Add long error explanation for E0576

Author: GuillaumeGomez

src/librustc_resolve/error_codes.rs

@@ -1797,6 +1797,31 @@ let _: <u8 as Age>::Empire; // ok!
 ```
 "##,
 
+E0576: r##"
+An associated item wasn't found in the given type.
+
+Erroneous code example:
+
+```compile_fail,E0576
+trait Hello {
+    type Who;
+
+    fn hello() -> <Self as Hello>::You; // error!
+}
+```
+
+In this example, we tried to use the non-existent associated type `You` of the
+`Hello` trait. To fix this error, use an existing associated type:
+
+```
+trait Hello {
+    type Who;
+
+    fn hello() -> <Self as Hello>::Who; // ok!
+}
+```
+"##,
+
 E0603: r##"
 A private item was used outside its scope.
 
@@ -1924,7 +1949,6 @@ struct Foo<X = Box<Self>> {
 //  E0427, merged into 530
 //  E0467, removed
 //  E0470, removed
-    E0576,
     E0577,
     E0578,
 }