Add error explanation for E0202.

Author: Nick Hamann

src/librustc_typeck/collect.rs

@@ -877,7 +877,7 @@ fn convert_item(ccx: &CrateCtxt, it: &ast::Item) {
                 if let ast::TypeImplItem(ref ty) = impl_item.node {
                     if opt_trait_ref.is_none() {
                         span_err!(tcx.sess, impl_item.span, E0202,
-                                  "associated items are not allowed in inherent impls");
+                                  "associated types are not allowed in inherent impls");
                     }
 
                     as_refsociated_type(ccx, ImplContainer(local_def(it.id)),

src/librustc_typeck/diagnostics.rs

@@ -573,6 +573,14 @@ impl Foo {
 ```
 "##,
 
+E0202: r##"
+Inherent associated types were part of [RFC 195] but are not yet implemented.
+See [the tracking issue][iss8995] for the status of this implementation.
+
+[RFC 195]: https://github.com/rust-lang/rfcs/pull/195
+[iss8995]: https://github.com/rust-lang/rust/issues/8995
+"##,
+
 E0204: r##"
 An attempt to implement the `Copy` trait for a struct failed because one of the
 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
@@ -881,7 +889,6 @@ register_diagnostics! {
     E0194,
     E0195, // lifetime parameters or bounds on method do not match the trait declaration
     E0196, // cannot determine a type for this closure
-    E0202, // associated items are not allowed in inherent impls
     E0203, // type parameter has more than one relaxed default bound,
            // and only one is supported
     E0207, // type parameter is not constrained by the impl trait, self type, or predicate

src/test/compile-fail/assoc-inherent.rs

@@ -13,7 +13,7 @@
 struct Foo;
 
 impl Foo {
-    type Bar = isize; //~ERROR associated items are not allowed in inherent impls
+    type Bar = isize; //~ERROR associated types are not allowed in inherent impls
 }
 
 fn main() {}