Issue #1118: A local bound from a sync function field loses sync: let r = b.run; r(1) is refused with NK2202

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ready · opened 2026-11-01 · updated 2026-11-01 · 0 comments

A field typed fn(i64) -> i64 sync (Part I, the Button example) can be called directly inside a sync function, but once it is read into a local, the call through the local is refused as one that can pause. The prover builder met it while moving prove.rs (#125).

Probe (0.0.1335):

struct Holder {
    run: fn(i64) -> i64 sync,
}

fn take(b: ref Holder) -> i64 sync {
    let r = b.run
    return r(1)
}

fn main() {
    let b = Holder { run: fn(x) { x + 1 } }
    print(take(b))
}

error[NK2202]: `take` is `sync`, but it calls `r`, which can pause.
 = note: `r` isn't declared `sync` in this program.

return b.run(1) in the same function compiles and prints 2. Without sync on the field type the refusal is right (Part II 12.2: where the compiler cannot tell it does not guess); with it, the local’s type carries the promise.

To build

• A local whose type is a sync function type (read from a field, a let with that type, a result) is callable inside a sync body; the sync check reads the callee’s type, not only its declaration.
• Test in tests/compiler/: the probe prints 2; the same with the field typed without sync keeps NK2202.

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