Add explanation for E0380

1 files changed, 12 insertions, 2 deletions

diff --git a/src/librustc_error_codes/error_codes/E0380.md b/src/librustc_error_codes/error_codes/E0380.md
index fe5de569339..638f0c8ecc6 100644
--- a/src/librustc_error_codes/error_codes/E0380.md
+++ b/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