a2840f3560
Add `#[rustc_error]` annotation, which causes trans to signal an error if found on the `main()` function. This lets you write tests that live in `compile-fail` but are expected to compile successfully. This is handy when you have many small variations on a theme that you want to keep together, and you are just testing the type checker, not the runtime semantics. r? @pnkfelix