non-scalar_cast.rs

trait Foo {}

struct Bar;

impl Foo for Bar {}

fn main() {
    let a: @Foo = @Bar as @Foo;
    let b = a as @Bar;
}

output

$ rustc non-scalar_cast.rs
non-scalar_cast.rs:9:12: 9:21 error: non-scalar cast: `@Foo:'static` as `@Bar`
non-scalar_cast.rs:9     let b = a as @Bar;
                                 ^~~~~~~~~
error: aborting due to previous error
task '<unnamed>' failed at 'explicit failure', /build/rust-git/src/rust/src/libsyntax/diagnostic.rs:98
task '<unnamed>' failed at 'explicit failure', /build/rust-git/src/rust/src/librustc/rustc.rs:391
$