One can put repr(align) into repr(packed) with generics

[RalfJung]

Usually, one cannot put repr(align) fields into repr(packed) structs:

#[repr(align(8))]
struct Align8(u32);

#[repr(packed)]
struct Packed(Align8); // error

However, this check can be circumvented using generics:

#[repr(align(8))]
pub struct Align8(u32);

#[repr(packed)]
pub struct Packed<T>(T);

pub type PackedAlign8 = Packed<Align8>;

pub fn foo(_x: PackedAlign8) {}

I don't know why this restriction exists in the first place, but I think we have one of two situations here:

• The restriction is truly needed, as parts of the compiler rely on it, and we have a problem here.
• The restriction is not really needed, and we could relax this check.

Cc @eddyb @nagisa

[RalfJung]

This is the exact same problem we had with repr(transparent) where we also had a check that could be circumvented with generics... we should probably do a proper audit of all these checks as that seems to be a weak spot.

Cc @nikomatsakis

[ghost]

Is this a duplicate of #80926?

[RalfJung]

It is, good point, thanks!