Issue #935: Mutual recursion of pausing functions across two files of a project fails in rustc (“Recursion in an async fn requires boxing”)

Open and answer this issue on GitHub.

ready · opened 2026-10-29 · updated 2026-10-29 · 1 comment

Found while changing the converter (crates/rs2nika/convert): todo in tr_core.nika started calling expr in tr_expr.nika, which already called todo. rustc then refused the lowered code. Once todo was moved into tr_expr.nika, the same code compiled.

Probe (a project; nikaia run):

src/main.nika

use std::fs

fn down(n: i64) -> i64 {
    if n == 99 {
        let s = fs::read_to_string("x", fs::Root::Anywhere) catch { "" }
        return s.len()
    }
    if n <= 0 {
        return 0
    }
    return up(n - 1) + 1
}

fn main() {
    println(f"{down(5)}")
}

src/other.nika

fn up(n: i64) -> i64 {
    if n <= 0 {
        return 0
    }
    return down(n - 1) + 1
}

What happens:

error: Recursion in an async fn requires boxing.
  --> src/main.nika:3:1

What should happen: it prints 5.

The variations narrow it down:

• Same code, one file: prints 5.
• Two files, no pausing call (no fs::read_to_string): prints 5.
• Methods (impl T split over the two files): fails the same way as free functions.

So the lowering finds the recursion cycle of pausing functions, and boxes their futures, only within one file. A cycle that crosses files is missed.

To build

• Find the recursion cycles of pausing functions over the whole project (every file’s call graph), not per file, and box the futures on every cycle that crosses files as it does within one.
• Add a test in tests/compiler/ with the two-file probe above, printing 5.

Comments

keywan-ghadami, 2026-10-29

Triage (#982), class B: waits for the rest of emit/mod.rs (E11 / E13, with_contracts’ analyses). emit/mod.rs:826 computes pausing_reach from one file’s items. Still reproduces at de0bb82.

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