Failed assert got 1400 instead of 14 after become tail call at -O3

-O3 passes the previous stack values instead of the tuple values passed to the tail call
I tried a patch but it's a non trivial one so I haven't submitted any PR

Repro:

#![feature(explicit_tail_calls, unboxed_closures)]
#![expect(incomplete_features)]
#![allow(dead_code)]

#[derive(Copy, Clone)]
struct Thing { a: u64, b: u64, c: u64, d: u64 }

#[inline(never)]
fn seed_stack() {
    let mut values = (
        Thing { a: 100, b: 200, c: 300, d: 400 },
        Thing { a: 500, b: 600, c: 700, d: 800 },
    );
    std::hint::black_box(&mut values);
}

#[inline(never)]
extern "rust-call" fn callee((left, right): (Thing, Thing)) -> u64 {
    left.a.wrapping_add(left.b).wrapping_add(right.a).wrapping_add(right.b)
}

#[inline(never)]
extern "rust-call" fn caller((left, right): (Thing, Thing)) -> u64 {
    become callee((right, left));
}

fn main() {
    seed_stack();
    let left = Thing { a: 1, b: 2, c: 3, d: 4 };
    let right = Thing { a: 5, b: 6, c: 7, d: 8 };
    println!("got={}", caller((left, right)));
}

$ rustc +nightly-2026-05-10 repro.rs -C opt-level=0 -o repro_o0
$ ./repro_o0
got=14
$ rustc +nightly-2026-05-10 repro.rs -C opt-level=3 -o repro_o3
$ ./repro_o3
got=1400

@dianqk

Possibly related to #155241.

cc @dianqk (author of #155343)

Slightly smaller repro:

#![feature(explicit_tail_calls, unboxed_closures)]
#![expect(incomplete_features)]

#[inline(never)]
fn seed_stack() {
    let mut values = [100_u64; 8];
    std::hint::black_box(&mut values);
}

#[inline(never)]
extern "rust-call" fn callee((value,): ([u64; 4],)) -> u64 {
    value[0]
}

#[inline(never)]
extern "rust-call" fn caller((_,): ([u64; 4],)) -> u64 {
    become callee(([5, 6, 7, 8],));
}

fn main() {
    seed_stack();
    println!("{}", caller(([1, 2, 3, 4],)));
}

Prints 5 in debug, prints 100 in release.

@kevin-valerio

@kevin-valerio is there any particular reason why you were experimenting with extern "rust-call" tail calls?

@WaffleLapkin

@kevin-valerio is there any particular reason why you were experimenting with extern "rust-call" tail calls?

@WaffleLapkin Yes! It's all described in this Zullip thread

Rollup merge of rust-lang#158374 - WaffleLapkin:no-tail-rust-call, r=…

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Failed assert got 1400 instead of 14 after `become` tail call at `-O3` #158017

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Failed assert got 1400 instead of 14 after become tail call at -O3 #158017

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theemathas commented on Jun 17, 2026

theemathas commented on Jun 17, 2026

WaffleLapkin commented on Jun 23, 2026

kevin-valerio commented on Jun 23, 2026

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Failed assert got 1400 instead of 14 after `become` tail call at `-O3` #158017