[Pitch] Constrain implicit raw pointer conversion to bitwise-copyable values

[tera]

+1 to this proposal. AFAIK it solves everything we discussed recently, does this to a much greater level of detail and is backed with the implementation already, so well done.

A type that contains no class references is referred to as bitwise copyable, because copying its value is equivalent to calling memcpy

memcpy works at the byte level (i.e. it can't copy 7 bits of a byte without copying the 8th (memory I/O is done at a byte granularity)) and thus memcpy doesn't even talk about bits in its man page. However I agree that "BitwiseCopyable" is a good name to use here and not any worse than a potential "BytewiseCopyable" alternative.

To mitigate source breakage, conversion of generic types that conform to FixedWidthInteger will be allowed.

I guess Double could follow the suit.

What about the tuples? Will there be some automatic conformance that says: the tuple of "BitwiseCopyable" elements is "BitwiseCopyable" itself? Or will it be possible to manually conform a tuple class to "BitwiseCopyable" (provided its elements are "BitwiseCopyable")?

On a point of clarification, would the "BitwiseCopyable" applied to a struct mean what "POD" currently means? In other words will it follow (1) or (2) below?

@tera

1. POD/PDS values (those can be recreated as "copies" from bits in a different process).
2. values that can be recreated as copies from bits in the same process only.
3. a value that can be recreated as a unique copy from bits in the same process (original value is destroyed). Attempt to create a second copy from the same bits should fail (somehow), or successfully create a new unique copy with the previously unique copy destroyed the moment the new unique copy is created (Edited).

On that front, could BitwiseCopyable be applied to a reference type? (obviously the one that doesn't contain reference fields in its payload). I guess – "no" and it is worth to mention it in the proposal (unless it is already mentioned and I just missed it).