refactor: reorganize crate hierarchy

fix-runtime/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "fix-runtime"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+gc-arena = { workspace = true }
+hashbrown = { workspace = true }
+num_enum = { workspace = true }
+smallvec = { workspace = true }
+string-interner = { workspace = true }
+likely_stable = { workspace = true }
+sptr = "0.3"
+
+fix-bytecode = { path = "../fix-bytecode" }
+fix-lang = { path = "../fix-lang" }
+fix-error = { path = "../fix-error" }

fix-runtime/src/boxing.rs

@@ -0,0 +1,502 @@
+#![allow(dead_code)]
+
+use std::fmt;
+use std::num::NonZeroU8;
+
+use sptr::Strict;
+
+const SIGN_MASK: u64 = 0x7FFF_FFFF_FFFF_FFFF;
+const QUIET_NAN: u64 = 0x7FF8_0000_0000_0000;
+const NEG_QUIET_NAN: u64 = 0xFFF8_0000_0000_0000;
+
+pub(crate) trait ArrayExt<const LEN: usize> {
+    type Elem;
+    fn truncate_to<const M: usize>(self) -> [Self::Elem; M];
+}
+
+impl<T: Default + Copy, const N: usize> ArrayExt<N> for [T; N] {
+    type Elem = T;
+    fn truncate_to<const M: usize>(self) -> [Self::Elem; M] {
+        let copy_len = usize::min(N, M);
+        let mut out = [T::default(); M];
+        out[0..copy_len].copy_from_slice(&self[0..copy_len]);
+        out
+    }
+}
+
+pub trait RawStore: Sized {
+    fn to_val(self, value: &mut Value);
+    fn from_val(value: &Value) -> Self;
+}
+
+impl RawStore for [u8; 6] {
+    #[inline]
+    fn to_val(self, value: &mut Value) {
+        value.set_data(self);
+    }
+
+    #[inline]
+    fn from_val(value: &Value) -> Self {
+        *value.data()
+    }
+}
+
+impl RawStore for bool {
+    #[inline]
+    fn to_val(self, value: &mut Value) {
+        value.set_data([u8::from(self)].truncate_to());
+    }
+
+    #[inline]
+    fn from_val(value: &Value) -> Self {
+        value.data()[0] == 1
+    }
+}
+
+macro_rules! int_store {
+    ($ty:ty) => {
+        impl RawStore for $ty {
+            #[inline]
+            fn to_val(self, value: &mut Value) {
+                let bytes = self.to_ne_bytes();
+                value.set_data(bytes.truncate_to());
+            }
+
+            #[inline]
+            fn from_val(value: &Value) -> Self {
+                <$ty>::from_ne_bytes(value.data().truncate_to())
+            }
+        }
+    };
+}
+
+int_store!(u8);
+int_store!(u16);
+int_store!(u32);
+
+int_store!(i8);
+int_store!(i16);
+int_store!(i32);
+
+fn store_ptr<P: Strict + Copy>(value: &mut Value, ptr: P) {
+    #[cfg(target_pointer_width = "64")]
+    {
+        assert!(
+            ptr.addr() <= 0x0000_FFFF_FFFF_FFFF,
+            "Pointer too large to store in NaN box"
+        );
+
+        let val = (unsafe { value.whole_mut() } as *mut [u8; 8]).cast::<P>();
+
+        let ptr = Strict::map_addr(ptr, |addr| {
+            addr | (usize::from(value.header().into_raw()) << 48)
+        });
+
+        unsafe { val.write(ptr) };
+    }
+
+    #[cfg(target_pointer_width = "32")]
+    {
+        let _ = (value, ptr);
+        unimplemented!("32-bit pointer storage not supported");
+    }
+}
+
+fn load_ptr<P: Strict>(value: &Value) -> P {
+    #[cfg(target_pointer_width = "64")]
+    {
+        let val = (unsafe { value.whole() } as *const [u8; 8]).cast::<P>();
+        let ptr = unsafe { val.read() };
+        Strict::map_addr(ptr, |addr| addr & 0x0000_FFFF_FFFF_FFFF)
+    }
+
+    #[cfg(target_pointer_width = "32")]
+    {
+        let _ = value;
+        unimplemented!("32-bit pointer storage not supported");
+    }
+}
+
+impl<T> RawStore for *const T {
