One Interpreter, Two Machines: How rustc's Const Eval Powers Miri Too

// compiler/rustc_const_eval/src/interpret/machine.rs:113-163

/// Methods of this trait signifies a point where CTFE evaluation would fail
/// and some use case dependent behaviour can instead be applied.
pub trait Machine<'tcx>: Sized {
    /// Additional memory kinds a machine wishes to distinguish from the builtin ones
    type MemoryKind: Debug + std::fmt::Display + MayLeak + Eq + 'static;

    /// Pointers are "tagged" with provenance information; typically the `AllocId` they belong to.
    type Provenance: Provenance + Eq + Hash + 'static;

    /// When getting the AllocId of a pointer, some extra data is also obtained from the provenance
    /// that is passed to memory access hooks so they can do things with it.
    type ProvenanceExtra: Copy + 'static;

    /// Machines can define extra (non-instance) things that represent values of function pointers.
    /// For example, Miri uses this to return a function pointer from `dlsym`
    /// that can later be called to execute the right thing.
    type ExtraFnVal: Debug + Copy;

    /// Extra data stored in every call frame.
    type FrameExtra;

    /// Extra data stored in every allocation.
    type AllocExtra: Debug + Clone + 'tcx;

    /// Type for the bytes of the allocation.
    type Bytes: AllocBytes + 'static;

    /// Memory's allocation map
    type MemoryMap: AllocMap<
            AllocId,
            (
                MemoryKind<Self::MemoryKind>,
                Allocation<Self::Provenance, Self::AllocExtra, Self::Bytes>,
            ),
        > + Default
        + Clone;

// compiler/rustc_const_eval/src/interpret/machine.rs:692-712

/// A lot of the flexibility above is just needed for `Miri`, but all "compile-time" machines
/// (CTFE and ConstProp) use the same instance. Here, we share that code.
pub macro compile_time_machine(<$tcx: lifetime>) {
    type Provenance = CtfeProvenance;
    type ProvenanceExtra = bool; // the "immutable" flag

    type ExtraFnVal = !;

    type MemoryKind = $crate::const_eval::MemoryKind;
    type MemoryMap =
        rustc_data_structures::fx::FxIndexMap<AllocId, (MemoryKind<Self::MemoryKind>, Allocation)>;
    const GLOBAL_KIND: Option<Self::MemoryKind> = None; // no copying of globals from `tcx` to machine memory

    type AllocExtra = ();
    type FrameExtra = ();
    type Bytes = Box<[u8]>;

// compiler/rustc_const_eval/src/interpret/operand.rs:25-41

/// An `Immediate` represents a single immediate self-contained Rust value.
///
/// For optimization of a few very common cases, there is also a representation for a pair of
/// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
/// operations and wide pointers. This idea was taken from rustc's codegen.
/// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
/// defined on `Immediate`, and do not have to work with a `Place`.
#[derive(Copy, Clone, Debug)]
pub enum Immediate<Prov: Provenance = CtfeProvenance> {
    /// A single scalar value (must have *initialized* `Scalar` ABI).
    Scalar(Scalar<Prov>),
    /// A pair of two scalar value (must have `ScalarPair` ABI where both fields are
    /// `Scalar::Initialized`).
    ScalarPair(Scalar<Prov>, Scalar<Prov>),
    /// A value of fully uninitialized memory. Can have arbitrary size and layout, but must be sized.
    Uninit,
}

// compiler/rustc_const_eval/src/interpret/operand.rs:476-489

/// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
/// or still in memory. The latter is an optimization, to delay reading that chunk of
/// memory and to avoid having to store arbitrary-sized data here.
#[derive(Copy, Clone, Debug)]
pub(super) enum Operand<Prov: Provenance = CtfeProvenance> {
    Immediate(Immediate<Prov>),
    Indirect(MemPlace<Prov>),
}

// compiler/rustc_const_eval/src/interpret/operand.rs:409-431

// This makes several assumptions about what layouts we will encounter; we match what
// codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
let inner_val: Immediate<_> = match (**self, self.layout.backend_repr) {
    // If the entire value is uninit, then so is the field (can happen in ConstProp).
    (Immediate::Uninit, _) => Immediate::Uninit,
    // If the field is uninhabited, we can forget the data (can happen in ConstProp).
    // `enum S { A(!), B, C }` is an example of an enum with Scalar layout that
    // has an uninhabited variant, which means this case is possible.
    _ if layout.is_uninhabited() => Immediate::Uninit,
    // the field contains no information, can be left uninit
    // (Scalar/ScalarPair can contain even aligned ZST, not just 1-ZST)
    _ if layout.is_zst() => Immediate::Uninit,
    ...
    // extract fields from types with `ScalarPair` ABI
    (
        Immediate::ScalarPair(a_val, b_val),
        BackendRepr::ScalarPair { a: _, b: _, b_offset },
    ) => Immediate::from(if offset.bytes() == 0 {
        a_val
    } else {
        assert_eq!(offset, b_offset);
        b_val
    }),

// compiler/rustc_const_eval/src/interpret/operand.rs:803-807

let mut op = self.local_to_op(mir_place.local, layout)?;
// Using `try_fold` turned out to be bad for performance, hence the loop.
for elem in mir_place.projection.iter() {
    op = self.project(&op, elem)?
}