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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. In particular, look for the green E-* labels which mark issues that should be rather well-suited for onboarding. For more ideas or help with hacking on Miri, you can contact us (oli-obk and RalfJ) on the Rust Zulip.

Preparing the build environment

Miri heavily relies on internal and unstable rustc interfaces to execute MIR, which means it is important that you install a version of rustc that Miri actually works with.

The rust-version file contains the commit hash of rustc that Miri is currently tested against. Other versions will likely not work. After installing rustup-toolchain-install-master, you can run the following command to install that exact version of rustc as a toolchain:

./rustup-toolchain

This will set up a rustup toolchain called miri and set it as an override for the current directory.

You can also create a .auto-everything file (contents don't matter, can be empty), which will cause any ./miri command to automatically call rustup-toolchain, clippy and rustfmt for you. If you don't want all of these to happen, you can add individual .auto-toolchain, .auto-clippy and .auto-fmt files respectively.

Building and testing Miri

Invoking Miri requires getting a bunch of flags right and setting up a custom sysroot with xargo. The miri script takes care of that for you. With the build environment prepared, compiling Miri is just one command away:

./miri build

Run ./miri without arguments to see the other commands our build tool supports.

Testing the Miri driver

The Miri driver compiled from src/bin/miri.rs 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.

For example, you can (cross-)run the driver on a particular file by doing

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

and you can (cross-)run the entire test suite using:

./miri test
MIRI_TEST_TARGET=i686-unknown-linux-gnu ./miri test

If your target doesn't support libstd, you can run miri with

MIRI_NO_STD=1 MIRI_TEST_TARGET=thumbv7em-none-eabihf ./miri test tests/fail/alloc/no_global_allocator.rs
MIRI_NO_STD=1 ./miri run tests/pass/no_std.rs --target thumbv7em-none-eabihf

to avoid attempting (and failing) to build libstd. Note that almost no tests will pass this way, but you can run individual tests.

./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/pass/vec.rs

Setting MIRI_LOG like this will configure logging for Miri itself as well as the rustc_middle::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/pass/vec.rs

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

UI testing

We use ui-testing in Miri, meaning we generate .stderr and .stdout files for the output produced by Miri. You can use ./miri bless to automatically (re)generate these files when you add new tests or change how Miri presents certain output.

Note that when you also use MIRIFLAGS to change optimizations and similar, the ui output will change in unexpected ways. In order to still be able to run the other checks while ignoring the ui output, use MIRI_SKIP_UI_CHECKS=1 ./miri test.

For more info on how to configure ui tests see the documentation on the ui test crate

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! Usually this means you have to write cargo +miri miri ... to select the miri toolchain that was installed by ./rustup-toolchain.

There's a test for the cargo wrapper in the test-cargo-miri directory; run ./run-test.py in there to execute it. Like ./miri test, this respects the MIRI_TEST_TARGET environment variable to execute the test for another target.

Using a modified standard library

Miri re-builds the standard library into a custom sysroot, so it is fairly easy to test Miri against a modified standard library -- you do not even have to build Miri yourself, the Miri shipped by rustup will work. All you have to do is set the MIRI_LIB_SRC environment variable to the library folder of a rust-lang/rust repository checkout. Note that changing files in that directory does not automatically trigger a re-build of the standard library; you have to clear the Miri build cache manually (on Linux, rm -rf ~/.cache/miri; and on Windows, rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache").

Benchmarking

Miri comes with a few benchmarks; you can run ./miri bench to run them with the locally built Miri. Note: this will run ./miri install as a side-effect. Also requires hyperfine to be installed (cargo install hyperfine).

Configuring rust-analyzer

To configure rust-analyzer and VS Code for working on Miri, save the following to .vscode/settings.json in your local Miri clone:

{
    "rust-analyzer.rustc.source": "discover",
    "rust-analyzer.linkedProjects": [
        "./Cargo.toml",
        "./cargo-miri/Cargo.toml"
    ],
    "rust-analyzer.checkOnSave.overrideCommand": [
        "env",
        "MIRI_AUTO_OPS=no",
        "./miri",
        "check",
        "--message-format=json"
    ],
    "rust-analyzer.buildScripts.overrideCommand": [
        "env",
        "MIRI_AUTO_OPS=no",
        "./miri",
        "check",
        "--message-format=json",
    ],
    "rust-analyzer.rustfmt.extraArgs": [
        "+nightly"
    ],
}

Note

If you are building Miri with a locally built rustc, set rust-analyzer.rustcSource to the relative path from your Miri clone to the root Cargo.toml of the locally built rustc. For example, the path might look like ../rust/Cargo.toml.

See the rustc-dev-guide's docs on "Configuring rust-analyzer for rustc" for more information about configuring VS Code and rust-analyzer.

Advanced topic: other build environments

We described above the simplest way to get a working build environment for Miri, which is to use the version of rustc indicated by rustc-version. But sometimes, that is not enough.

Updating rustc-version

The rustc-version file is regularly updated to keep Miri close to the latest version of rustc. Usually, new contributors do not have to worry about this. But sometimes a newer rustc is needed for a patch, and sometimes Miri needs fixing for changes in rustc. In both cases, rustc-version needs updating.

To update the rustc-version file and install the latest rustc, you can run:

./rustup-toolchain HEAD

Now edit Miri until ./miri test passes, and submit a PR. Generally, it is preferred to separate updating rustc-version and doing what it takes to get Miri working again, from implementing new features that rely on the updated rustc. This avoids blocking all Miri development on landing a big PR.

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:

# Clone the rust-lang/rust repo.
git clone https://github.com/rust-lang/rust rustc
cd rustc
# Create a config.toml with defaults for working on Miri.
./x.py setup compiler
 # Now edit `config.toml` and under `[rust]` set `debug-assertions = true`.

# Build a stage 2 rustc, and build the rustc libraries with that rustc.
# This step can take 30 minutes or more.
./x.py build --stage 2 compiler/rustc
# If you change something, you can get a faster rebuild by doing
./x.py build --keep-stage 0 --stage 2 compiler/rustc
# You may have to change the architecture in the next command
rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2
# Now cd to your Miri directory, then configure rustup
rustup override set stage2

Note: When you are working with a locally built rustc or any other toolchain that is not the same as the one in rust-version, you should not have .auto-everything or .auto-toolchain as that will keep resetting your toolchain.

rm -f .auto-everything .auto-toolchain

Important: You need to delete the Miri cache when you change the stdlib; otherwise the old, chached version will be used. On Linux, the cache is located at ~/.cache/miri, and on Windows, it is located at %LOCALAPPDATA%\rust-lang\miri\cache; the exact location is printed after the library build: "A libstd for Miri is now available in ...".

Note: ./x.py --stage 2 compiler/rustc currently errors with thread 'main' panicked at 'fs::read(stamp) failed with No such file or directory (os error 2), you can simply ignore that error; Miri will build anyway.

For more information about building and configuring a local compiler, see https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html.

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