+    fn to_val(self, value: &mut Value) {
+        store_ptr::<*const T>(value, self);
+    }
+
+    fn from_val(value: &Value) -> Self {
+        load_ptr::<*const T>(value)
+    }
+}
+
+impl<T> RawStore for *mut T {
+    fn to_val(self, value: &mut Value) {
+        store_ptr::<*mut T>(value, self);
+    }
+
+    fn from_val(value: &Value) -> Self {
+        load_ptr::<*mut T>(value)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+enum TagVal {
+    _P1,
+    _P2,
+    _P3,
+    _P4,
+    _P5,
+    _P6,
+    _P7,
+
+    _N1,
+    _N2,
+    _N3,
+    _N4,
+    _N5,
+    _N6,
+    _N7,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub struct RawTag(TagVal);
+
+impl RawTag {
+    pub const P1: RawTag = RawTag(TagVal::_P1);
+    pub const P2: RawTag = RawTag(TagVal::_P2);
+    pub const P3: RawTag = RawTag(TagVal::_P3);
+    pub const P4: RawTag = RawTag(TagVal::_P4);
+    pub const P5: RawTag = RawTag(TagVal::_P5);
+    pub const P6: RawTag = RawTag(TagVal::_P6);
+    pub const P7: RawTag = RawTag(TagVal::_P7);
+
+    pub const N1: RawTag = RawTag(TagVal::_N1);
+    pub const N2: RawTag = RawTag(TagVal::_N2);
+    pub const N3: RawTag = RawTag(TagVal::_N3);
+    pub const N4: RawTag = RawTag(TagVal::_N4);
+    pub const N5: RawTag = RawTag(TagVal::_N5);
+    pub const N6: RawTag = RawTag(TagVal::_N6);
+    pub const N7: RawTag = RawTag(TagVal::_N7);
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn new(neg: bool, val: NonZeroU8) -> RawTag {
+        unsafe { Self::new_unchecked(neg, val.get() & 0x07) }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn new_checked(neg: bool, val: u8) -> Option<RawTag> {
+        Some(RawTag(match (neg, val) {
+            (false, 1) => TagVal::_P1,
+            (false, 2) => TagVal::_P2,
+            (false, 3) => TagVal::_P3,
+            (false, 4) => TagVal::_P4,
+            (false, 5) => TagVal::_P5,
+            (false, 6) => TagVal::_P6,
+            (false, 7) => TagVal::_P7,
+
+            (true, 1) => TagVal::_N1,
+            (true, 2) => TagVal::_N2,
+            (true, 3) => TagVal::_N3,
+            (true, 4) => TagVal::_N4,
+            (true, 5) => TagVal::_N5,
+            (true, 6) => TagVal::_N6,
+            (true, 7) => TagVal::_N7,
+
+            _ => return None,
+        }))
+    }
+
+    /// # Safety
+    ///
+    /// `val` must be in the range `1..8`
+    #[inline]
+    #[must_use]
+    pub(crate) const unsafe fn new_unchecked(neg: bool, val: u8) -> RawTag {
+        RawTag(match (neg, val) {
+            (false, 1) => TagVal::_P1,
+            (false, 2) => TagVal::_P2,
+            (false, 3) => TagVal::_P3,
+            (false, 4) => TagVal::_P4,
+            (false, 5) => TagVal::_P5,
+            (false, 6) => TagVal::_P6,
+            (false, 7) => TagVal::_P7,
+
+            (true, 1) => TagVal::_N1,
+            (true, 2) => TagVal::_N2,
+            (true, 3) => TagVal::_N3,
+            (true, 4) => TagVal::_N4,
+            (true, 5) => TagVal::_N5,
+            (true, 6) => TagVal::_N6,
+            (true, 7) => TagVal::_N7,
+
+            _ => unsafe { core::hint::unreachable_unchecked() },
+        })
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn is_neg(self) -> bool {
+        matches!(self.0, |TagVal::_N1| TagVal::_N2
+            | TagVal::_N3
+            | TagVal::_N4
+            | TagVal::_N5
+            | TagVal::_N6
+            | TagVal::_N7)
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn val(self) -> NonZeroU8 {
+        match self.0 {
