# 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]. [Rust Zulip]: https://rust-lang.zulipchat.com ## 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. [`rustup-toolchain-install-master`]: https://github.com/kennytm/rustup-toolchain-install-master ## 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][miri-flags]. [miri-flags]: README.md#miri--z-flags-and-environment-variables For example, you can (cross-)run the driver on a particular file by doing ```sh ./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: ```sh 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: ```sh 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][ui_test] [ui_test]: ui_test/README.md ### 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: ```json { "rust-analyzer.rustc.source": "discover", "rust-analyzer.linkedProjects": [ "./Cargo.toml", "./cargo-miri/Cargo.toml" ], "rust-analyzer.checkOnSave.overrideCommand": [ "env", "MIRI_AUTO_OPS=no", "./miri", "cargo", "clippy", // make this `check` when working with a locally built rustc "--message-format=json" ], // 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", ], } ``` > #### 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`"][rdg-r-a] for more information about configuring VS Code and `rust-analyzer`. [rdg-r-a]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc ## 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 [building Miri with a locally built rustc]: #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: ```sh # 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 . With this, you should now have a working development setup! See [above](#building-and-testing-miri) for how to proceed working on Miri.