Implement constant folding in the frontend

[pcwalton]

We should be able to do constant folding in the frontend to eliminate "if" conditions that are never executed. It would also be nice to be able to call size_of at compile time in constants. (This latter part is an RFC.)

[alexcrichton]

If size_of were always invoked as calling the compiler intrinsic directly, it might be fairly simple to evaluate things like compiler intrinsics at compile-time. It's behind the sys::size_of function, however, which the compiler knows nothing about. Basically I think that if we get size_of (unless we do something like pub use intrinsics::size_of in sys), we get generic CTFE (maybe a separate issue?)

[nikomatsakis]

I imagine that size_of might have to be a keyword or some kind of built-in? This really depends on how hard we want to hold the line about what kinds of things get evaluated at compile-time.

[pnkfelix]

visiting for (long long delayed) triage. Should we consider differentiating between the two issues being described here? (I.e. one is a pure front-end optimization which could have great follow-on effects improving rustc performance; the second sounds like a user-visible feature, at least in its most general form...)

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#685