On some platforms, a type can have two alignment: one is when it is used as itself, and the other is when it is used as a struct field. And alignof and alignas reports the first one, which isn't really useful for Maybe, because we are using it inside a struct. For example, on 32bit Linux, uint64_t and double have 8byte alignment as their own, but when they are inside a struct, they are aligned to 4byte instead. [1] Make Maybe::mStroage use the second alignment helps better packing in those cases, and that can also workaround a Rust inconsistency with C++ [2] which causes bindgen issue [3] on 32bit Linux. [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79024 [2] https://github.com/rust-lang/rust/issues/43899 [3] https://github.com/rust-lang-nursery/rust-bindgen/issues/917

Note that MOZ_ALIGN_OF is different. https://bugzilla.mozilla.org/show_bug.cgi?id=1288016

Perhaps it should be called MOZ_ALIGNAS_IN_STRUCT to make it more clear that it's intended to be different from alignas (rather than, say, a replacement for use for compilers that don't support it)?

I cannot really think of a use case where the original alignas is more useful than this macro...

Everything seems to be fine with this patch: https://treeherder.mozilla.org/#/jobs?repo=try&revision=ad70130ca3164aadc325ef007cd026bfe938f28e

Comment on attachment 8898050 [details] Bug 1391103 - Align Maybe::mStorage like when the type is in a struct. https://reviewboard.mozilla.org/r/169356/#review175934 ::: mfbt/Alignment.h:55 (Diff revision 1) > + * > + * Known examples are 64bit types (uint64_t, double) on 32bit Linux, > + * where they have 8byte alignment on their own, and 4byte alignment > + * when in struct. > + */ > +#define MOZ_ALIGNAS(T) alignas(mozilla::detail::AlignasHelper<T>) I think dbaron's suggestion of `MOZ_ALIGNAS_IN_STRUCT` is a better name for this macro, for the reasons he cites.

Comment on attachment 8898050 [details] Bug 1391103 - Align Maybe::mStorage like when the type is in a struct. https://reviewboard.mozilla.org/r/169356/#review175938 r=me with the below change.

Pushed by xquan@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/06d2f579a7f3 Align Maybe::mStorage like when the type is in a struct. r=froydnj