Remove invalid suggestions for assoc consts on placeholder type error

Author: JohnTitor

compiler/rustc_typeck/src/collect.rs

@@ -180,8 +180,7 @@ crate fn placeholder_type_error(
     // Suggest, but only if it is not a function in const or static
     if suggest {
         let mut is_fn = false;
-        let mut is_const = false;
-        let mut is_static = false;
+        let mut is_const_or_static = false;
 
         if let Some(hir_ty) = hir_ty {
             if let hir::TyKind::BareFn(_) = hir_ty.kind {
@@ -191,19 +190,26 @@ crate fn placeholder_type_error(
                 let parent_id = tcx.hir().get_parent_node(hir_ty.hir_id);
                 let parent_node = tcx.hir().get(parent_id);
 
-                if let hir::Node::Item(item) = parent_node {
-                    if let hir::ItemKind::Const(_, _) = item.kind {
-                        is_const = true;
-                    } else if let hir::ItemKind::Static(_, _, _) = item.kind {
-                        is_static = true;
-                    }
-                }
+                is_const_or_static = match parent_node {
+                    Node::Item(&hir::Item {
+                        kind: hir::ItemKind::Const(..) | hir::ItemKind::Static(..),
+                        ..
+                    })
+                    | Node::TraitItem(&hir::TraitItem {
+                        kind: hir::TraitItemKind::Const(..),
+                        ..
+                    })
+                    | Node::ImplItem(&hir::ImplItem {
+                        kind: hir::ImplItemKind::Const(..), ..
+                    }) => true,
+                    _ => false,
+                };
             }
         }
 
         // if function is wrapped around a const or static,
         // then don't show the suggestion
-        if !(is_fn && (is_const || is_static)) {
+        if !(is_fn && is_const_or_static) {
             err.multipart_suggestion(
                 "use type parameters instead",
                 sugg,

src/test/ui/typeck/type-placeholder-fn-in-const.rs

@@ -0,0 +1,14 @@
+struct MyStruct;
+
+trait Test {
+    const TEST: fn() -> _;
+    //~^ ERROR: the type placeholder `_` is not allowed within types on item signatures [E0121]
+    //~| ERROR: the type placeholder `_` is not allowed within types on item signatures [E0121]
+}
+
+impl Test for MyStruct {
+    const TEST: fn() -> _ = 42;
+    //~^ ERROR: the type placeholder `_` is not allowed within types on item signatures [E0121]
+}
+
+fn main() {}

src/test/ui/typeck/type-placeholder-fn-in-const.stderr

@@ -0,0 +1,21 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/type-placeholder-fn-in-const.rs:4:25
+   |
+LL |     const TEST: fn() -> _;
+   |                         ^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/type-placeholder-fn-in-const.rs:4:25
+   |
+LL |     const TEST: fn() -> _;
+   |                         ^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/type-placeholder-fn-in-const.rs:10:25
+   |
+LL |     const TEST: fn() -> _ = 42;
+   |                         ^ not allowed in type signatures
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0121`.