[ICE]: operand.rs:114:32: OperandValue cannot be a pointer: Uninit

[tmiasko]

Code

use std::mem::MaybeUninit;

fn main() {
    let a: Box<[u8; 0]> = unsafe { MaybeUninit::uninit().assume_init() };
    std::hint::black_box(a as Box<[u8]>);
}

$ rustc z.rs -O
...
error: internal compiler error: /rustc-dev/89c61a7545da48b06116675b888398d02a4064c7/compiler/rustc_codegen_ssa/src/mir/operand.rs:114:32: OperandValue cannot be a pointer: Uninit

An additional code path from #157797 (cc @glandium, @oli-obk) that was assumed to be unreachable.

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (89c61a754 2026-07-23)
binary: rustc
commit-hash: 89c61a7545da48b06116675b888398d02a4064c7
commit-date: 2026-07-23
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

[glandium]

@oli-obk

This is the second ICE from OperandValue::Uninit, after #159815 (fix in #159825), plus tmiasko's case (#157797 (comment)) where an uninit intrinsic argument gets dropped and we emit invalid bitcode. Rather than keep patching holes hoping there aren't any more left, should we go back to the original targeted version, 18d4354ad0fb2b923a0016968a01a3cd412618da?

[oli-obk]

There aren't that many cases in the original PR where it was marked as unreachable. I think whackamole will get us to an exhaustively tested impl quickly

[scottmcm]

I'd be tempted to go back. Losing the "It's BackendRepr::Scalar so it's always OperandValue::Immediate" property is pretty impactful -- anything that uses the backend repr to look at things is now potentially wrong.

[glandium]

I went ahead and pointed Claude code at the whack-a-mole. The result is neither enticing nor nice. It found 30 distinct reachable ICEs, plus @tmiasko's invalid-bitcode case, plus two cases of a silently dropped volatile store. Fixing all those that it identified is +165/-54 over 9 files across 3 backends, and considering how each time I made it try again, it found new ones, I can't even be sure this is the end of it. So I would tend to agree with scottmcm, and I think going back to the original targeted fix would be better.

[oli-obk]

Okidoki. Let's do that