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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:

./miri 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 ./miri 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. 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

Tests in pass-dep need to be run using ./miri run --dep <filename>.
For example:

./miri run --dep tests/pass-dep/shims/libc-fs.rs

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

./miri test
./miri test --target i686-unknown-linux-gnu

./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). Multiple filters are supported: ./miri test FILTER1 FILTER2 runs all tests that contain either string.

Fine grained logging

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

Note that you will only get info, warn or error messages if you use a prebuilt compiler. In order to get debug and trace level messages, you need to build miri with a locally built compiler that has debug=true set in config.toml.

Debugging error messages

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 test --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 as a component of the miri toolchain. Note that the miri and cargo-miri executables are placed in the miri toolchain's sysroot to prevent conflicts with other toolchains. The Miri binaries in the cargo bin directory (usually ~/.cargo/bin) are managed by rustup.

There's a test for the cargo wrapper in the test-cargo-miri directory; run ./run-test.py in there to execute it. You can pass --target 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; on Windows, rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"; and on macOS, rm -rf ~/Library/Caches/org.rust-lang.miri).

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",
        "miri-script/Cargo.toml",
    ],
    "rust-analyzer.check.overrideCommand": [
        "env",
        "MIRI_AUTO_OPS=no",
        "./miri",
        "cargo",
        "clippy", // make this `check` when working with a locally built rustc
        "--message-format=json",
        "--all-targets",
    ],
    // Contrary to what the name suggests, this also affects proc macros.
    "rust-analyzer.cargo.buildScripts.overrideCommand": [
        "env",
        "MIRI_AUTO_OPS=no",
        "./miri",
        "cargo",
        "check",
        "--message-format=json",
        "--all-targets",
    ],
}

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: Working on Miri in the rustc tree

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.

A big part of the Miri driver is shared with rustc, so working on Miri will sometimes require also working on rustc itself. In this case, you should not work in a clone of the Miri repository, but in a clone of the main Rust repository. There is a copy of Miri located at src/tools/miri that you can work on directly. A maintainer will eventually sync those changes back into this repository.

When working on Miri in the rustc tree, here's how you can run tests:

./x.py test miri --stage 0

--bless will work, too.

You can also directly run Miri on a Rust source file:

./x.py run miri --stage 0 --args src/tools/miri/tests/pass/hello.rs

Advanced topic: Syncing with the rustc repo

We use the josh proxy to transmit changes between the rustc and Miri repositories. You can install it as follows:

RUSTFLAGS="--cap-lints=warn" cargo +stable install josh-proxy --git https://github.com/josh-project/josh --tag r23.12.04

Josh will automatically be started and stopped by ./miri.

Importing changes from the rustc repo

Note: this usually happens automatically, so these steps rarely have to be done by hand.

We assume we start on an up-to-date master branch in the Miri repo.

# Fetch and merge rustc side of the history. Takes ca 5 min the first time.
# This will also update the `rustc-version` file.
./miri rustc-pull
# Update local toolchain and apply formatting.
./miri toolchain && ./miri fmt
git commit -am "rustup"

Now push this to a new branch in your Miri fork, and create a PR. It is worth running ./miri test locally in parallel, since the test suite in the Miri repo is stricter than the one on the rustc side, so some small tweaks might be needed.

Exporting changes to the rustc repo

We will use the josh proxy to push to your fork of rustc. Run the following in the Miri repo, assuming we are on an up-to-date master branch:

# Push the Miri changes to your rustc fork (substitute your github handle for YOUR_NAME).
./miri rustc-push YOUR_NAME miri

This will create a new branch called miri in your fork, and the output should include a link that creates a rustc PR to integrate those changes into the main repository. If that PR has conflicts, you need to pull rustc changes into Miri first, and then re-do the rustc push.

If this fails due to authentication problems, it can help to make josh push via ssh instead of https. Add the following to your .gitconfig:

[url "git@github.com:"]
    pushInsteadOf = https://github.com/

Further environment variables

The following environment variables are relevant to ./miri:

  • MIRI_AUTO_OPS indicates whether the automatic execution of rustfmt, clippy and toolchain setup (as controlled by the ./auto-* files) should be skipped. If it is set to no, they are skipped. This is used to allow automated IDE actions to avoid the auto ops.
  • MIRI_LOG, MIRI_BACKTRACE control logging and backtrace printing during Miri executions.
  • MIRI_TEST_THREADS (recognized by ./miri test) sets the number of threads to use for running tests. By default, the number of cores is used.
  • MIRI_SKIP_UI_CHECKS (recognized by ./miri test) disables checking that the stderr or stdout files match the actual output.

Furthermore, the usual environment variables recognized by cargo miri also work for ./miri, e.g. MIRI_LIB_SRC. Note that MIRIFLAGS is ignored by ./miri test as each test controls the flags it is run with.

The following environment variables are internal and must not be used by anyone but Miri itself. They are used to communicate between different Miri binaries, and as such worth documenting:

  • CARGO_EXTRA_FLAGS is understood by ./miri and passed to all host cargo invocations.
  • MIRI_BE_RUSTC can be set to host or target. It tells the Miri driver to actually not interpret the code but compile it like rustc would. With target, Miri sets some compiler flags to prepare the code for interpretation; with host, this is not done. This environment variable is useful to be sure that the compiled rlibs are compatible with Miri.
  • MIRI_CALLED_FROM_SETUP is set during the Miri sysroot build, which will re-invoke cargo-miri as the rustc to use for this build.
  • MIRI_CALLED_FROM_RUSTDOC when set to any value tells cargo-miri that it is running as a child process of rustdoc, which invokes it twice for each doc-test and requires special treatment, most notably a check-only build before interpretation. This is set by cargo-miri itself when running as a rustdoc-wrapper.
  • MIRI_CWD when set to any value tells the Miri driver to change to the given directory after loading all the source files, but before commencing interpretation. This is useful if the interpreted program wants a different working directory at run-time than at build-time.
  • MIRI_LOCAL_CRATES is set by cargo-miri to tell the Miri driver which crates should be given special treatment in diagnostics, in addition to the crate currently being compiled.
  • MIRI_ORIG_RUSTDOC is set and read by different phases of cargo-miri to remember the value of RUSTDOC from before it was overwritten.
  • MIRI_REPLACE_LIBRS_IF_NOT_TEST when set to any value enables a hack that helps bootstrap run the standard library tests in Miri.
  • MIRI_TEST_TARGET is set by ./miri test (and ./x.py test miri) to tell the test harness about the chosen target.
  • MIRI_VERBOSE when set to any value tells the various cargo-miri phases to perform verbose logging.
  • MIRI_HOST_SYSROOT is set by bootstrap to tell cargo-miri which sysroot to use for host operations.
  • RUSTC_BLESS is set by ./miri test (and ./x.py test miri) to indicate bless-mode to the test harness.