Rollup merge of #69697 - GuillaumeGomez:explanation-e0380, r=Dylan-DPC

Add explanation for E0380

r? @Dylan-DPC

Author: Dylan-DPC

src/librustc_error_codes/error_codes/E0380.md

@@ -1,4 +1,14 @@
-Auto traits cannot have methods or associated items.
-For more information see the [opt-in builtin traits RFC][RFC 19].
+An auto trait was declared with a method or an associated item.
+
+Erroneous code example:
+
+```compile_fail,E0380
+unsafe auto trait Trait {
+    type Output; // error!
+}
+```
+
+Auto traits cannot have methods or associated items. For more information see
+the [opt-in builtin traits RFC][RFC 19].
 
 [RFC 19]: https://github.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md