::GenericParam<'_>],
     placeholder_types: Vec<Span>,
     suggest: bool,
+    is_fn: bool,
 ) {
     if placeholder_types.is_empty() {
         return;
@@ -171,7 +172,9 @@ crate fn placeholder_type_error(
     }
 
     let mut err = bad_placeholder_type(tcx, placeholder_types);
-    if suggest {
+
+    // Suggest, but only if it is not a function
+    if suggest && !is_fn {
         err.multipart_suggestion(
             "use type parameters instead",
             sugg,
@@ -198,7 +201,14 @@ fn reject_placeholder_type_signatures_in_item(tcx: TyCtxt<'tcx>, item: &'tcx hir
     let mut visitor = PlaceholderHirTyCollector::default();
     visitor.visit_item(item);
 
-    placeholder_type_error(tcx, Some(generics.span), &generics.params[..], visitor.0, suggest);
+    placeholder_type_error(
+        tcx,
+        Some(generics.span),
+        &generics.params[..],
+        visitor.0,
+        suggest,
+        false,
+    );
 }
 
 impl Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
@@ -743,7 +753,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
             // Account for `const C: _;`.
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(tcx, None, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false, false);
         }
 
         hir::TraitItemKind::Type(_, Some(_)) => {
@@ -752,7 +762,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
             // Account for `type T = _;`.
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(tcx, None, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false, false);
         }
 
         hir::TraitItemKind::Type(_, None) => {
@@ -761,7 +771,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
             // even if there is no concrete type.
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(tcx, None, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false, false);
         }
     };
 
@@ -782,7 +792,7 @@ fn convert_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::HirId) {
             // Account for `type T = _;`
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_impl_item(impl_item);
-            placeholder_type_error(tcx, None, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false, false);
         }
         hir::ImplItemKind::Const(..) => {}
     }

src/test/ui/did_you_mean/bad-assoc-ty.stderr

@@ -129,33 +129,18 @@ LL | fn foo<X: K<_, _>>(x: X) {}
    |             ^  ^ not allowed in type signatures
    |             |
    |             not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL | fn foo<X: K<T, T>, T>(x: X) {}
-   |             ^  ^ ^^^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/bad-assoc-ty.rs:52:34
    |
 LL | fn bar<F>(_: F) where F: Fn() -> _ {}
    |                                  ^ not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL | fn bar<F, T>(_: F) where F: Fn() -> T {}
-   |         ^^^                         ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/bad-assoc-ty.rs:55:19
    |
 LL | fn baz<F: Fn() -> _>(_: F) {}
    |                   ^ not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL | fn baz<F: Fn() -> T, T>(_: F) {}
-   |                   ^^^^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/bad-assoc-ty.rs:58:33
@@ -217,11 +202,6 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
    |
 LL |     fn foo<F>(_: F) where F: Fn() -> _ {}
    |                                      ^ not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL |     fn foo<F, T>(_: F) where F: Fn() -> T {}
-   |             ^^^                         ^
 
 error: aborting due to 28 previous errors
 

src/test/ui/issues/issue-74086.stderr

@@ -2,10 +2,7 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
   --> $DIR/issue-74086.rs:2:20
    |
 LL |     static BUG: fn(_) -> u8 = |_| 8;
-   |                    ^
-   |                    |
-   |                    not allowed in type signatures
-   |                    help: use type parameters instead: `T`
+   |                    ^ not allowed in type signatures
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-81885.rs

@@ -0,0 +1,10 @@
+const TEST4: fn() -> _ = 42;
+                  //~^ ERROR the type placeholder `_` is not allowed within types on item
+                  //signatures
+
+fn main() {
+    const TEST5: fn() -> _ = 42;
+                      //~^ ERROR the type placeholder `_` is not allowed within types on item
+                      //signatures
+
+}

src/test/ui/issues/issue-81885.stderr

@@ -0,0 +1,15 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-81885.rs:1:22
+   |
+LL | const TEST4: fn() -> _ = 42;
+   |                      ^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-81885.rs:6:26
+   |
+LL |     const TEST5: fn() -> _ = 42;
+   |                          ^ not allowed in type signatures
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.

src/test/ui/self/self-infer.stderr

@@ -3,22 +3,12 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
    |
 LL |     fn f(self: _) {}
    |                ^ not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL |     fn f<T>(self: T) {}
-   |         ^^^       ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/self-infer.rs:5:17
    |
 LL |     fn g(self: &_) {}
    |                 ^ not allowed in type signatures
-   |
-help: use type parameters instead
-   |
-LL |     fn g<T>(self: &T) {}
-   |         ^^^        ^
 
 error: aborting due to 2 previous errors