What guarantees are provided by repr(packed)?

[joshlf]

Per the nomicon, repr(packed) guarantees that all inter-field padding is removed. However, the reference merely says that "packed may also alter the padding between fields" (emphasis added).

What guarantees does repr(packed) provide? In particular:

• Does repr(packed) guarantee that all inter-field padding is removed?
• Does repr(packed) guarantee that a type's layout is fixed in any way other than removing padding (e.g., field ordering)?

[RalfJung]

Does repr(packed) guarantee that all inter-field padding is removed?

I would strongly think so, that is the entire point after all. Things become a bit more tricky to describe with packed(N) though. Still I think this can be just fixed via a PR against the reference.

As for field ordering, I don't know if it has any effect.

[Diggsey]

#[repr(packed)] means the struct won't add any padding as part of its own layout, but there might still effectively be padding between fields, eg. if the type of a field alignment=8 but only actually uses 4 bytes to store data, then its my understanding that there would still be 4 bytes of padding because that padding belongs to the field itself, and is not added by the struct.

[RalfJung]

Yes, the padding contained inside field types (including trailing padding to make the size a multiple of the alignment) is unaffected.

[joshlf]

Submitted: rust-lang/reference#1163