Improve diagnostics for function passed when a type was expected.

compiler/rustc_typeck/src/astconv/generics.rs

@@ -109,6 +109,20 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     );
                 }
             }
+            (GenericArg::Const(cnst), GenericParamDefKind::Type { .. }) => {
+                let body = tcx.hir().body(cnst.value.body);
+                if let rustc_hir::ExprKind::Path(rustc_hir::QPath::Resolved(_, path)) =
+                    body.value.kind
+                {
+                    if let Res::Def(DefKind::Fn { .. }, id) = path.res {
+                        err.help(&format!(
+                            "`{}` is a function item, not a type",
+                            tcx.item_name(id)
+                        ));
+                        err.help("function item types cannot be named directly");
+                    }
+                }
+            }
             _ => {}
         }
 

src/test/ui/generics/generic-function-item-where-type.rs

@@ -0,0 +1,6 @@
+fn foo<U>() {}
+
+fn main() {
+    foo::<main>()
+    //~^ ERROR constant provided when a type was expected
+}

src/test/ui/generics/generic-function-item-where-type.stderr

@@ -0,0 +1,12 @@
+error[E0747]: constant provided when a type was expected
+  --> $DIR/generic-function-item-where-type.rs:4:11
+   |
+LL |     foo::<main>()
+   |           ^^^^
+   |
+   = help: `main` is a function item, not a type
+   = help: function item types cannot be named directly
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0747`.

src/test/ui/privacy/privacy-ns1.stderr

@@ -57,6 +57,9 @@ error[E0747]: constant provided when a type was expected
    |
 LL |     let _x: Box<Bar>;
    |                 ^^^
+   |
+   = help: `Bar` is a function item, not a type
+   = help: function item types cannot be named directly
 
 error: aborting due to 4 previous errors
 

src/test/ui/privacy/privacy-ns2.stderr

@@ -83,12 +83,18 @@ error[E0747]: constant provided when a type was expected
    |
 LL |     let _x : Box<Bar>;
    |                  ^^^
+   |
+   = help: `Bar` is a function item, not a type
+   = help: function item types cannot be named directly
 
 error[E0747]: constant provided when a type was expected
   --> $DIR/privacy-ns2.rs:48:17
    |
 LL |     let _x: Box<Bar>;
    |                 ^^^
+   |
+   = help: `Bar` is a function item, not a type
+   = help: function item types cannot be named directly
 
 error: aborting due to 8 previous errors