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An experimental interpreter for Rust's mid-level intermediate representation (MIR). It can run binaries and test suites of cargo projects and detect certain classes of undefined behavior, for example:

  • Out-of-bounds memory accesses and use-after-free
  • Invalid use of uninitialized data
  • Violation of intrinsic preconditions (an unreachable_unchecked being reached, calling copy_nonoverlapping with overlapping ranges, ...)
  • Not sufficiently aligned memory accesses and references
  • Violation of some basic type invariants (a bool that is not 0 or 1, for example, or an invalid enum discriminant)
  • Experimental: Violations of the Stacked Borrows rules governing aliasing for reference types

On top of that, Miri will also tell you about memory leaks: when there is memory still allocated at the end of the execution, and that memory is not reachable from a global static, Miri will raise an error.

Miri has already discovered some real-world bugs. If you found a bug with Miri, we'd appreciate if you tell us and we'll add it to the list!

However, be aware that Miri will not catch all cases of undefined behavior in your program, and cannot run all programs:

  • There are still plenty of open questions around the basic invariants for some types and when these invariants even have to hold. Miri tries to avoid false positives here, so if you program runs fine in Miri right now that is by no means a guarantee that it is UB-free when these questions get answered.

    In particular, Miri does currently not check that integers/floats are initialized or that references point to valid data.

  • If the program relies on unspecified details of how data is laid out, it will still run fine in Miri -- but might break (including causing UB) on different compiler versions or different platforms.

  • Program execution is non-deterministic when it depends, for example, on where exactly in memory allocations end up. Miri tests one of many possible executions of your program. If your code is sensitive to allocation base addresses or other non-deterministic data, try running Miri with different values for -Zmiri-seed to test different executions.

  • Miri runs the program as a platform-independent interpreter, so the program has no access to most platform-specific APIs or FFI. A few APIs have been implemented (such as printing to stdout) but most have not: for example, Miri currently does not support SIMD or networking.

  • Miri currently does not check for data-races and most other concurrency-related issues.

Using Miri

Install Miri on Rust nightly via rustup:

rustup +nightly component add miri

If rustup says the miri component is unavailable, that's because not all nightly releases come with all tools. Check out this website to determine a nightly version that comes with Miri and install that using rustup toolchain install nightly-YYYY-MM-DD.

Now you can run your project in Miri:

  1. Run cargo clean to eliminate any cached dependencies. Miri needs your dependencies to be compiled the right way, that would not happen if they have previously already been compiled.
  2. To run all tests in your project through Miri, use cargo miri test.
  3. If you have a binary project, you can run it through Miri using cargo miri run.

The first time you run Miri, it will perform some extra setup and install some dependencies. It will ask you for confirmation before installing anything.

cargo miri run/test supports the exact same flags as cargo run/test. You can pass arguments to Miri via MIRIFLAGS. For example, MIRIFLAGS="-Zmiri-disable-stacked-borrows" cargo miri run runs the program without checking the aliasing of references.

When compiling code via cargo miri, the cfg(miri) config flag is set. You can use this to ignore test cases that fail under Miri because they do things Miri does not support:

#[cfg_attr(miri, ignore)]
fn does_not_work_on_miri() {
    std::thread::spawn(|| println!("Hello Thread!"))

There is no way to list all the infinite things Miri cannot do, but the interpreter will explicitly tell you when it finds something unsupported:

error: unsupported operation: can't call foreign function: bind
    = help: this is likely not a bug in the program; it indicates that the program \
            performed an operation that the interpreter does not support

Cross-interpretation: running for different targets

Miri can not only run a binary or test suite for your host target, it can also perform cross-interpretation for arbitrary foreign targets: cargo miri run --target x86_64-unknown-linux-gnu will run your program as if it was a Linux program, no matter your host OS. This is particularly useful if you are using Windows, as the Linux target is much better supported than Windows targets.

You can also use this to test platforms with different properties than your host platform. For example cargo miri test --target mips64-unknown-linux-gnuabi64 will run your test suite on a big-endian target, which is useful for testing endian-sensitive code.

Running Miri on CI

To run Miri on CI, make sure that you handle the case where the latest nightly does not ship the Miri component because it currently does not build. For example, you can use the following snippet to always test with the latest nightly that does come with Miri:

MIRI_NIGHTLY=nightly-$(curl -s
echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
rustup set profile minimal
rustup default "$MIRI_NIGHTLY"
rustup component add miri

cargo miri test

Common Problems

When using the above instructions, you may encounter a number of confusing compiler errors.

