Allow for representing Option<uint> as an integer with an excluded sentinel value

[zwarich]

We should have some way of using Option<uint> (or a moral equivalent) for integers with an excluded sentinel value, e.g. MovePathIndex in librustc. It's frustrating that Option isn't actually a zero-cost abstraction, forcing you to fall back to manually checking sentinel values.

[zwarich]

This would require giving up one representable value, of course.

[huonw]

This should be an RFC... and it has already been filed: rust-lang/rfcs#41

[ben0x539]

A problem here is that you take a mutable reference to that uint and now all arithmetic on it has to be checked to make sure you don't fall into the None value.

[steveklabnik]

Closing in favor of that RFC.