Add type hint struct

[brendanzab]

Adds a zero-sized type hint that can be used to access static methods that don't mention Self. For example:

trait A {
    fn a(_: ForType<Self>) -> uint;
}

impl A for int {
    fn a(_: ForType<int>) -> uint { 6 }
}

assert_eq!(A::a(ForType::<int>), 6);

Previously Option<Self> was usually used, but this makes the intent clearer.

[thestinger]

I really think we need to fix the language here, rather than working around it more. I know @pcwalton really hates the idea of making :: lookup magical but the library is ending up filled with ugly workarounds.

[brendanzab]

@thestinger Yeah. But as an interim fix? I could maybe add a FIXME referencing #8888 and #6894.

[sfackler]

Is there any reason that the A::<for int>::a() syntax hasn't been added? I thought that that was the planned fix for this.

[brendanzab]

@sfackler see the issues mentioned above: #6894 and #8888

[alexcrichton]

I would personally much rather see an actual fix for this issue (something along the A::<for int>::a() syntax) rather than a band-aid over the actual problem. It seems like we've already got enough band-aids as is and it's always sad to add more if we can avoid it.

[alexcrichton]

Closing due to inactivity, and I also believe that this would be better served with actually fixing the language rather than having these type hint structures.