+            TagVal::_P1 | TagVal::_N1 => NonZeroU8::MIN,
+            TagVal::_P2 | TagVal::_N2 => NonZeroU8::MIN.saturating_add(1),
+            TagVal::_P3 | TagVal::_N3 => NonZeroU8::MIN.saturating_add(2),
+            TagVal::_P4 | TagVal::_N4 => NonZeroU8::MIN.saturating_add(3),
+            TagVal::_P5 | TagVal::_N5 => NonZeroU8::MIN.saturating_add(4),
+            TagVal::_P6 | TagVal::_N6 => NonZeroU8::MIN.saturating_add(5),
+            TagVal::_P7 | TagVal::_N7 => NonZeroU8::MIN.saturating_add(6),
+        }
+    }
+
+    #[inline]
+    #[must_use]
+    pub const fn neg_val(self) -> (bool, u8) {
+        match self.0 {
+            TagVal::_P1 => (false, 1),
+            TagVal::_P2 => (false, 2),
+            TagVal::_P3 => (false, 3),
+            TagVal::_P4 => (false, 4),
+            TagVal::_P5 => (false, 5),
+            TagVal::_P6 => (false, 6),
+            TagVal::_P7 => (false, 7),
+            TagVal::_N1 => (true, 1),
+            TagVal::_N2 => (true, 2),
+            TagVal::_N3 => (true, 3),
+            TagVal::_N4 => (true, 4),
+            TagVal::_N5 => (true, 5),
+            TagVal::_N6 => (true, 6),
+            TagVal::_N7 => (true, 7),
+        }
+    }
+}
+
+impl fmt::Debug for RawTag {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("RawTag")
+            .field("neg", &self.is_neg())
+            .field("val", &self.val())
+            .finish()
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+#[repr(transparent)]
+struct Header(u16);
+
+impl Header {
+    #[inline]
+    fn new(tag: RawTag) -> Header {
+        let (neg, val) = tag.neg_val();
+        Header(0x7FF8 | (u16::from(neg) << 15) | u16::from(val))
+    }
+
+    #[inline]
+    const fn tag(self) -> RawTag {
+        unsafe { RawTag::new_unchecked(self.get_sign(), self.get_tag()) }
+    }
+
+    #[inline]
+    const fn get_sign(self) -> bool {
+        self.0 & 0x8000 != 0
+    }
+
+    #[inline]
+    const fn get_tag(self) -> u8 {
+        (self.0 & 0x0007) as u8
+    }
+
+    #[inline]
+    fn into_raw(self) -> u16 {
+        self.0
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+#[repr(C, align(8))]
+pub struct Value {
+    #[cfg(target_endian = "big")]
+    header: Header,
+    data: [u8; 6],
+    #[cfg(target_endian = "little")]
+    header: Header,
+}
+
+impl Value {
+    #[inline]
+    pub(crate) fn new(tag: RawTag, data: [u8; 6]) -> Value {
+        Value {
+            header: Header::new(tag),
+            data,
+        }
+    }
+
+    #[inline]
+    pub(crate) fn empty(tag: RawTag) -> Value {
+        Value::new(tag, [0; 6])
+    }
+
+    pub(crate) fn store<T: RawStore>(tag: RawTag, val: T) -> Value {
+        let mut v = Value::new(tag, [0; 6]);
+        T::to_val(val, &mut v);
+        v
+    }
+
+    pub(crate) fn load<T: RawStore>(self) -> T {
+        T::from_val(&self)
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) const fn tag(&self) -> RawTag {
+        self.header.tag()
+    }
+
+    #[inline]
+    fn header(&self) -> &Header {
+        &self.header
+    }
+
+    #[inline]
+    pub(crate) fn set_data(&mut self, val: [u8; 6]) {
+        self.data = val;
+    }
+
+    #[inline]
+    #[must_use]
+    pub fn data(&self) -> &[u8; 6] {
+        &self.data
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn data_mut(&mut self) -> &mut [u8; 6] {
+        &mut self.data
+    }
+
+    #[inline]
+    #[must_use]
+    unsafe fn whole(&self) -> &[u8; 8] {
+        let ptr = (self as *const Value).cast::<[u8; 8]>();
