Safe code can implement Message and therefore Object traits for any type to invoke undefined behavior

[SSheldon]

Implementing Message doesn't require actually implementing anything, and then implementing INSObject doesn't require any unsafe code. Safe code therefore can implement INSObject for types that it shouldn't and invoke undefined behavior by calling the INSObject methods.

impl Message for uint { }

impl INSObject for uint {
    fn class_name() -> ClassName<uint> {
        ClassName("uint")
    }
}

fn main() {
    let x = 0u;
    let hash_code = x.hash_code();
}

[SSheldon]

This could be resolved by rust-lang/rust#19703 and marking the traits as unsafe.

[SSheldon]

Message was made unsafe in 776486e; the object traits may need to be marked unsafe as well, but that isn't as pressing because invoking an unsupported selector on an object in Objective-C is defined behavior.