3 If you want to hack on Miri yourself, great! Here are some resources you might
8 Check out the issues on this GitHub repository for some ideas. In particular,
9 look for the green `E-*` labels which mark issues that should be rather
10 well-suited for onboarding. For more ideas or help with hacking on Miri, you can
11 contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
13 [Rust Zulip]: https://rust-lang.zulipchat.com
15 ## Preparing the build environment
17 Miri heavily relies on internal and unstable rustc interfaces to execute MIR,
18 which means it is important that you install a version of rustc that Miri
21 The `rust-version` file contains the commit hash of rustc that Miri is currently
22 tested against. Other versions will likely not work. After installing
23 [`rustup-toolchain-install-master`], you can run the following command to
24 install that exact version of rustc as a toolchain:
28 This will set up a rustup toolchain called `miri` and set it as an override for
29 the current directory.
31 You can also create a `.auto-everything` file (contents don't matter, can be empty), which
32 will cause any `./miri` command to automatically call `./miri toolchain`, `clippy` and `rustfmt`
33 for you. If you don't want all of these to happen, you can add individual `.auto-toolchain`,
34 `.auto-clippy` and `.auto-fmt` files respectively.
36 [`rustup-toolchain-install-master`]: https://github.com/kennytm/rustup-toolchain-install-master
38 ## Building and testing Miri
40 Invoking Miri requires getting a bunch of flags right and setting up a custom
41 sysroot. The `miri` script takes care of that for you. With the
42 build environment prepared, compiling Miri is just one command away:
48 Run `./miri` without arguments to see the other commands our build tool
51 ### Testing the Miri driver
53 The Miri driver compiled from `src/bin/miri.rs` is the "heart" of Miri: it is
54 basically a version of `rustc` that, instead of compiling your code, runs it.
55 It accepts all the same flags as `rustc` (though the ones only affecting code
56 generation and linking obviously will have no effect) [and more][miri-flags].
58 [miri-flags]: README.md#miri--z-flags-and-environment-variables
60 For example, you can (cross-)run the driver on a particular file by doing
63 ./miri run tests/pass/format.rs
64 ./miri run tests/pass/hello.rs --target i686-unknown-linux-gnu
67 and you can (cross-)run the entire test suite using:
71 MIRI_TEST_TARGET=i686-unknown-linux-gnu ./miri test
74 If your target doesn't support libstd, you can run miri with
77 MIRI_NO_STD=1 MIRI_TEST_TARGET=thumbv7em-none-eabihf ./miri test tests/fail/alloc/no_global_allocator.rs
78 MIRI_NO_STD=1 ./miri run tests/pass/no_std.rs --target thumbv7em-none-eabihf
81 to avoid attempting (and failing) to build libstd. Note that almost no tests will pass
82 this way, but you can run individual tests.
84 `./miri test FILTER` only runs those tests that contain `FILTER` in their
85 filename (including the base directory, e.g. `./miri test fail` will run all
88 You can get a trace of which MIR statements are being executed by setting the
89 `MIRI_LOG` environment variable. For example:
92 MIRI_LOG=info ./miri run tests/pass/vec.rs
95 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
96 the `rustc_middle::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
97 can also do more targeted configuration, e.g. the following helps debug the
98 stacked borrows implementation:
101 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/pass/vec.rs
104 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
105 evaluation error was originally raised.
109 We use ui-testing in Miri, meaning we generate `.stderr` and `.stdout` files for the output
110 produced by Miri. You can use `./miri bless` to automatically (re)generate these files when
111 you add new tests or change how Miri presents certain output.
113 Note that when you also use `MIRIFLAGS` to change optimizations and similar, the ui output
114 will change in unexpected ways. In order to still be able
115 to run the other checks while ignoring the ui output, use `MIRI_SKIP_UI_CHECKS=1 ./miri test`.
117 For more info on how to configure ui tests see [the documentation on the ui test crate][ui_test]
119 [ui_test]: ui_test/README.md
121 ### Testing `cargo miri`
123 Working with the driver directly gives you full control, but you also lose all
124 the convenience provided by cargo. Once your test case depends on a crate, it
125 is probably easier to test it with the cargo wrapper. You can install your
126 development version of Miri using
132 and then you can use it as if it was installed by `rustup`. Make sure you use
133 the same toolchain when calling `cargo miri` that you used when installing Miri!
134 Usually this means you have to write `cargo +miri miri ...` to select the `miri`
135 toolchain that was installed by `./miri toolchain`.
137 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
138 `./run-test.py` in there to execute it. Like `./miri test`, this respects the
139 `MIRI_TEST_TARGET` environment variable to execute the test for another target.
141 Note that installing Miri like this will "take away" Miri management from `rustup`.
142 If you want to later go back to a rustup-installed Miri, run `rustup update`.
144 ### Using a modified standard library
146 Miri re-builds the standard library into a custom sysroot, so it is fairly easy
147 to test Miri against a modified standard library -- you do not even have to
148 build Miri yourself, the Miri shipped by `rustup` will work. All you have to do
149 is set the `MIRI_LIB_SRC` environment variable to the `library` folder of a
150 `rust-lang/rust` repository checkout. Note that changing files in that directory
151 does not automatically trigger a re-build of the standard library; you have to
152 clear the Miri build cache manually (on Linux, `rm -rf ~/.cache/miri`;
153 and on Windows, `rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"`).
157 Miri comes with a few benchmarks; you can run `./miri bench` to run them with the locally built
158 Miri. Note: this will run `./miri install` as a side-effect. Also requires `hyperfine` to be
159 installed (`cargo install hyperfine`).
