MIR const-prop not putting `usize` into `AggregateKind::RawPtr`?

[scottmcm]

Discord Context: https://discord.com/channels/273534239310479360/273541522815713281/1236936970912137236

I was surprised in https://rust.godbolt.org/z/hnMT6K4qh to see

        _18 = const 128_usize;
        StorageLive(_24);
        _24 = _22 as *const () (PtrToPtr);
        _23 = *const [u8] from (_24, _18);

Because I would have expected constprop into the Aggregate, like

        StorageLive(_24);
        _24 = _22 as *const () (PtrToPtr);
        _23 = *const [u8] from (_24, const 128_usize);

As happened in other places, like

        _20 = Offset(_16, const 128_usize);
        StorageLive(_21);
        _21 = Sub(_13, const 128_usize);

AggregateKind::RawPtr is relatively new (#123840), so maybe it needs to be handled properly somewhere else?

[saethlin]

I think the problem is this FIXME that you added:

rust/compiler/rustc_mir_transform/src/gvn.rs

Lines 930 to 931 in 8b64adc

	// FIXME: Do the extra work to GVN `from_raw_parts`
	AggregateKind::RawPtr(..) => return None,

Constant propagation is done by GVN.

[scottmcm]

Foiled once again by my greatest nemesis: my past self.

Thanks, @saethlin