An interpreter for Rust's mid-level intermediate representation
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RalfJung Merge pull request #488 from RalfJung/start-fn
Automatically use start-fn if we have all the MIR
Latest commit 53dc505 Oct 19, 2018

Miri [slides] [report] Build Status Windows build status

An experimental interpreter for Rust's mid-level intermediate representation (MIR). This project began as part of my work for the undergraduate research course at the University of Saskatchewan.

Building Miri

I recommend that you install rustup to obtain Rust. Miri comes with a rust-toolchain file so rustup will automatically pick a suitable nightly version. Then all you have to do is:

cargo build

Running Miri

cargo run tests/run-pass/ # Or whatever test you like.

Running Miri with full libstd

Per default libstd does not contain the MIR of non-polymorphic functions. When Miri hits a call to such a function, execution terminates. To fix this, it is possible to compile libstd with full MIR:

rustup component add rust-src
cargo install xargo

Now you can run Miri against the libstd compiled by xargo:

MIRI_SYSROOT=~/.xargo/HOST cargo run tests/run-pass-fullmir/

Notice that you will have to re-run the last step of the preparations above when your toolchain changes (e.g., when you update the nightly).

You can also set -Zmiri-start-fn to make Miri start evaluation with the start_fn lang item, instead of starting at the main function.

Running Miri on your own project('s test suite)

Install Miri as a cargo subcommand with cargo install --all-features, and install a full libstd as described above.

Then, inside your own project, use MIRI_SYSROOT=~/.xargo/HOST cargo +nightly miri to run your project, if it is a bin project, or run MIRI_SYSROOT=~/.xargo/HOST cargo +nightly miri test to run all tests in your project through Miri.

Miri -Z flags

Miri adds some extra -Z flags to control its behavior:

  • -Zmiri-start-fn: This makes interpretation start with lang_start (defined in libstd) instead of starting with main. Requires full MIR!
  • -Zmiri-disable-validation disables enforcing the validity invariant.

Development and Debugging

Since the heart of Miri (the main interpreter engine) lives in rustc, working on Miri will often require using a locally built rustc. This includes getting a trace of the execution, as distributed rustc has trace! disabled.

The first-time setup for a local rustc looks as follows:

git clone rustc
cd rustc
cp config.toml.example config.toml
# Now edit `config.toml` and set `debug-assertions = true`
./ build src/rustc
# You may have to change the architecture in the next command
rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
# Now cd to your Miri directory
rustup override set custom

The build step can take 30 minutes and more.

Now you can cargo build Miri, and you can cargo test --tests. (--tests is needed to skip doctests because we have not built rustdoc for your custom toolchain.) You can also set RUST_LOG=rustc_mir::interpret=trace as environment variable to get a step-by-step trace.

If you changed something in rustc and want to re-build, run

./ build src/rustc --keep-stage 0

This avoids rebuilding the entire stage 0, which can save a lot of time.

Contributing and getting help

Check out the issues on this GitHub repository for some ideas. There's lots that needs to be done that I haven't documented in the issues yet, however. For more ideas or help with running or hacking on Miri, you can contact me (scott) on Mozilla IRC in any of the Rust IRC channels (#rust, #rust-offtopic, etc).


Licensed under either of


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.