New enum discrimintant check doesn't work for negative numbers

[gwilymk]

I tried this code:

#[derive(Debug, PartialEq, Eq)]
enum Foo {
    Negative = -1,
    Positive = 1,
}

#[test]
fn t() {
    let transmuted: Foo = unsafe {
        std::mem::transmute::<i8, Foo>(-1)
    };
    
    assert_eq!(transmuted, Foo::Negative);
}

I expected to see this happen: The test passes

Instead, this happened:

thread 't' panicked at src/lib.rs:10:9:
trying to construct an enum from an invalid value 0xffffffffffffffffffffffffffffffff
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.

Meta

rustc --version --verbose:

rustc 1.90.0-nightly (11ad40bb8 2025-06-28)
binary: rustc
commit-hash: 11ad40bb839ca16f74784b4ab72596ad85587298
commit-date: 2025-06-28
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

Playground link: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=c5012dd5ec0afc1bea52e85e81ff28e9

Seems to be related to #141759

[ProgramCrafter]

If so, it must be related to https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir_transform/src/check_enums.rs#L111.

[QuineDot]

This comment implies the values are not sign extended in Discr<'_>, but the check seems to be doing an int-to-int cast.

[saethlin]

cc @1c3t3a

[1c3t3a]

Good catch! I know what's the issue and I'll take a look!

[1c3t3a]

@rustbot assign @1c3t3a