+        unsafe { &*ptr }
+    }
+
+    #[inline]
+    #[must_use]
+    unsafe fn whole_mut(&mut self) -> &mut [u8; 8] {
+        let ptr = (self as *mut Value).cast::<[u8; 8]>();
+        unsafe { &mut *ptr }
+    }
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub(crate) union RawBox {
+    float: f64,
+    value: Value,
+    bits: u64,
+    #[cfg(target_pointer_width = "64")]
+    ptr: *const (),
+    #[cfg(target_pointer_width = "32")]
+    ptr: (u32, *const ()),
+}
+
+impl RawBox {
+    #[inline]
+    #[must_use]
+    pub(crate) fn from_float(val: f64) -> RawBox {
+        match (val.is_nan(), val.is_sign_positive()) {
+            (true, true) => RawBox {
+                float: f64::from_bits(QUIET_NAN),
+            },
+            (true, false) => RawBox {
+                float: f64::from_bits(NEG_QUIET_NAN),
+            },
+            (false, _) => RawBox { float: val },
+        }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn from_value(value: Value) -> RawBox {
+        RawBox { value }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) const fn tag(&self) -> Option<RawTag> {
+        if self.is_value() {
+            Some(unsafe { self.value.tag() })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn is_float(&self) -> bool {
+        (unsafe { !self.float.is_nan() } || unsafe { self.bits & SIGN_MASK == QUIET_NAN })
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) const fn is_value(&self) -> bool {
+        (unsafe { self.float.is_nan() } && unsafe { self.bits & SIGN_MASK != QUIET_NAN })
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn float(&self) -> Option<&f64> {
+        if self.is_float() {
+            Some(unsafe { &self.float })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn value(&self) -> Option<&Value> {
+        if self.is_value() {
+            Some(unsafe { &self.value })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    pub(crate) fn into_float_unchecked(self) -> f64 {
+        unsafe { self.float }
+    }
+
+    #[inline]
+    #[must_use]
+    pub(crate) fn to_bits(self) -> u64 {
+        unsafe { self.bits }
+    }
+}
+
+impl fmt::Debug for RawBox {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.float() {
+            Some(val) => f.debug_tuple("RawBox::Float").field(val).finish(),
+            None => {
+                let val = self.value().expect("RawBox is neither float nor value");
+
+                f.debug_struct("RawBox::Data")
+                    .field("tag", &val.tag())
+                    .field("data", val.data())
+                    .finish()
+            }
+        }
+    }
+}

fix-runtime/src/forced.rs

@@ -0,0 +1,272 @@
+use fix_lang::StringId;
+use gc_arena::{Gc, Mutation};
+
+use crate::{
+    AttrSet, Break, BytecodeReader, Closure, List, Machine, NixNum, NixString, NixType, Null,
+    PrimOp, PrimOpApp, Step, StrictValue,
+};
+
+pub trait Forced<'gc>: Sized {
+    const WIDTH: usize;
+
+    /// Force and type-check the `WIDTH` slots starting at `base_depth` from
+    /// TOS, deepest-first. If a slot holds a thunk, enter it and return
+    /// `Break::Force`. If a slot holds a value of the wrong type, call
+    /// `finish_type_err` and return `Break::Done`.
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+        resume_pc: usize,
+    ) -> Step;
+
+    /// After `force_and_check` returned `Continue`, pop `WIDTH` slots
+    /// (TOS first) and convert. Type assertions are infallible because
+    /// `force_and_check` already validated every slot.
