Miri (MIR Interpreter) is a virtual machine for executing MIR without compiling to machine code. It is usually invoked via tcx.const_eval.

If you start out with a constant

# #![allow(unused_variables)]
#fn main() {
const FOO: usize = 1 << 12;

rustc doesn't actually invoke anything until the constant is either used or placed into metadata.

Once you have a use-site like

type Foo = [u8; FOO - 42];

The compiler needs to figure out the length of the array before being able to create items that use the type (locals, constants, function arguments, ...).

To obtain the (in this case empty) parameter environment, one can call let param_env = tcx.param_env(length_def_id);. The GlobalId needed is

let gid = GlobalId {
    promoted: None,
    instance: Instance::mono(length_def_id),

Invoking tcx.const_eval(param_env.and(gid)) will now trigger the creation of the MIR of the array length expression. The MIR will look something like this:

const Foo::{{initializer}}: usize = {
    let mut _0: usize;                   // return pointer
    let mut _1: (usize, bool);

    bb0: {
        _1 = CheckedSub(const Unevaluated(FOO, Slice([])), const 42usize);
        assert(!(_1.1: bool), "attempt to subtract with overflow") -> bb1;

    bb1: {
        _0 = (_1.0: usize);

Before the evaluation, a virtual memory location (in this case essentially a vec![u8; 4] or vec![u8; 8]) is created for storing the evaluation result.

At the start of the evaluation, _0 and _1 are ConstValue::Scalar(Scalar::Undef). When the initialization of _1 is invoked, the value of the FOO constant is required, and triggers another call to tcx.const_eval, which will not be shown here. If the evaluation of FOO is successful, 42 will be subtracted by its value 4096 and the result stored in _1 as ConstValue::ScalarPair(Scalar::Bytes(4054), Scalar::Bytes(0)). The first part of the pair is the computed value, the second part is a bool that's true if an overflow happened.

The next statement asserts that said boolean is 0. In case the assertion fails, its error message is used for reporting a compile-time error.

Since it does not fail, ConstValue::Scalar(Scalar::Bytes(4054)) is stored in the virtual memory was allocated before the evaluation. _0 always refers to that location directly.

After the evaluation is done, the virtual memory allocation is interned into the TyCtxt. Future evaluations of the same constants will not actually invoke miri, but just extract the value from the interned allocation.

The tcx.const_eval function has one additional feature: it will not return a ByRef(interned_allocation_id), but a Scalar(computed_value) if possible. This makes using the result much more convenient, as no further queries need to be executed in order to get at something as simple as a usize.


Miri's core datastructures can be found in librustc/mir/interpret. This is mainly the error enum and the ConstValue and Scalar types. A ConstValue can be either Scalar (a single Scalar), ScalarPair (two Scalars, usually fat pointers or two element tuples) or ByRef, which is used for anything else and refers to a virtual allocation. These allocations can be accessed via the methods on tcx.interpret_interner.

If you are expecting a numeric result, you can use unwrap_usize (panics on anything that can't be representad as a u64) or assert_usize which results in an Option<u128> yielding the Scalar if possible.


A miri allocation is either a byte sequence of the memory or an Instance in the case of function pointers. Byte sequences can additionally contain relocations that mark a group of bytes as a pointer to another allocation. The actual bytes at the relocation refer to the offset inside the other allocation.

These allocations exist so that references and raw pointers have something to point to. There is no global linear heap in which things are allocated, but each allocation (be it for a local variable, a static or a (future) heap allocation) gets its own little memory with exactly the required size. So if you have a pointer to an allocation for a local variable a, there is no possible (no matter how unsafe) operation that you can do that would ever change said pointer to a pointer to b.


Although the main entry point to constant evaluation is the tcx.const_eval query, there are additional functions in librustc_mir/const_eval.rs that allow accessing the fields of a ConstValue (ByRef or otherwise). You should never have to access an Allocation directly except for translating it to the compilation target (at the moment just LLVM).

Miri starts by creating a virtual stack frame for the current constant that is being evaluated. There's essentially no difference between a constant and a function with no arguments, except that constants do not allow local (named) variables at the time of writing this guide.

A stack frame is defined by the Frame type in librustc_mir/interpret/eval_context.rs and contains all the local variables memory (None at the start of evaluation). Each frame refers to the evaluation of either the root constant or subsequent calls to const fn. The evaluation of another constant simply calls tcx.const_eval, which produces an entirely new and independent stack frame.

The frames are just a Vec<Frame>, there's no way to actually refer to a Frame's memory even if horrible shenanigans are done via unsafe code. The only memory that can be referred to are Allocations.

Miri now calls the step method (in librustc_mir/interpret/step.rs ) until it either returns an error or has no further statements to execute. Each statement will now initialize or modify the locals or the virtual memory referred to by a local. This might require evaluating other constants or statics, which just recursively invokes tcx.const_eval.