"note: run with RUST_BACKTRACE=1 environment variable to display a backtrace"

You may see this when trying to get Miri to display a backtrace. By default, Miri doesn't expose any environment to the program, so running RUST_BACKTRACE=1 cargo miri test will not do what you expect.

To get a backtrace, you need to disable isolation using -Zmiri-disable-isolation:

RUST_BACKTRACE=1 MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test

"found possibly newer version of crate std which <dependency> depends on"

Your build directory may contain artifacts from an earlier build that have/have not been built for Miri. Run cargo clean before switching from non-Miri to Miri builds and vice-versa.

"found crate std compiled by an incompatible version of rustc"

You may be running cargo miri with a different compiler version than the one used to build the custom libstd that Miri uses, and Miri failed to detect that. Try deleting ~/.cache/miri.

"no mir for std::rt::lang_start_internal"

This means the sysroot you are using was not compiled with Miri in mind. This should never happen when you use cargo miri because that takes care of setting up the sysroot. If you are using miri (the Miri driver) directly, see the contributors' guide for how to use ./miri to best do that.

Miri -Z flags and environment variables

Miri adds its own set of -Z flags, which are usually set via the MIRIFLAGS environment variable:

  • -Zmiri-disable-alignment-check disables checking pointer alignment, so you can focus on other failures, but it means Miri can miss bugs in your program. Using this flag is unsound.
  • -Zmiri-disable-stacked-borrows disables checking the experimental Stacked Borrows aliasing rules. This can make Miri run faster, but it also means no aliasing violations will be detected. Using this flag is unsound (but the affected soundness rules are experimental).
  • -Zmiri-disable-validation disables enforcing validity invariants, which are enforced by default. This is mostly useful to focus on other failures (such as out-of-bounds accesses) first. Setting this flag means Miri can miss bugs in your program. However, this can also help to make Miri run faster. Using this flag is unsound.
  • -Zmiri-disable-isolation disables host isolation. As a consequence, the program has access to host resources such as environment variables, file systems, and randomness.
  • -Zmiri-env-exclude=<var> keeps the var environment variable isolated from the host so that it cannot be accessed by the program. Can be used multiple times to exclude several variables. On Windows, the TERM environment variable is excluded by default.
  • -Zmiri-ignore-leaks disables the memory leak checker.
  • -Zmiri-seed=<hex> configures the seed of the RNG that Miri uses to resolve non-determinism. This RNG is used to pick base addresses for allocations. When isolation is enabled (the default), this is also used to emulate system entropy. The default seed is 0. NOTE: This entropy is not good enough for cryptographic use! Do not generate secret keys in Miri or perform other kinds of cryptographic operations that rely on proper random numbers.
  • -Zmiri-symbolic-alignment-check makes the alignment check more strict. By default, alignment is checked by casting the pointer to an integer, and making sure that is a multiple of the alignment. This can lead to cases where a program passes the alignment check by pure chance, because things "happened to be" sufficiently aligned -- there is no UB in this execution but there would be UB in others. To avoid such cases, the symbolic alignment check only takes into account the requested alignment of the relevant allocation, and the offset into that allocation. This avoids missing such bugs, but it also incurs some false positives when the code does manual integer arithmetic to ensure alignment. (The standard library align_to method works fine in both modes; under symbolic alignment it only fills the middle slice when the allocation guarantees sufficient alignment.)
  • -Zmiri-track-alloc-id=<id> shows a backtrace when the given allocation is being allocated or freed. This helps in debugging memory leaks and use after free bugs.
  • -Zmiri-track-call-id=<id> shows a backtrace when the given call id is assigned to a stack frame. This helps in debugging UB related to Stacked Borrows "protectors".
  • -Zmiri-track-pointer-tag=<tag> shows a backtrace when the given pointer tag is popped from a borrow stack (which is where the tag becomes invalid and any future use of it will error). This helps you in finding out why UB is happening and where in your code would be a good place to look for it.
  • -Zmiri-track-raw-pointers makes Stacked Borrows track a pointer tag even for raw pointers. This can make valid code fail to pass the checks, but also can help identify latent aliasing issues in code that Miri accepts by default. You can recognize false positives by "" occurring in the message -- this indicates a pointer that was cast from an integer, so Miri was unable to track this pointer. Make sure to use a non-Windows target with this flag, as the Windows runtime makes use of integer-pointer casts.

