Cases like `Either<@int,()>` have a null case with at most one value but
a nonzero number of fields; if we misreport this, then bad things can
happen inside of, for example, pattern matching.
Closes#6117.
A struct (inc. tuple struct) can be annotated with #[packed], so that there
is no padding between its elements, like GCC's `__attribute__((packed))`.
Closes#1704
The only thing we really lose is that C-like enums with one variant and a
non-zero discriminant now take up space, but I do not think this is a
common usage. As previously noted, that was mostly there for
transitional compatibility with the pre-adt.rs codebase.
Out goes the extra layer of struct wrapping; the destructedness flag is
added to the end of the struct. This means that, if the struct
previously had alignment padding at the end, the flag will live there
instead of increasing the struct size.
Note that in the ByValue case (which can't happen? yet?) we're still
effectively bitcasting, I think. So this change adds a way to assert
that that's safe.
Note also, for future reference, that LLVM's instcombine pass will turn
a bitcast into a GEP(0, 0, ...) if possible.
This change remains separate from the addition of adt.rs, even though
it's necessary for compatibility with pre-trans::adt representation,
to serve as an example of a change to the representation logic.
Later changes on this branch adapt the rest of rustc::middle::trans
to use this module instead of scattered hard-coded knowledge of
representations; a few of them also have improvements or cleanup for
adt.rs (and many added comments) that weren't drastic enough to justify
changing history to move them into this commit.
