Proposal: do not allow ++ on many-ptrs

[mlugg]

In status quo Zig, you can use ++ with many-ptr operands. The intention (as indicated by comments on the implementation) is that these many-ptrs should be sentinel-terminated, and that the compiler would essentially do std.mem.span for you - in practice, the compiler currently does something much stranger to do with its internal representation of pointers.

Both the actual and intended behavior here seem incorrect. The process of effectively converting a many-ptr to a slice (by virtue of assigning it a length) is not one the compiler should perform implicitly: the user should make this intent clear by performing this conversion in userland, such as with std.mem.span, as is the case for quite literally every other construct in Zig.

I therefore propose that this feature be removed, and that instead the operands to ++ must have a known length (i.e. they must be arrays, vectors, tuples, or slices). The migration path for any userland code relying on this feature (if any exists, that is; I highly doubt it does) will simply be to add calls to std.mem.span where needed.