Add long error explanation for E0573

Author: GuillaumeGomez

src/librustc_resolve/error_codes.rs

@@ -1611,6 +1611,80 @@ fn print_on_failure(state: &State) {
 ```
 "##,
 
+E0573: r##"
+Something other than a type has been used when one was expected.
+
+Erroneous code examples:
+
+```compile_fail,E0573
+enum Dragon {
+    Born,
+}
+
+fn oblivion() -> Dragon::Born { // error!
+    Dragon::Born
+}
+
+const HOBBIT: u32 = 2;
+impl HOBBIT {} // error!
+
+enum Wizard {
+    Gandalf,
+    Saruman,
+}
+
+trait Isengard {
+    fn wizard(_: Wizard::Saruman); // error!
+}
+```
+
+In all these errors, a type was expected. For example, in the first error, if
+we want to return the `Born` variant from the `Dragon` enum, we must set the
+function to return the enum and not its variant:
+
+```
+enum Dragon {
+    Born,
+}
+
+fn oblivion() -> Dragon { // ok!
+    Dragon::Born
+}
+```
+
+In the second error, you can't implement something on an item, only on types.
+We would need to create a new type if we wanted to do something similar:
+
+```
+struct Hobbit(u32); // we create a new type
+
+const HOBBIT: Hobbit = Hobbit(2);
+impl Hobbit {} // ok!
+```
+
+In the third case, we tried to only expect one variant of the `Wizard` enum,
+which is not possible. To make this work, we need to using pattern matching
+over the `Wizard` enum:
+
+```
+enum Wizard {
+    Gandalf,
+    Saruman,
+}
+
+trait Isengard {
+    fn wizard(w: Wizard) { // error!
+        match w {
+            Wizard::Saruman => {
+                // do something
+            }
+            _ => {} // ignore everything else
+        }
+    }
+}
+```
+"##,
+
 E0574: r##"
 Something other than a struct, variant or union has been used when one was
 expected.
@@ -1788,7 +1862,6 @@ struct Foo<X = Box<Self>> {
 //  E0427, merged into 530
 //  E0467, removed
 //  E0470, removed
-    E0573,
     E0575,
     E0576,
     E0577,