Potential unsound casting from volatile_register::RW to the wrapped type

[shinmao]

Hi, I am wondering whether the casting here is unsound:

cortex-m/src/peripheral/scb.rs

Lines 172 to 174 in 1746a63

	pub fn vect_active() -> VectActive {
	let icsr =
	unsafe { ptr::read_volatile(&(*SCB::PTR).icsr as *const _ as *const u32) } & 0x1FF;

Based on the definition, the wrapper struct volatile_register::RW (let me abbreviate it as vr here) didn't have stable layout, and rustc would preserve the right to insert padding or reorder the structure. vr<u32> should be declared as transparent; otherwise, casting it to u32 could lead to UB such as uninitialized memory exposure to ptr::read_volatile.

[adamgreig]

Yep, you're right. In practice this is very unlikely to lead to any issues but in principle volatile_register should add a repr to the structs. There's an open PR to do so here: rust-embedded/volatile-register#9 but it's not merged yet.

[Dirbaio]

volatile-register v0.2.2 is now out, it adds #[repr(transparent)] so the pointer cast is now sound.