Clarify std::mem-docs wrt host/target

[lukaslueg]

Functions like std::mem::align_of (and also parts of std::alloc::Layout) which are const can be used to initialize other constants and, soon, types like [u8; { std::mem::align_of::<String>() }]. As far as I can see the docs currently do not explicitly make it clear that the compiler guarantees (including future versions) that the return values of those functions are valid for the target-platform and not for the host-platform, even in a const-context.

This is probably everybody's expectation. If this behavior is guaranteed, the docs should mention that, though.

[nagisa]

host vs target vs build is a concern build system needs to deal when selecting toolchains, libraries and flags to use. Such a distinction doesn't exist within the toolchain itself, however… if you ignore accidental breakage once in a while due to failures to normalize for byte order when handling the libraries, that is.

With that in mind I feel like documenting this on potentially every const function is excessive. It might make sense to put a line that states something along these lines in the reference somewhere, however.

[lukaslueg]

@nagisa I agree. My concern came up specifically with std::mem::align_of, where one might wonder if const foo: usize = std::mem::align_of::<String>() is String's alignment on the target-platform or it's alignment on the host-platform, which is definitely different.