Basic usage

Using cargo-bisect-rustc simply involves running it inside a Cargo project that reproduces the regression:

cargo bisect-rustc

For a quick introduction, see the Tutorial.

Note: On Windows, due to an issue with rustup, you will need to execute cargo-bisect-rustc with a - between cargo and bisect.

cargo-bisect-rustc works by building a Cargo project, and detecting if it succeeds or fails. It will download and use nightly Rust toolchains. It begins with two nightly boundaries, known as the start where the project successfully builds (the baseline), and the end where it is known to fail (the regression). It will then do a binary search between those dates to find the nightly where the project started to fail.

Once it finds the nightly where it started to fail, cargo-bisect-rustc will then try to find the individual PR where it regressed. The Rust project keeps the builds of every merged PR for the last 167 days. If the nightly is within that range, then it will bisect between those PRs.

And even further, if the regression is in a rollup PR, then it will bisect the individual PRs within the rollup. This final bisection is only available for x86_64-unknown-linux-gnu since it is using the builds made for the rustc performance tracker.

Rust src repo

cargo-bisect-rustc needs to read the git log of the rust-lang/rust repo in order to scan individual commits. See the Rust src repo chapter for details on how to configure how it finds the git repo.


Without setting any options, cargo-bisect-rustc will try to automatically find the start where the build succeeds and the end where it fails. This can take some time, depending on how far back it needs to scan. It is recommended to use the --start and --end CLI options to tell it where the boundaries are.

cargo bisect-rustc --start=2022-11-01 --end=2023-02-14

See the Bisection boundaries chapter for more details on setting these options.

Regression check

By default, cargo-bisect-rustc assumes the start boundary successfully builds, and the end boundary fails to build. You can change this using the --regress CLI option. For example, you can tell it that the start should fail, and the end should pass. There are several options you can use with the --regress flag:

errorSucceedFailThe default setting checks for a failure as the regression.
successFailSucceedReverses the check to find when something is fixed.
iceNo ICEICEScans when an Internal Compiler Error (ICE) was introduced.
non-iceICENo ICEScans when an ICE was fixed.
non-errorNon-ICE FailureSucceed or ICEScans when an ill-formed program stops being properly rejected, or the compiler starts generating an ICE.

See Scripting for customizing this behavior.

Custom commands

By default, cargo-bisect-rustc runs cargo build. You can change which cargo command is run by passing additional arguments after --:

cargo bisect rustc -- test --test mytest


You can use an arbitrary script for determining what is a baseline and regression. This is an extremely flexible option that allows you to perform any action automatically. Just pass the path to the script to the --script CLI command:

cargo bisect-rustc --script ./

The script should exit 0 for the baseline, and nonzero for a regression. Since cargo-bisect-rustc sets RUSTUP_TOOLCHAIN (see Rustup toolchains), all you need to do is call cargo or rustc, and the script should automatically use the toolchain that is currently being tested.


set -ex

# This checks that a warning is only printed once.
# See for a regression where it
# started printing twice.

OUTPUT=`cargo check 2>&1`
COUNT=`echo "$OUTPUT" | grep -c "unnecessary parentheses"`
test $COUNT -eq 1

If you need to use the targets directly without using cargo in the script, they are available in $CARGO_TARGET_DIR/[release|debug]/..., since cargo-bisect-rustc sets $CARGO_TARGET_DIR.

Check out the examples chapters for several examples of how to use this option.