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self;
+}
+
+impl<'gc> Forced<'gc> for StrictValue<'gc> {
+    const WIDTH: usize = 1;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+        resume_pc: usize,
+    ) -> Step {
+        m.force_slot_to_pc(base_depth, reader, mc, resume_pc)
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        m.pop_forced()
+    }
+}
+
+macro_rules! impl_forced_inline {
+    ($($ty:ty => $nix_ty:expr),* $(,)?) => {
+        $(
+        impl<'gc> Forced<'gc> for $ty {
+            const WIDTH: usize = 1;
+
+            #[inline(always)]
+            fn force_and_check<M: Machine<'gc>>(
+                m: &mut M,
+                reader: &mut BytecodeReader<'_>,
+                mc: &Mutation<'gc>,
+                base_depth: usize,
+                resume_pc: usize,
+            ) -> Step {
+                m.force_slot_to_pc(base_depth, reader, mc, resume_pc)?;
+                let v = m.peek_forced(base_depth);
+                if v.as_inline::<$ty>().is_none() {
+                    let _: Step = m.finish_type_err($nix_ty, v.ty());
+                    return Step::Break(Break::Done);
+                }
+                Step::Continue(())
+            }
+
+            #[inline(always)]
+            fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+                m.pop_forced()
+                    .as_inline::<$ty>()
+                    .expect("type checked in force_and_check")
+            }
+        }
+        )*
+    };
+}
+
+macro_rules! impl_forced_gc {
+    ($($ty:ty => $nix_ty:expr),* $(,)?) => {
+        $(
+        impl<'gc> Forced<'gc> for Gc<'gc, $ty> {
+            const WIDTH: usize = 1;
+
+            #[inline(always)]
+            fn force_and_check<M: Machine<'gc>>(
+                m: &mut M,
+                reader: &mut BytecodeReader<'_>,
+                mc: &Mutation<'gc>,
+                base_depth: usize,
+                resume_pc: usize,
+            ) -> Step {
+                m.force_slot_to_pc(base_depth, reader, mc, resume_pc)?;
+                let v = m.peek_forced(base_depth);
+                if v.as_gc::<$ty>().is_none() {
+                    let _: Step = m.finish_type_err($nix_ty, v.ty());
+                    return Step::Break(Break::Done);
+                }
+                Step::Continue(())
+            }
+
+            #[inline(always)]
+            fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+                m.pop_forced()
+                    .as_gc::<$ty>()
+                    .expect("type checked in force_and_check")
+            }
+        }
+        )*
+    };
+}
+
+impl_forced_inline! {
+    i32 => NixType::Int,
+    bool => NixType::Bool,
+    Null => NixType::Null,
+    StringId => NixType::String,
+    PrimOp => NixType::PrimOp,
+}
+
+impl_forced_gc! {
+    i64 => NixType::Int,
+    NixString => NixType::String,
+    AttrSet<'gc> => NixType::AttrSet,
+    List<'gc> => NixType::List,
+    Closure<'gc> => NixType::Closure,
+    PrimOpApp<'gc> => NixType::PrimOpApp,
+}
+
+impl<'gc> Forced<'gc> for NixNum {
+    const WIDTH: usize = 1;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+        resume_pc: usize,
+    ) -> Step {
+        m.force_slot_to_pc(base_depth, reader, mc, resume_pc)?;
+        let v = m.peek_forced(base_depth);
+        if v.as_num().is_none() {
+            let _: Step = m.finish_type_err(NixType::Int, v.ty());
+            return Step::Break(Break::Done);
+        }
+        Step::Continue(())
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        m.pop_forced()
+            .as_num()
+            .expect("type checked in force_and_check")
+    }
+}
+
+impl<'gc> Forced<'gc> for f64 {
+    const WIDTH: usize = 1;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+        resume_pc: usize,
+    ) -> Step {
+        m.force_slot_to_pc(base_depth, reader, mc, resume_pc)?;
+        let v = m.peek_forced(base_depth);
+        if v.as_float().is_none() {
+            let _: Step = m.finish_type_err(NixType::Float, v.ty());
+            return Step::Break(Break::Done);
+        }
+        Step::Continue(())
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        m.pop_forced()
+            .as_float()
+            .expect("type checked in force_and_check")
+    }
+}
+
+impl<'gc, A: Forced<'gc>, B: Forced<'gc>> Forced<'gc> for (A, B) {