Some native rustc -Z flags are also very relevant for Miri:

  • -Zmir-opt-level controls how many MIR optimizations are performed. Miri overrides the default to be 0; be advised that using any higher level can make Miri miss bugs in your program because they got optimized away.
  • -Zalways-encode-mir makes rustc dump MIR even for completely monomorphic functions. This is needed so that Miri can execute such functions, so Miri sets this flag per default.
  • -Zmir-emit-retag controls whether Retag statements are emitted. Miri enables this per default because it is needed for Stacked Borrows.

Moreover, Miri recognizes some environment variables:

  • MIRI_LOG, MIRI_BACKTRACE control logging and backtrace printing during Miri executions, also see "Testing the Miri driver" in
  • MIRIFLAGS (recognized by cargo miri and the test suite) defines extra flags to be passed to Miri.
  • MIRI_SYSROOT (recognized by cargo miri and the test suite) indicates the sysroot to use. To do the same thing with miri directly, use the --sysroot flag.
  • MIRI_TEST_TARGET (recognized by the test suite) indicates which target architecture to test against. miri and cargo miri accept the --target flag for the same purpose.

The following environment variables are internal, but used to communicate between different Miri binaries, and as such worth documenting:

  • MIRI_BE_RUSTC when set to any value tells the Miri driver to actually not interpret the code but compile it like rustc would. This is useful to be sure that the compiled rlibs are compatible with Miri. When set while running cargo-miri, it indicates that we are part of a sysroot build (for which some crates need special treatment).
  • MIRI_CWD when set to any value tells the Miri driver to change to the given directory after loading all the source files, but before commencing interpretation. This is useful if the interpreted program wants a different working directory at run-time than at build-time.

Miri extern functions

Miri provides some extern functions that programs can import to access Miri-specific functionality:

extern "Rust" {
    /// Miri-provided extern function to mark the block `ptr` points to as a "root"
    /// for some static memory. This memory and everything reachable by it is not
    /// considered leaking even if it still exists when the program terminates.
    /// `ptr` has to point to the beginning of an allocated block.
    fn miri_static_root(ptr: *const u8);

    /// Miri-provided extern function to obtain a backtrace of the current call stack.
    /// This returns a boxed slice of pointers - each pointer is an opaque value
    /// that is only useful when passed to `miri_resolve_frame`
    /// The `flags` argument must be `0`.
    fn miri_get_backtrace(flags: u64) -> Box<[*mut ()]>;

    /// Miri-provided extern function to resolve a frame pointer obtained
    /// from `miri_get_backtrace`. The `flags` argument must be `0`,
    /// and `MiriFrame` should be declared as follows:
    /// ```rust
    /// #[repr(C)]
    /// struct MiriFrame {
    ///     // The name of the function being executed, encoded in UTF-8
    ///     name: Box<[u8]>,
    ///     // The filename of the function being executed, encoded in UTF-8
    ///     filename: Box<[u8]>,
    ///     // The line number currently being executed in `filename`, starting from '1'.
    ///     lineno: u32,
    ///     // The column number currently being executed in `filename`, starting from '1'.
    ///     colno: u32,
    ///     // The function pointer to the function currently being executed.
    ///     // This can be compared against function pointers obtained by
    ///     // casting a function (e.g. `my_fn as *mut ()`)
    ///     fn_ptr: *mut ()
    /// }
    /// ```
    /// The fields must be declared in exactly the same order as they appear in `MiriFrame` above.
    /// This function can be called on any thread (not just the one which obtained `frame`).
    fn miri_resolve_frame(frame: *mut (), flags: u64) -> MiriFrame;

    /// Miri-provided extern function to begin unwinding with the given payload.
    /// This is internal and unstable and should not be used; we give it here
    /// just to be complete.
    fn miri_start_panic(payload: *mut u8) -> !;

Contributing and getting help

If you want to contribute to Miri, great! Please check out our contribution guide.

For help with running Miri, you can open an issue here on GitHub or contact us (oli-obk and RalfJ) on the Rust Zulip.


This project began as part of an undergraduate research course in 2015 by @solson at the University of Saskatchewan. There are slides and a report available from that project. In 2016, @oli-obk joined to prepare miri for eventually being used as const evaluator in the Rust compiler itself (basically, for const and static stuff), replacing the old evaluator that worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla and began developing miri towards a tool for detecting undefined behavior, and also using miri as a way to explore the consequences of various possible definitions for undefined behavior in Rust. @oli-obk's move of the miri engine into the compiler finally came to completion in early 2018. Meanwhile, later that year, @RalfJung did a second internship, developing miri further with support for checking basic type invariants and verifying that references are used according to their aliasing restrictions.

Bugs found by Miri

Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.

Definite bugs found:

Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):


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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.


An interpreter for Rust's mid-level intermediate representation




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