---

yaml
---
r: 172842
b: refs/heads/try
c: 02d0a8b
h: refs/heads/master
v: v3

Author: Manishearth

[refs]

@@ -2,7 +2,7 @@
 refs/heads/master: 170c4399e614fe599c3d41306b3429ca8b3b68c6
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: 5b3cd3900ceda838f5798c30ab96ceb41f962534
-refs/heads/try: ad7e33efee023822bd63fa6f942c25582ef12a1e
+refs/heads/try: 02d0a8bbcf0a64339e4279c4ddb4841189ba5069
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596

branches/try/src/doc/reference.md

@@ -2117,6 +2117,13 @@ macro scope.
   destructors from being run twice. Destructors might be run multiple times on
   the same object with this attribute.
 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
+- `rustc_on_unimplemented` - Write a custom note to be shown along with the error
+   when the trait is found to be unimplemented on a type.
+   You may use format arguments like `{T}`, `{A}` to correspond to the
+   types at the point of use corresponding to the type parameters of the
+   trait of the same name. `{Self}` will be replaced with the type that is supposed
+   to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
+   must be enabled.
 
 ### Conditional compilation
 

branches/try/src/test/compile-fail/on-unimplemented-bad-anno.rs

@@ -29,6 +29,9 @@ trait BadAnnotation1 {}
 //~^ ERROR there is no type parameter C on trait BadAnnotation2
 trait BadAnnotation2<A,B> {}
 
+#[rustc_on_unimplemented = "Unimplemented trait error on `{Self}` with params `<{A},{B},{}>`"]
+//~^ only named substitution parameters are allowed
+trait BadAnnotation3<A,B> {}
 
 pub fn main() {
 }