+    const WIDTH: usize = A::WIDTH + B::WIDTH;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+        resume_pc: usize,
+    ) -> Step {
+        A::force_and_check(m, reader, mc, base + B::WIDTH, resume_pc)?;
+        B::force_and_check(m, reader, mc, base, resume_pc)
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        let b = B::pop_converted(m);
+        let a = A::pop_converted(m);
+        (a, b)
+    }
+}
+
+impl<'gc, A: Forced<'gc>, B: Forced<'gc>, C: Forced<'gc>> Forced<'gc> for (A, B, C) {
+    const WIDTH: usize = A::WIDTH + B::WIDTH + C::WIDTH;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+        resume_pc: usize,
+    ) -> Step {
+        A::force_and_check(m, reader, mc, base + B::WIDTH + C::WIDTH, resume_pc)?;
+        B::force_and_check(m, reader, mc, base + C::WIDTH, resume_pc)?;
+        C::force_and_check(m, reader, mc, base, resume_pc)
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        let c = C::pop_converted(m);
+        let b = B::pop_converted(m);
+        let a = A::pop_converted(m);
+        (a, b, c)
+    }
+}
+
+impl<'gc, A: Forced<'gc>, B: Forced<'gc>, C: Forced<'gc>, D: Forced<'gc>> Forced<'gc>
+    for (A, B, C, D)
+{
+    const WIDTH: usize = A::WIDTH + B::WIDTH + C::WIDTH + D::WIDTH;
+
+    #[inline(always)]
+    fn force_and_check<M: Machine<'gc>>(
+        m: &mut M,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+        resume_pc: usize,
+    ) -> Step {
+        A::force_and_check(
+            m,
+            reader,
+            mc,
+            base + B::WIDTH + C::WIDTH + D::WIDTH,
+            resume_pc,
+        )?;
+        B::force_and_check(m, reader, mc, base + C::WIDTH + D::WIDTH, resume_pc)?;
+        C::force_and_check(m, reader, mc, base + D::WIDTH, resume_pc)?;
+        D::force_and_check(m, reader, mc, base, resume_pc)
+    }
+
+    #[inline(always)]
+    fn pop_converted<M: Machine<'gc>>(m: &mut M) -> Self {
+        let d = D::pop_converted(m);
+        let c = C::pop_converted(m);
+        let b = B::pop_converted(m);
+        let a = A::pop_converted(m);
+        (a, b, c, d)
+    }
+}

fix-runtime/src/host.rs

@@ -0,0 +1,165 @@
+use fix_bytecode::InstructionPtr;
+use fix_error::Source;
+use fix_lang::{self, BUILTINS, StringId};
+use hashbrown::HashSet;
+
+use crate::{
+    AttrSet, Closure, ExtraScope, List, NixString, NixType, Null, Path, PrimOp, PrimOpApp,
+    StaticValue, StrictValue, StringContext, Thunk, ThunkState, Value,
+};
+
+pub trait VmContext {
+    fn split(&mut self) -> (&mut impl VmCode, &mut impl VmRuntimeCtx);
+}
+
+pub trait VmRuntimeCtx {
+    fn intern_string(&mut self, s: impl AsRef<str>) -> StringId;
+    fn resolve_string(&self, id: StringId) -> &str;
+    fn get_const(&self, id: u32) -> StaticValue;
+    fn add_const(&mut self, val: StaticValue) -> u32;
+}
+
+pub trait VmCode {
+    fn bytecode(&self) -> &[u8];
+    fn compile_with_scope(
+        &mut self,
+        source: Source,
+        extra_scope: Option<ExtraScope>,
+        ctx: &mut impl VmRuntimeCtx,
+    ) -> fix_error::Result<InstructionPtr>;
+}
+
+pub trait VmRuntimeCtxExt: VmRuntimeCtx {
+    fn get_string<'a, 'gc: 'a>(&'a self, val: StrictValue<'gc>) -> Option<&'a str>;
+    fn get_string_or_path<'a, 'gc: 'a>(&'a self, val: StrictValue<'gc>) -> Option<&'a str>;
+    fn get_string_id<'a, 'gc: 'a>(
+        &'a mut self,
+        val: StrictValue<'gc>,
+    ) -> std::result::Result<StringId, NixType>;
+    /// Returns the string context attached to `val`, or `&[]` if `val` is
+    /// either a non-string or a string without context.
+    fn get_string_context<'gc>(&self, val: StrictValue<'gc>) -> &'gc StringContext;
+    fn convert_value(&self, val: Value) -> fix_lang::Value;
+}
+
+impl<T: VmRuntimeCtx> VmRuntimeCtxExt for T {
+    fn get_string<'a, 'gc: 'a>(&'a self, val: StrictValue<'gc>) -> Option<&'a str> {
+        if let Some(sid) = val.as_inline::<StringId>() {
+            Some(self.resolve_string(sid))
+        } else {
+            val.as_gc::<NixString>().map(|ns| ns.as_ref().as_str())
+        }
+    }
+
+    /// Like `get_string`, but also accepts `Path` values (returning their
+    /// underlying canonical-path string). Use this in places where Nix
+    /// would coerce a path to a string (string interpolation, file IO
+    /// builtins, etc.).
