Rollup merge of #28234 - AlisdairO:diagnostics247, r=Manishearth

As title :-)
Part of #24407.

r? @Manishearth

src/librustc_typeck/diagnostics.rs

@@ -2475,6 +2475,24 @@ struct Bar<S, T> { x: Foo<S, T> }
 ```
 "##,
 
+//NB: not currently reachable
+E0247: r##"
+This error indicates an attempt to use a module name where a type is expected.
+For example:
+
+```
+mod MyMod {
+    mod MySubMod { }
+}
+
+fn do_something(x: MyMod::MySubMod) { }
+```
+
+In this example, we're attempting to take a parameter of type `MyMod::MySubMod`
+in the do_something function. This is not legal: `MyMod::MySubMod` is a module
+name, not a type.
+"##,
+
 E0248: r##"
 This error indicates an attempt to use a value where a type is expected. For
 example:
@@ -3291,7 +3309,6 @@ register_diagnostics! {
     E0242, // internal error looking up a definition
     E0245, // not a trait
 //  E0246, // invalid recursive type
-    E0247, // found module name used as a type
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
     E0320, // recursive overflow during dropck
     E0321, // extended coherence rules for defaulted traits violated