KnownLayout::validate_cast

[joshlf]

This method is a core primitive that other parts of the codebase can build on to validate casts and compute the appropriate pointer metadata which should result from those casts. Future commits will build on top of this one to make use of it.

[joshlf]

@jswrenn Another thing to call out: I still need to determine whether there are isize overflow issues. The function itself doesn't have any, of course, but will it ever be used in a context where it needs to either a) handle checking for isize overflow or, b) require that the caller make isize-related guarantees?

I think the answer is no: Given an address/length of a sound allocation (ie, one that safe Rust could produce):

• The length cannot overflow isize
• The address plus length cannot overflow usize

Those are the preconditions to the pointer add method. Note that the address + length sum doesn't have to fit in an isize, just in a usize. The isize overflow issues only apply to offsets within the allocation, not to addresses.

That's certainly consistent with the use in the TryFromBytes PR. Just wanted to put it in your head in case you see something that I'm missing.