Triaging Crater Runs

Running crater

We regularly run Crater runs, and this documents the procedure for triaging a beta run; it may also be applicable to non-release team runs (e.g., PR crater runs) with minor modifications.

First, file a new issue titled "Crater runs for 1.x" (example)

A crater run for beta should be started as soon as we have beta out. Use the following craterbot invocations.

$BETA_VERSION is e.g. 1.40.0-1, increment the 1 if it's not the first beta crater run, you can also use the auto-incremented counter on the beta rustc --version.

$STABLE is e.g. 1.39.0 (the stable release) $BETA is beta-YYYY-MM-DD, get the date by looking at and get the date of the most recent channel-rust-beta.toml.

@craterbot run name=beta-$BETA_VERSION start=$STABLE end=$BETA mode=build-and-test cap-lints=warn p=10
@craterbot run name=beta-rustdoc-$BETA_VERSION start=$STABLE end=$BETA mode=rustdoc cap-lints=warn p=5

Once the runs complete, you want to triage them


These steps should generally be done for the normal rustc run, and then followed up by a triage of the rustdoc run. Ignore failures in rustdoc that look to be rooted in rustc (i.e., duplicate failures).

There will usually be quite a few regressions -- there are a couple tools that can help reduce the amount of work that you need to do. It's mostly a matter of personal preference which is more helpful.

    • This groups regressions by 'root' by parsing the logs to look for the compilation failed messages printed by Cargo
    • This groups regressions by the "error" message, also by parsing logs

If you've written a tool, feel free to add it here! We're still figuring out what the best UI for this is.

Regardless of the tool you've run, you ultimately need to read through a bunch of logs and try to quickly determine if they're genuine failures or spurious. Most of the time, a compiler failure is genuine, and test failures are mostly spurious, but this usually requires some level of guessing.

Once you've determined that something is a genuine failure, add it to a list somewhere (local file, HackMD, whatever) with the error "category." Mostly, you're trying to group things such that the regressions in a single group are all caused by the same set of commits, and different groups have different causes.

Once this is done, and you have all the regressions triaged into their separate groups, you want to file a new issue for each group. It should have the regression-from-stable-to-beta and T-compiler label by default, possibly T-libs if it's a standard library regression, but that's relatively rare. If you happen to think you know the PR that caused the failure, cc the PR author in a separate comment and link to the PR; otherwise compiler team will triage the issue soon.

Leave a comment on the original issue with the crater runs linking to all the issues you just opened, ideally with the issue titles as well.

You're done!

Re-running rustc on a crate

For the crates which we're not sure about, you can try running crater locally, or build the crate directly (cratesio-curl can be helpful). Be careful -- regardless of what you do, you are running arbitrary code locally. It's also fine to file issues for the crates you're not sure about and let the triage process naturally categorize the error, though it's not good to do this for all the crates. Once you've triaged a crater run a couple times you get a pretty good sense of what is spurious and what isn't, too.

You can run crater on just a single crate by doing something like this (at least, as of now). Note that this will download several gigabytes (on first use) and requires Docker to be running.

git clone
cd crater
cargo run -- prepare-local
CRATES="crates-io-crate-0.4.0,owner/repository-name" # Edit this.
cargo run -- define-ex --crate-select=list:$CRATES --cap-lints=forbid 1.38.0 beta # Edit the stable version.
cargo run -- run-graph --threads 4
cargo run -- gen-report work/ex/default/
# view report for this crate

It's also possible to re-queue a subset of crates onto the official builders, for which that take a look at:

Determining the root cause of the regression

It's not always apparent why a crate stopped building. This isn't generally something done as part of crater triage -- but can be a good followup. Here, cargo-bisect-rustc and Felix's minimization guide are excellent tools to apply.