encode should return ? for types that don't implement Encode

[SSheldon]

Implementing #10 would currently require that any type used in a method argument must implement Encode. This is difficult for cases like functions, whose ObjC type encoding is just '?' but it's hard to implement Encode for all fns (because there's fn() -> R, extern fn() -> R, unsafe extern fn() -> R, unsafe fn() -> R, fn(A) -> R, etc).

It'd be simplest if encode<fn()> == "?" without implementing Encode for fn(), and then if Encode isn't implemented when it should be, it would just be an error at runtime.

[SSheldon]

This could be possible with negative bounds:

trait EncodePrivate {
    fn private_code() -> &'static str;
}

impl EncodePrivate for T where T: Encode {
    fn private_code() -> &'static str { <T as Encode>::code() }
}

impl EncodePrivate for T where T: !Encode {
    fn private_code() -> &'static str { "?" }
}

fn encode<T>() -> &'static str {
    <T as EncodePrivate>::private_code()
}

Negative bounds are being discussed in rust-lang/rfcs#586.

[SSheldon]

Turns out encode for a function actually returns "^?", so this won't do any good. Closing.