same `X`, both `X == 0` and `X != 0` are triggered

[loynoir]

I tried this code:

    if status == napi_ok {
        eprintln!("[reproduce] status == napi_ok, should not error");
    } else {
        eprintln!("[reproduce] status != napi_ok, should be error");
    }

    if status != napi_ok {
        unsafe {
            napi_throw_error(env, null_mut(), c"[reproduce] Error: status != napi_ok".as_ptr());
        }
        return null_mut();
    }

    let result = MaybeUninit::<napi_value>::uninit();

I expected to see this happen: explanation

X == 0 and X != 0 can not be both triggered

=== Calling debug reproduce ===
[reproduce] status == napi_ok, should not error

=== Calling release workaround ===
[workaround] status == napi_ok, should not error

Instead, this happened: explanation

X == 0 and X != 0 can be both triggered

=== Calling release reproduce ===
[reproduce] status == napi_ok, should not error
Error: [reproduce] Error: status != napi_ok
    at Object.<anonymous> (/workspaces/loynoir/repo/reproduce-rust-145163/reproduce.js:9:15)
    at Module._compile (node:internal/modules/cjs/loader:1738:14)
    at Object..js (node:internal/modules/cjs/loader:1871:10)
    at Module.load (node:internal/modules/cjs/loader:1470:32)
    at Module._load (node:internal/modules/cjs/loader:1290:12)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:238:24)
    at Module.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:154:5)
    at node:internal/main/run_main_module:33:47

workaround

let result = MaybeUninit::<napi_value>::uninit();

let result = MaybeUninit::<[napi_value; 1]>::uninit();

Meta

rustc --version --verbose:

$ rustc --version --verbose
rustc 1.91.0-nightly (ec7c02612 2025-08-05)
binary: rustc
commit-hash: ec7c02612527d185c379900b613311bc1dcbf7dc
commit-date: 2025-08-05
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 20.1.8

Backtrace

<backtrace>

reproduce

https://github.com/loynoir/reproduce-rust-145163

[fmease]

Unless this is some miscompilation, this is very likely happening due to Undefined Behavior in your code. Have you already run Miri on it?

[loynoir]

Code updated

workaround

let result = MaybeUninit::<napi_value>::uninit();

let result = MaybeUninit::<[napi_value; 1]>::uninit();

[loynoir]

Hi @fmease

I am first time using miri.

Correct me if I'm wrong.

I run

# note: workaround miri tmp exe within scratch devcontainer
# could not execute process `/tmp/.tmpXXX/target/custom_sysroot/build/std-XXX/build-script-build`
# sudo mount -o remount exec /tmp

cargo miri setup
cargo miri test

cargo build
cargo build -r

• Nothing is reported.

• Nothing is changed.

[theemathas]

Your code has undefined behavior. This is not a rust bug. For an explanation, please read https://www.ralfj.de/blog/2019/07/14/uninit.html

[fmease]

Oh, yeah:

let args = MaybeUninit::<[napi_value; 2]>::uninit();
let mut args = unsafe { args.assume_init() };

[loynoir]

Oh, yeah:

let args = MaybeUninit::<[napi_value; 2]>::uninit();
let mut args = unsafe { args.assume_init() };

So, what is the recommened rust way, allocate memory, no need for zero, before calling C api get output value?

[theemathas]

let args = MaybeUninit::<[napi_value; 2]>::uninit();};
let status = unsafe { napi_get_cb_info(env, info, argc.as_mut_ptr(), args.as_mut_ptr().cast::<napi_value>(), null_mut(), null_mut()) };

[loynoir]

And what is the general way to lint UB without open an issue?

cargo miri test

miri report nothing.

And I cannot find this kind maybe uninit UB document location.

All I see is

MaybeUninit::uninit().assume_init()

Above is UB.

[theemathas]

What cargo miri test does is that it effectively runs cargo test, but with UB checks. Your code doesn't have tests, so this does nothing.

I don't know how to run Miri with your setup though. But Miri currently doesn't support FFI anyway, so Miri is probably not useful for your use case, unfortunately.

A list of things that are undefined behavior can be found in the reference. In particular, your program produced an invalid value.