Cleanup E0749

src/librustc_error_codes/error_codes/E0749.md

@@ -1,4 +1,19 @@
-Negative impls are not allowed to have any items. Negative impls
-declare that a trait is **not** implemented (and never will be) and
-hence there is no need to specify the values for trait methods or
-other items.
+An item was added on a negative impl.
+
+Erroneous code example:
+
+```compile_fail,E0749
+# #![feature(negative_impls)]
+trait MyTrait {
+    type Foo;
+}
+
+impl !MyTrait for u32 {
+    type Foo = i32; // error!
+}
+# fn main() {}
+```
+
+Negative impls are not allowed to have any items. Negative impls declare that a
+trait is **not** implemented (and never will be) and hence there is no need to
+specify the values for trait methods or other items.

src/tools/tidy/src/error_codes_check.rs

@@ -16,7 +16,7 @@ const EXEMPTED_FROM_TEST: &[&str] = &[
 ];
 
 // Some error codes don't have any tests apparently...
-const IGNORE_EXPLANATION_CHECK: &[&str] = &["E0570", "E0601", "E0602", "E0639", "E0729", "E0749"];
+const IGNORE_EXPLANATION_CHECK: &[&str] = &["E0570", "E0601", "E0602", "E0639", "E0729"];
 
 fn check_error_code_explanation(
     f: &str,