Issue #1068: Bogus “Unused import: ProbePlain” warning on spawn fn { f(s) } where f throws and calls process::run

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compiler, ready · opened 2026-10-31 · updated 2026-10-31 · 0 comments

Problem. A bogus warning Unused import: ProbePlain (a name the program never wrote) is printed for spawn fn { work(s) } where work throws and calls process::run. The program runs correctly. Found by the supervisor’s script builders (2026-10-31). spawn is to be removed by #991 (async, ADR-async-1..4); if #991 lands first, check whether async fn { work(s) } shows the same warning, and close this if not.

Probe (0.0.1322):

use std::process

fn work(s: String) -> bool throws {
    let out = process::run("true", [s])
    return out.code == 0
}

fn main() throws {
    let s = f"a{1}"
    let h = spawn fn { work(s) }
    println(f"{h.join()}")
}

nikaia run prints warning: Unused import: ProbePlain. at line 10, then true. Expected: no warning.

Suspected location (low confidence): the lowering of a spawn lambda emits a use ...::ProbePlain (the throws-probe of a task’s error) that the lambda’s body does not need, and the warning from rustc is mapped back to the source; the import should be written only where it is used.

To build

1. No warning for the probe. Test tests/compiler/tests/spawn_throwing_process_no_warning.nika + .stdout (true), and the .stderr empty.

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