Miri and nextest

Nextest works with the Miri interpreter for Rust. This interpreter can check for certain classes of undefined behavior. It can also run your tests for (almost) arbitrary targets.


The main benefit of using nextest with Miri is that each test runs in its own process. This means that it's easier to identify memory leaks, for example.

Miri can be very taxing on most computers. If nextest is run under Miri, it configures itself to use 1 thread by default. This mirrors cargo miri test. You can customize this with the --test-threads/-j option.


  • cargo-nextest 0.9.29 or above
  • Miri from Rust nightly 2022-07-24 or after


After installing Miri, run:

cargo miri nextest run

You may need to specify the toolchain to run as, using cargo +nightly-YYYY-MM-DD miri nextest run.

Miri supports cross-interpretation, so e.g. to run your tests on a big-endian target, run:

cargo miri nextest run --target mips64-unknown-linux-gnuabi64

This does not require installing any special toolchain, and will work even if you are using macOS or Windows.

Note: Archiving and reusing builds is not supported under Miri.

Configuring nextest running under Miri

If nextest detects a Miri environment, it uses the default-miri profile by default. Add repository-specific Miri configuration to this profile. For example, to terminate tests after 2 minutes, add this to .config/nextest.toml:

slow-timeout = { period = "60s", terminate-after = 2 }