miscompilation with incremental + unpacked debug + macos

[ehuss]

The following script illustrates a miscompilation on macOS when using incremental and unpacked debug:

I tried this code:

#!/bin/bash 

rm -rf *.o incremental foo

echo "fn main() { let a: i64 = 1 << 64; }" > foo1.rs
echo "fn main() { let a: i64 = 1 << 63; }" > foo2.rs

ARGS="--crate-name foo -C split-debuginfo=unpacked -C debuginfo=2 -C incremental=incremental"

for i in {1..20}
do
    rustc foo1.rs $ARGS && { echo "ERROR: first build should have failed"; exit 1; }
    rustc foo2.rs $ARGS || { echo "ERROR: second build should have passed"; exit 1; }
    ./foo || { echo "ERROR: executing should have passed"; exit 1; }
done

The first compilation should fail due to the overflow error:

error: this arithmetic operation will overflow
 --> src/main.rs:1:26
  |
1 | fn main() { let a: i64 = 1 << 64; }
  |                          ^^^^^^^ attempt to shift left by `64_i32`, which would overflow
  |
  = note: `#[deny(arithmetic_overflow)]` on by default

The second compilation should succeed since the overflow has been removed.
Then executing the binary should pass, since there is no overflow.

Instead, this happened:

Sometimes executing the binary will fail, as-if it compiled with the incorrect constant value:

thread 'main' panicked at 'attempt to shift left with overflow', foo1.rs:1:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
ERROR: executing should have passed

This doesn't happen always, although it usually happens right away (thus the reason for the loop in the script).

Testing on macOS 13.2 aarch64 and macOS 12.6.3 x86_64 with either Xcode 14 or 13.

I couldn't get it to fail on Linux.

Bisection

This appears to be a regression introduced in #87337. Presumably because before that it wouldn't write any .o files (or write anything to the incremental db) due to the error. Strangely, I cannot get it to fail with #![allow(arithmetic_overflow)], and I don't really have a theory as to why.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[ldm0]

This can be reproduced easily by rustc with toolchain: nightly-2023-02-08-aarch64-apple-darwin.
But cannot be reproduced by rustc with toolchain: nightly-2023-07-03-aarch64-apple-darwin.

environment: macOS 13.4.1 + Xcode 14.3

[lqd]

I'll try to reproduce the issue as well.