when a trait bound can't be fulfilled the compiler should tell you why

[metajack]

If you call some function with a trait bound (or trigger it some other way like capturing in a closure), the compiler will yell that some type doesn't fulfill the bound. It won't tell you what in that type is failing the bound, and in the case of non-trival structures this makes it extremely frustrating to figure out what's wrong.

Case in point: can you tell me why ScriptMsg isn't send here? https://github.com/mozilla/servo/blob/master/src/components/script/script_task.rs#L57-L78

[alexcrichton]

Closing, this will get covered by #13231