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 If you want to also have `clippy` installed, you need to run this:
33 ./rustup-toolchain "" -c clippy
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 with xargo. 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!
135 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
136 `./run-test.py` in there to execute it. Like `./miri test`, this respects the
137 `MIRI_TEST_TARGET` environment variable to execute the test for another target.
139 ### Using a modified standard library
141 Miri re-builds the standard library into a custom sysroot, so it is fairly easy
142 to test Miri against a modified standard library -- you do not even have to
143 build Miri yourself, the Miri shipped by `rustup` will work. All you have to do
144 is set the `MIRI_LIB_SRC` environment variable to the `library` folder of a
145 `rust-lang/rust` repository checkout. Note that changing files in that directory
146 does not automatically trigger a re-build of the standard library; you have to
147 clear the Miri build cache manually (on Linux, `rm -rf ~/.cache/miri`;
148 and on Windows, `rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"`).
152 Miri comes with a few benchmarks; you can run `./miri bench` to run them with the locally built
153 Miri. Note: this will run `./miri install` as a side-effect. Also requires `hyperfine` to be
154 installed (`cargo install hyperfine`).
156 ## Configuring `rust-analyzer`
158 To configure `rust-analyzer` and VS Code for working on Miri, save the following
159 to `.vscode/settings.json` in your local Miri clone:
163 "rust-analyzer.rustc.source": "discover",
164 "rust-analyzer.linkedProjects": [
166 "./cargo-miri/Cargo.toml"
168 "rust-analyzer.checkOnSave.overrideCommand": [
171 "--message-format=json"
173 "rust-analyzer.buildScripts.overrideCommand": [
176 "--message-format=json",
178 "rust-analyzer.rustfmt.extraArgs": [
186 > If you are [building Miri with a locally built rustc][], set
187 > `rust-analyzer.rustcSource` to the relative path from your Miri clone to the
188 > root `Cargo.toml` of the locally built rustc. For example, the path might look
189 > like `../rust/Cargo.toml`.
191 See the rustc-dev-guide's docs on ["Configuring `rust-analyzer` for `rustc`"][rdg-r-a]
192 for more information about configuring VS Code and `rust-analyzer`.
194 [rdg-r-a]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rust-analyzer-for-rustc
196 ## Advanced topic: other build environments
198 We described above the simplest way to get a working build environment for Miri,
199 which is to use the version of rustc indicated by `rustc-version`. But
200 sometimes, that is not enough.
202 ### Updating `rustc-version`
204 The `rustc-version` file is regularly updated to keep Miri close to the latest
205 version of rustc. Usually, new contributors do not have to worry about this. But
206 sometimes a newer rustc is needed for a patch, and sometimes Miri needs fixing
207 for changes in rustc. In both cases, `rustc-version` needs updating.
209 To update the `rustc-version` file and install the latest rustc, you can run:
211 ./rustup-toolchain HEAD
214 Now edit Miri until `./miri test` passes, and submit a PR. Generally, it is
215 preferred to separate updating `rustc-version` and doing what it takes to get
216 Miri working again, from implementing new features that rely on the updated
217 rustc. This avoids blocking all Miri development on landing a big PR.
219 ### Building Miri with a locally built rustc
221 [building Miri with a locally built rustc]: #building-miri-with-a-locally-built-rustc
223 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
224 require using a locally built rustc. The bug you want to fix may actually be on
225 the rustc side, or you just need to get more detailed trace of the execution
226 than what is possible with release builds -- in both cases, you should develop
227 Miri against a rustc you compiled yourself, with debug assertions (and hence
230 The setup for a local rustc works as follows:
232 # Clone the rust-lang/rust repo.
233 git clone https://github.com/rust-lang/rust rustc
235 # Create a config.toml with defaults for working on Miri.
236 ./x.py setup compiler
237 # Now edit `config.toml` and under `[rust]` set `debug-assertions = true`.
239 # Build a stage 2 rustc, and build the rustc libraries with that rustc.
240 # This step can take 30 minutes or more.
241 ./x.py build --stage 2 compiler/rustc
242 # If you change something, you can get a faster rebuild by doing
243 ./x.py build --keep-stage 0 --stage 2 compiler/rustc
244 # You may have to change the architecture in the next command
245 rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2
246 # Now cd to your Miri directory, then configure rustup
247 rustup override set stage2
250 Important: You need to delete the Miri cache when you change the stdlib; otherwise the
251 old, chached version will be used. On Linux, the cache is located at `~/.cache/miri`,
252 and on Windows, it is located at `%LOCALAPPDATA%\rust-lang\miri\cache`; the exact
253 location is printed after the library build: "A libstd for Miri is now available in ...".
255 Note: `./x.py --stage 2 compiler/rustc` currently errors with `thread 'main'
256 panicked at 'fs::read(stamp) failed with No such file or directory (os error 2)`,
257 you can simply ignore that error; Miri will build anyway.
259 For more information about building and configuring a local compiler,
260 see <https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html>.
262 With this, you should now have a working development setup! See
263 [above](#building-and-testing-miri) for how to proceed working on Miri.