Regression rust ICE involving const generic expressions and rayon.

[GuillaumeDIDIER]

I got a regression building some of my PhD tooling.

The bug has been reproduced on a rust playground, cf #150162 (comment) (it could perhaps still be minimised further to remove the rayon dependency)

I do note that I use generic_const_exprs, which is known for its instability.

I have identified that the bug doesn't occur on nightly-2025-10-10, I'll bisect the start of the issue.

Code

See #150162 (comment)

https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=05f0c7faa14b351d2ca03482a32904da

Meta

rustc --version --verbose:

rustc 1.94.0-nightly (fcf67da03 2025-12-18)
binary: rustc
commit-hash: fcf67da039f42e3905cf6f69e33304299c45149f
commit-date: 2025-12-18
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Error output

rustc-ice-2025-12-19T15_02_28-2676931.txt

[GuillaumeDIDIER]

I have narrowed down the regression to :
nightly-2025-11-02: WORKS
nightly-2025-11-03: FAILS
rustc-ice-2025-12-19T15_20_23-2880123.txt
Attached the ICE report from the earliest failing version too.

(And a rusty ICE, courtesy of Deutsch Bahn too)

[lqd]

You can use https://github.com/rust-lang/cargo-bisect-rustc to locate the PR causing the regression. Also, as you said, we’d need the full code that reproduces the issue, thanks!

[GuillaumeDIDIER]

Bisection Done

searched toolchains bd3ac0330018c23b111bbee176f32c377be7b319 through b15a874aafe7eab9ea3ac2c1d59c7b03e1425027


********************************************************************************
Regression in 8483293b3b4eb209e8e8bd0d069e61de790018a8
********************************************************************************

Attempting to search unrolled perf builds
Found commits ["0defc2de", "85faa626", "0bf42788", "66cb3d9d", "44c96e88", "f01ac443", "9ebb0c3a", "ff83676e", "1feff6c2"]
installing 0defc2de2b710c10e3d1fc6c17e99803e206c745
rust-std-nightly-x86_64-unknown-linux-gnu: 31.72 MB / 31.72 MB [===============================================================================================] 100.00 % 11.16 MB/s testing...
RESULT: 0defc2de2b710c10e3d1fc6c17e99803e206c745, ===> Script returned success
uninstalling 0defc2de2b710c10e3d1fc6c17e99803e206c745

installing 85faa6269b753bfbfff78fa584c0b2272d941c8b
rust-std-nightly-x86_64-unknown-linux-gnu: 31.75 MB / 31.75 MB [===============================================================================================] 100.00 % 12.02 MB/s testing...
RESULT: 85faa6269b753bfbfff78fa584c0b2272d941c8b, ===> Script returned success
uninstalling 85faa6269b753bfbfff78fa584c0b2272d941c8b

installing 0bf427888c1d1c553c02fe65f147543c9ef7929d
rust-std-nightly-x86_64-unknown-linux-gnu: 31.77 MB / 31.77 MB [================================================================================================] 100.00 % 9.31 MB/s testing...
RESULT: 0bf427888c1d1c553c02fe65f147543c9ef7929d, ===> Script returned error
uninstalling 0bf427888c1d1c553c02fe65f147543c9ef7929d

Regression in https://github.com/rust-lang/rust/commit/0bf427888c1d1c553c02fe65f147543c9ef7929d. Note that if it is a legacy rollup build, it might be available in https://github.com/rust-lang-ci/rust/commit/0bf427888c1d1c553c02fe65f147543c9ef7929d.
The PR introducing the regression in this rollup is #147642: Miscellaneous const-generics-related fixes
==================================================================================
= Please file this regression report on the rust-lang/rust GitHub repository     =
=        New issue: https://github.com/rust-lang/rust/issues/new                 =
=     Known issues: https://github.com/rust-lang/rust/issues                     =
= Copy and paste the text below into the issue report thread.  Thanks!           =
==================================================================================

searched nightlies: from nightly-2025-10-31 to nightly-2025-11-10
regressed nightly: nightly-2025-11-03
searched commit range: bd3ac03...b15a874
regressed commit: 8483293

(I'll no start looking into creating a minimal example).

bisected with cargo-bisect-rustc v0.6.11

[GuillaumeDIDIER]

I have reduced the regression down to a manageable rust file + Cargo.toml. (60 lines) https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=05f0c7faa14b351d2ca03482a32904da

[package]
name = "calibration_analysis"
version = "0.1.0"
edition = "2024"

[dependencies]
rayon = { version = "1.10.0" }

#![feature(generic_const_exprs)]

use std::default::Default;
use std::vec::Vec;
use std::collections::HashMap;
use rayon::prelude::*;
use core::fmt::Debug;

#[derive(Debug, Clone, PartialEq)]
struct StaticHistogram<const WIDTH: u64, const N: usize> {
    data: [u32; N],
}

impl<const WIDTH: u64, const N: usize> Default for StaticHistogram<WIDTH, N> {
    fn default() -> Self {
        Self { data: [0; N] }
    }
}

#[derive(Debug, PartialEq)]
struct CalibrateResult2TNuma<const WIDTH: u64, const N: usize> {
    pub res: Vec<()>, // Formerly another type with const generic parameters <const WIDTH: u64, const N: usize>.
}

fn calibration_result_to_location_map_parallel<
    const WIDTH: u64,
    const N: usize,
    T: Send,
    Analysis: Send + Sync + Fn() -> T,
>(
    results: Vec<CalibrateResult2TNuma<WIDTH, N>>,
    analysis: &Analysis,
) -> HashMap<u8, T> {
    let result = results
        .into_par_iter()
        .flat_map(|calibrate_2t_result| {
            let analysed = analysis();
            vec![(0u8, analysed)]
        })
        .collect();
    result
}

pub fn run_analysis_from_file<const WIDTH: u64, const N: usize>() -> Result<(), ()>
where
    [(); { N + N } as usize]:,
    [(); { WIDTH + WIDTH } as usize]:,
{

    let results : Vec<CalibrateResult2TNuma<WIDTH, { N + N }>> = vec![];

    let _location_map = calibration_result_to_location_map_parallel(
        results,
        &|| {
            let result = StaticHistogram::<WIDTH, { N + N }>::default();
            result
        },
    );

    Ok(())
}

This causes an ICE when building with `cargo +nightly build --release```

[GuillaumeDIDIER]

searched nightlies: from nightly-2025-11-01 to nightly-2025-12-20
regressed nightly: nightly-2025-11-03
searched commit range: bd3ac03...b15a874
regressed commit: 8483293

bisected with cargo-bisect-rustc v0.6.11

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2025-11-01 --script ./test.sh 

Attaching the regression commit and the latest nightly ICE reports on this specific test, and the test script.

test.sh

rustc-ice-2025-12-20T14_11_38-2631197.txt

rustc-ice-2025-12-20T14_13_08-2631947.txt

[GuillaumeDIDIER]

@lqd @cyrgani the Need Repro label can go away now, right ?

[lqd]

Sure. You can also easily remove rayon.