Reserve `sizeof` keyword

[brson]

Nominating.

(Clarification from Felix: We already have sys::size_of::<T>(), but you cannot use that in const-expressions. Thus the proposal that we grab sizeof as a keyword, so that we can incorporate it into an extension of the current set of const-expressions. At least, that's how Felix understands the situation.)

[nikomatsakis]

Might as well be the same as C (i.e., sizeof). Also, let's
reserve offsetof while we're at it.

[thestinger]

C also has alignof (and alignas, but that would be an attribute) now. The offsetof implementation is just a macro in C and could likely be one in Rust too.

[nikomatsakis]

On Fri, Sep 20, 2013 at 02:41:21AM -0700, Daniel Micay wrote:

C also has alignof (and alignas, but that would be an attribute)
now.

Yes.

The offsetof implementation is just a macro in C and could likely
be one in Rust too.

Hmm, I considered that. I was thinking that it could not be a macro if
we wanted it to appear in constant expressions. But I guess that's not
true, since all the macro does is something like

&((T*)NULL)->f

which I believe is legal in our constant expressions -- or at least
might as well be legal in any world that permits sizeof in a
constant expression.

In that case, we don't need to reserve offsetof, except that it would
mean you write sizeof(T) (or whatever) but offsetof!(T,f). I guess
that's fine.