Ensure the neon vector aggregates like float32x4x4_t are #[repr(C)]

[thomcc]

These are C structs from arm_neon.h, so having them be #[repr(Rust)] is wrong. Also they are used by certain intrinsics, and I suspect if they had a weird layout, it would be very broken.

Moreover, it seems reasonable for someone to want to do things like transmute a float32x4x4_t into a [float32x4_t; 4] and such, which requires #[repr(C)].

[rust-highfive]

r? @Amanieu

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[infinity0]

@thomcc could this be related to rust-lang/rust#96983 ?

[thomcc]

That certainly wouldn't make sense to me. In practice under default flags this doesn't change anything -- they already had the same layout that C would use, I think. But I don't know, I guess it's possible.

TBH, I'd be somewhat opposed to reverting this change either way, as it would defeat the point of having these types.

[infinity0]

That bug is for 1.59 which predates this PR, so I was wondering if backporting this PR to 1.59 would fix this issue for us in Debian. However this doesn't match your explanation either, so I'll have to continue investigating. Anyway, thanks for the comment.

[thomcc]

Ah, hm, yeah, I don't think it would do that, but could be mistaken.