Add an assert_size attribute for structs

[kmcallister]

size_of is maybe the top use case for function evaluation in static asserts, which doesn't seem like it will happen any time soon. We can satisfy this use case with a very simple and self-contained feature, e.g.

#[assert_size(48)]
struct StringCacheEntry {
    next_in_bucket: *mut StringCacheEntry,
    hash: u64,
    ref_count: AtomicInt,
    string: String,
}

I think making this kind of assertion painless will help programs avoid performance regressions.

[mahkoh]

More special cases due to the lack of CTFE.

[thestinger]

I don't think we should add a bunch of very special cased hacks to work around a clearly desirable language feature.

[steveklabnik]

closing in favor of rust-lang/rfcs#591

[mahkoh]

I don't see how that RFC solves this issue.