rust/CONTRIBUTING.md

Contribution Guide

If you want to hack on miri yourself, great! Here are some resources you might find useful.

Getting started

Check out the issues on this GitHub repository for some ideas. There's lots that needs to be done that we haven't documented in the issues yet, however. For more ideas or help with hacking on Miri, you can contact us (oli-obk and RalfJ) on the Rust Zulip.

Fixing Miri when rustc changes

Miri is heavily tied to rustc internals, so it is very common that rustc changes break Miri. Fixing those is a good way to get starting working on Miri. Usually, Miri will require changes similar to the other consumers of the changed rustc API, so reading the rustc PR diff is a good way to get an idea for what is needed.

When submitting a PR against Miri after fixing it for rustc changes, make sure you update the rust-version file. That file always contains the exact rustc git commit with which Miri works, and it is the version that our CI tests Miri against.

Building Miri with a nightly rustc

Miri heavily relies on internal rustc interfaces to execute MIR. Still, some things (like adding support for a new intrinsic or a shim for an external function being called) can be done by working just on the Miri side.

To prepare, make sure you are using a nightly Rust compiler. Then you should be able to just cargo build Miri.

In case this fails, your nightly might be incompatible with Miri master. The rust-version file contains the commit hash of rustc that Miri is currently tested against; you can use that to find a nightly that works or you might have to wait for the next nightly to get released. You can also use rustup-toolchain-install-master to install that exact version of rustc as a toolchain:

rustup-toolchain-install-master $(cat rust-version) -c rust-src

Another common problem is outdated dependencies: Miri does not come with a lockfile (it cannot, due to how it gets embedded into the rustc build). So you have to run cargo update every now and then yourself to make sure you are using the latest versions of everything (which is what gets tested on CI).

Testing the Miri driver

The Miri driver in the miri binary is the "heart" of Miri: it is basically a version of rustc that, instead of compiling your code, runs it. It accepts all the same flags as rustc (though the ones only affecting code generation and linking obviously will have no effect) [and more][miri-flags].

Running the Miri driver requires some fiddling with environment variables, so the miri script helps you do that. For example, you can run the driver on a particular file by doing

./miri run tests/run-pass/format.rs
./miri run tests/run-pass/hello.rs --target i686-unknown-linux-gnu

and you can run the test suite using:

./miri test

./miri test FILTER only runs those tests that contain FILTER in their filename (including the base directory, e.g. ./miri test fail will run all compile-fail tests).

You can get a trace of which MIR statements are being executed by setting the MIRI_LOG environment variable. For example:

MIRI_LOG=info ./miri run tests/run-pass/vecs.rs

Setting MIRI_LOG like this will configure logging for Miri itself as well as the rustc::mir::interpret and rustc_mir::interpret modules in rustc. You can also do more targeted configuration, e.g. the following helps debug the stacked borrows implementation:

MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs

In addition, you can set MIRI_BACKTRACE=1 to get a backtrace of where an evaluation error was originally raised.

Testing cargo miri

Working with the driver directly gives you full control, but you also lose all the convenience provided by cargo. Once your test case depends on a crate, it is probably easier to test it with the cargo wrapper. You can install your development version of Miri using

./miri install

and then you can use it as if it was installed by rustup. Make sure you use the same toolchain when calling cargo miri that you used when installing Miri!

There's a test for the cargo wrapper in the test-cargo-miri directory; run ./run-test.py in there to execute it.

Building Miri with a locally built rustc

A big part of the Miri driver lives in rustc, so working on Miri will sometimes require using a locally built rustc. The bug you want to fix may actually be on the rustc side, or you just need to get more detailed trace of the execution than what is possible with release builds -- in both cases, you should develop miri against a rustc you compiled yourself, with debug assertions (and hence tracing) enabled.

The setup for a local rustc works as follows:

git clone https://github.com/rust-lang/rust/ rustc
cd rustc
cp config.toml.example config.toml
# Now edit `config.toml` and set `debug-assertions = true`.
# This step can take 30 minutes and more.
./x.py build src/rustc
# If you change something, you can get a faster rebuild by doing
./x.py --keep-stage 0 build src/rustc
# You may have to change the architecture in the next command
rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
# Now cd to your Miri directory, then configure rustup
rustup override set custom

With this, you should now have a working development setup! See above for how to proceed working with the Miri driver.