Values and Sized types are not larger than isize::MAX

[joshlf]

See #t-opsem > Constraints on abstract values? @ 💬 for context.

For Sized types, we nearly already guarantee this by implication. If we assume that Layout::new<T>() -> Layout can't fail to compile or panic at runtime, then it implies that, for any T: Sized, it is possible to acquire a Layout describing that T. Layout, in turn, guarantees that:

The size, when rounded up to the nearest multiple of align, does not overflow isize (i.e., the rounded value will always be less than or equal to isize::MAX).

This is also already guaranteed for any Rust allocation, although in theory it's possible that:

• A value could live somewhere other than an allocation
• A type could be compiled that is never instantiated, and thus the allocation constraint doesn't constrain the size of the type itself

[joshlf]

r? @ehuss

cc @RalfJung

[programmerjake]

lgtm!

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