#[non_exhaustive] affects whether code has UB or not, according to Miri #147722
Description
bar/src/lib.rs:
#[non_exhaustive] #[repr(u8)] pub enum Thing { One(u8) = 0, }src/main.rs:
use std::mem::MaybeUninit; use bar::Thing; fn main() { let buffer: [MaybeUninit<u8>; 2] = [MaybeUninit::uninit(), MaybeUninit::new(0u8)]; let ptr: *const Thing = (&raw const buffer).cast(); unsafe { match *ptr { Thing::One(ref _val) => {} _ => {} } } }Running the above code in Miri detects UB as follows:
error: Undefined Behavior: reading memory at alloc168[0x0..0x1], but memory is uninitialized at [0x0..0x1], and this operation requires initialized memory --> src/main.rs:9:15 | 9 | match *ptr { | ^^^^ Undefined Behavior occurred here | = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information = note: BACKTRACE: = note: inside `main` at src/main.rs:9:15: 9:19 Uninitialized memory occurred at alloc168[0x0..0x1], in this allocation: alloc168 (stack variable, size: 2, align: 1) { __ 00 │ ░. } note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace error: aborting due to 1 previous error However, if I delete the #[non_exhaustive] line, Miri does not detect any UB.
This seems incorrect. I believe that #[non_exhaustive] is supposed to only affect whether code compiles. It should not affect how code behaves.
It seems that, without #[non_exhaustive], rust does not even read the discriminant. But with #[non_exhaustive], rust does.
Related to:
matchon uninhabited type does not trigger UB in Miri #142394- Adding generics affect whether code has UB or not, according to Miri #146803
- Uninhabited types have strange borrow-checking behavior, even behind references #146590
- Make closure capturing have consistent and correct behaviour around patterns #138961
- Document how closure capturing interacts with discriminant reads reference#1837
Meta
rustc --version --verbose:
rustc 1.92.0-nightly (844264add 2025-10-14) binary: rustc commit-hash: 844264adda6f41ca6d0d61c4bcac0f263fc5072f commit-date: 2025-10-14 host: aarch64-apple-darwin release: 1.92.0-nightly LLVM version: 21.1.3 cargo miri --version:
miri 0.1.0 (844264adda 2025-10-14)