Stabilizing a const generics MVP

[oli-obk]

@eddyb

We would still need an WF constant value check that enforces the value is actually an integer (and not e.g. a CTFE "abstract memory address", or anything weirder). Presumably some of that is already enforced by the current implementation for integer types, but not ADTs?

This already exists, even for aggregates. The only thing where it doesn't work is through references to other statics, we don't follow references that point into statics. But since the static must be WF itself, you need unsafe code to construct a bad reference. Also constants can't refer to statics, so this is a non-issue for const generics.

const FOO: usize = unsafe { std::mem::transmute(&42) };

will give an error that can't be worked around