161 ## Configuring `rust-analyzer`
163 To configure `rust-analyzer` and VS Code for working on Miri, save the following
164 to `.vscode/settings.json` in your local Miri clone:
168 "rust-analyzer.rustc.source": "discover",
169 "rust-analyzer.linkedProjects": [
171 "./cargo-miri/Cargo.toml"
173 "rust-analyzer.checkOnSave.overrideCommand": [
178 "clippy", // make this `check` when working with a locally built rustc
179 "--message-format=json"
181 // Contrary to what the name suggests, this also affects proc macros.
182 "rust-analyzer.cargo.buildScripts.overrideCommand": [
188 "--message-format=json",
195 > If you are [building Miri with a locally built rustc][], set
196 > `rust-analyzer.rustcSource` to the relative path from your Miri clone to the
197 > root `Cargo.toml` of the locally built rustc. For example, the path might look
198 > like `../rust/Cargo.toml`.
200 See the rustc-dev-guide's docs on ["Configuring `rust-analyzer` for `rustc`"][rdg-r-a]
201 for more information about configuring VS Code and `rust-analyzer`.
203 [rdg-r-a]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc
205 ## Advanced topic: other build environments
207 We described above the simplest way to get a working build environment for Miri,
208 which is to use the version of rustc indicated by `rustc-version`. But
209 sometimes, that is not enough.
211 ### Updating `rustc-version`
213 The `rustc-version` file is regularly updated to keep Miri close to the latest
214 version of rustc. Usually, new contributors do not have to worry about this. But
215 sometimes a newer rustc is needed for a patch, and sometimes Miri needs fixing
216 for changes in rustc. In both cases, `rustc-version` needs updating.
218 To update the `rustc-version` file and install the latest rustc, you can run:
220 ./miri toolchain HEAD
223 Now edit Miri until `./miri test` passes, and submit a PR. Generally, it is
224 preferred to separate updating `rustc-version` and doing what it takes to get
225 Miri working again, from implementing new features that rely on the updated
226 rustc. This avoids blocking all Miri development on landing a big PR.
228 ### Building Miri with a locally built rustc
230 [building Miri with a locally built rustc]: #building-miri-with-a-locally-built-rustc
232 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
233 require using a locally built rustc. The bug you want to fix may actually be on
234 the rustc side, or you just need to get more detailed trace of the execution
235 than what is possible with release builds -- in both cases, you should develop
236 Miri against a rustc you compiled yourself, with debug assertions (and hence
239 The setup for a local rustc works as follows:
241 # Clone the rust-lang/rust repo.
242 git clone https://github.com/rust-lang/rust rustc
244 # Create a config.toml with defaults for working on Miri.
245 ./x.py setup compiler
246 # Now edit `config.toml` and under `[rust]` set `debug-assertions = true`.
248 # Build a stage 2 rustc, and build the rustc libraries with that rustc.
249 # This step can take 30 minutes or more.
250 ./x.py build --stage 2 compiler/rustc
251 # If you change something, you can get a faster rebuild by doing
252 ./x.py build --keep-stage 0 --stage 2 compiler/rustc
253 # You may have to change the architecture in the next command
254 rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2
255 # Now cd to your Miri directory, then configure rustup
256 rustup override set stage2
259 Note: When you are working with a locally built rustc or any other toolchain that
260 is not the same as the one in `rust-version`, you should not have `.auto-everything` or
261 `.auto-toolchain` as that will keep resetting your toolchain.
264 rm -f .auto-everything .auto-toolchain
267 Important: You need to delete the Miri cache when you change the stdlib; otherwise the
268 old, chached version will be used. On Linux, the cache is located at `~/.cache/miri`,
269 and on Windows, it is located at `%LOCALAPPDATA%\rust-lang\miri\cache`; the exact
270 location is printed after the library build: "A libstd for Miri is now available in ...".
272 Note: `./x.py --stage 2 compiler/rustc` currently errors with `thread 'main'
273 panicked at 'fs::read(stamp) failed with No such file or directory (os error 2)`,
274 you can simply ignore that error; Miri will build anyway.
276 For more information about building and configuring a local compiler,
277 see <https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html>.
279 With this, you should now have a working development setup! See
280 [above](#building-and-testing-miri) for how to proceed working on Miri.
282 ## Advanced topic: Syncing with the rustc repo
284 We use the [`josh` proxy](https://github.com/josh-project/josh) to transmit
285 changes between the rustc and Miri repositories. For now, a fork of josh needs to be built
286 from source. This downloads and runs josh:
289 git clone https://github.com/RalfJung/josh
291 cargo run --release -p josh-proxy -- --local=$(pwd)/local --remote=https://github.com --no-background
294 ### Importing changes from the rustc repo
296 Josh needs to be running, as described above.
297 We assume we start on an up-to-date master branch in the Miri repo.
300 # Fetch and merge rustc side of the history. Takes ca 5 min the first time.
302 # Update toolchain reference and apply formatting.
303 ./miri toolchain HEAD && ./miri fmt
304 git commit -am "rustup"
307 Now push this to a new branch in your Miri fork, and create a PR. It is worth
308 running `./miri test` locally in parallel, since the test suite in the Miri repo
309 is stricter than the one on the rustc side, so some small tweaks might be
312 ### Exporting changes to the rustc repo
314 Josh needs to be running, as described above. We will use the josh proxy to push
315 to your fork of rustc. Run the following in the Miri repo, assuming we are on an
316 up-to-date master branch:
319 # Push the Miri changes to your rustc fork (substitute your github handle for YOUR_NAME).
320 ./miri rustc-push YOUR_NAME miri
323 This will create a new branch called 'miri' in your fork, and the output should
324 include a link to create a rustc PR that will integrate those changes into the