The interpreter is a virtual machine for executing MIR without compiling to machine code. It is usually invoked via tcx.const_eval_* functions. The interpreter is shared between the compiler (for compile-time function evaluation, CTFE) and the tool Miri, which uses the same virtual machine to detect Undefined Behavior in (unsafe) Rust code.
If you start out with a constant:
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:
Foo::{{constant}}#0: usize = { let mut _0: usize; let mut _1: (usize, bool); bb0: { _1 = CheckedSub(const FOO, const 42usize); assert(!move (_1.1: bool), "attempt to subtract with overflow") -> bb1; } bb1: { _0 = move (_1.0: usize); return; } }
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 Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Undef)). This is quite a mouthful: Operand can represent either data stored somewhere in the interpreter memory (Operand::Indirect), or (as an optimization) immediate data stored in-line. And Immediate can either be a single (potentially uninitialized) scalar value (integer or thin pointer), or a pair of two of them. In our case, the single scalar value is not (yet) initialized.
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 from its value 4096 and the result stored in _1 as Operand::Immediate(Immediate::ScalarPair(Scalar::Raw { data: 4054, .. }, Scalar::Raw { data: 0, .. }). The first part of the pair is the computed value, the second part is a bool that's true if an overflow happened. A Scalar::Raw also stores the size (in bytes) of this scalar value; we are eliding that here.
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, Operand::Immediate(Immediate::Scalar(Scalar::Raw { data: 4054, .. })) is stored in the virtual memory it was allocated before the evaluation. _0 always refers to that location directly.
After the evaluation is done, the return value is converted from Operand to ConstValue by op_to_const: the former representation is geared towards what is needed during const evaluation, while ConstValue is shaped by the needs of the remaining parts of the compiler that consume the results of const evaluation. As part of this conversion, for types with scalar values, even if the resulting Operand is Indirect, it will return an immediate ConstValue::Scalar(computed_value) (instead of the usual ConstValue::ByRef). This makes using the result much more efficient and also more convenient, as no further queries need to be executed in order to get at something as simple as a usize.
Future evaluations of the same constants will not actually invoke the interpreter, but just use the cached result.
The interpreter's outside-facing datastructures can be found in rustc_middle/src/mir/interpret. This is mainly the error enum and the ConstValue and Scalar types. A ConstValue can be either Scalar (a single Scalar, i.e., integer or thin pointer), Slice (to represent byte slices and strings, as needed for pattern matching) 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. A Scalar is either some Raw integer or a pointer; see the next section for more on that.
If you are expecting a numeric result, you can use eval_usize (panics on anything that can't be represented as a u64) or try_eval_usize which results in an Option<u64> yielding the Scalar if possible.
To support any kind of pointers, the interpreter needs to have a “virtual memory” that the pointers can point to. This is implemented in the Memory type. In the simplest model, every global variable, stack variable and every dynamic allocation corresponds to an Allocation in that memory. (Actually using an allocation for every MIR stack variable would be very inefficient; that's why we have Operand::Immediate for stack variables that are both small and never have their address taken. But that is purely an optimization.)
Such an Allocation is basically just a sequence of u8 storing the value of each byte in this allocation. (Plus some extra data, see below.) Every Allocation has a globally unique AllocId assigned in Memory. With that, a Pointer consists of a pair of an AllocId (indicating the allocation) and an offset into the allocation (indicating which byte of the allocation the pointer points to). It may seem odd that a Pointer is not just an integer address, but remember that during const evaluation, we cannot know at which actual integer address the allocation will end up -- so we use AllocId as symbolic base addresses, which means we need a separate offset. (As an aside, it turns out that pointers at run-time are more than just integers, too.)
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 a different local variable b. Pointer arithmetic on a will only ever change its offset; the AllocId stays the same.
This, however, causes a problem when we want to store a Pointer into an Allocation: we cannot turn it into a sequence of u8 of the right length! AllocId and offset together are twice as big as a pointer “seems” to be. This is what the relocation field of Allocation is for: the byte offset of the Pointer gets stored as a bunch of u8, while its AllocId gets stored out-of-band. The two are reassembled when the Pointer is read from memory. The other bit of extra data an Allocation needs is undef_mask for keeping track of which of its bytes are initialized.
Memory exists only during evaluation; it gets destroyed when the final value of the constant is computed. In case that constant contains any pointers, those get “interned” and moved to a global “const eval memory” that is part of TyCtxt. These allocations stay around for the remaining computation and get serialized into the final output (so that dependent crates can use them).
Moreover, to also support function pointers, the global memory in TyCtxt can also contain “virtual allocations”: instead of an Allocation, these contain an Instance. That allows a Pointer to point to either normal data or a function, which is needed to be able to evaluate casts from function pointers to raw pointers.
Finally, the GlobalAlloc type used in the global memory also contains a variant Static that points to a particular const or static item. This is needed to support circular statics, where we need to have a Pointer to a static for which we cannot yet have an Allocation as we do not know the bytes of its value.
One common cause of confusion in the interpreter is that being a pointer value and having a pointer type are entirely independent properties. By “pointer value”, we refer to a Scalar::Ptr containing a Pointer and thus pointing somewhere into the interpreter's virtual memory. This is in contrast to Scalar::Raw, which is just some concrete integer.
However, a variable of pointer or reference type, such as *const T or &T, does not have to have a pointer value: it could be obtained by casting or transmuting an integer to a pointer. And similarly, when casting or transmuting a reference to some actual allocation to an integer, we end up with a pointer value (Scalar::Ptr) at integer type (usize). This is a problem because we cannot meaningfully perform integer operations such as division on pointer values.
Although the main entry point to constant evaluation is the tcx.const_eval_* functions, there are additional functions in rustc_const_eval/src/const_eval 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).
The interpreter 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 rustc_const_eval/src/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 produce 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.
The interpreter now calls the step method (in rustc_const_eval/src/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_*.