+    fn get_string_or_path<'a, 'gc: 'a>(&'a self, val: StrictValue<'gc>) -> Option<&'a str> {
+        if let Some(p) = val.as_inline::<Path>() {
+            Some(self.resolve_string(p.0))
+        } else {
+            self.get_string(val)
+        }
+    }
+
+    fn get_string_id<'a, 'gc: 'a>(
+        &'a mut self,
+        val: StrictValue<'gc>,
+    ) -> std::result::Result<StringId, NixType> {
+        if let Some(sid) = val.as_inline::<StringId>() {
+            Ok(sid)
+        } else if let Some(s) = val.as_gc::<NixString>().map(|ns| ns.as_ref().as_str()) {
+            Ok(self.intern_string(s))
+        } else {
+            Err(val.ty())
+        }
+    }
+
+    fn get_string_context<'gc>(&self, val: StrictValue<'gc>) -> &'gc StringContext {
+        if let Some(ns) = val.as_gc::<NixString>() {
+            ns.as_ref().context()
+        } else {
+            StringContext::empty()
+        }
+    }
+
+    fn convert_value(&self, val: Value) -> fix_lang::Value {
+        self.convert_value_with_seen(val, &mut HashSet::new())
+    }
+}
+
+pub(crate) trait ConvertValueWithSeen: VmRuntimeCtx {
+    fn convert_value_with_seen(&self, val: Value, seen: &mut HashSet<u64>) -> fix_lang::Value;
+}
+
+impl<T: VmRuntimeCtx> ConvertValueWithSeen for T {
+    fn convert_value_with_seen(&self, val: Value, seen: &mut HashSet<u64>) -> fix_lang::Value {
+        use fix_lang::Value;
+        if let Some(i) = val.as_inline::<i32>() {
+            Value::Int(i as i64)
+        } else if let Some(gc_i) = val.as_gc::<i64>() {
+            Value::Int(*gc_i)
+        } else if let Some(f) = val.as_float() {
+            Value::Float(f)
+        } else if let Some(b) = val.as_inline::<bool>() {
+            Value::Bool(b)
+        } else if val.is::<Null>() {
+            Value::Null
+        } else if let Some(sid) = val.as_inline::<StringId>() {
+            let s = self.resolve_string(sid).to_owned();
+            Value::String(s)
+        } else if let Some(ns) = val.as_gc::<NixString>() {
+            Value::String(ns.as_str().to_owned())
+        } else if let Some(p) = val.as_inline::<Path>() {
+            Value::Path(self.resolve_string(p.0).to_owned())
+        } else if let Some(attrs) = val.as_gc::<AttrSet>() {
+            let bits = val.to_bits();
+            if attrs.entries.is_empty() {
+                return Value::AttrSet(Default::default());
+            }
+            if !seen.insert(bits) {
+                return Value::Repeated;
+            }
+            let mut map = std::collections::BTreeMap::new();
+            for &(key, val) in attrs.entries.iter() {
+                let key = self.resolve_string(key).to_owned();
+                let converted = self.convert_value_with_seen(val, seen);
+                map.insert(fix_lang::Symbol::from(key), converted);
+            }
+            Value::AttrSet(fix_lang::AttrSet::new(map))
+        } else if let Some(list) = val.as_gc::<List>() {
+            let bits = val.to_bits();
+            if list.inner.borrow().is_empty() {
+                return Value::List(Default::default());
+            }
+            if !seen.insert(bits) {
+                return Value::Repeated;
+            }
+            let items: Vec<_> = list
+                .inner
+                .borrow()
+                .iter()
+                .copied()
+                .map(|v| self.convert_value_with_seen(v, seen))
+                .collect();
+            Value::List(fix_lang::List::new(items))
+        } else if val.is::<Closure>() {
+            Value::Func
+        } else if let Some(thunk) = val.as_gc::<Thunk>() {
+            if let ThunkState::Evaluated(v) = *thunk.borrow() {
+                self.convert_value_with_seen(v.relax(), seen)
+            } else {
+                Value::Thunk
+            }
+        } else if let Some(primop) = val.as_inline::<PrimOp>() {
+            let name = BUILTINS[primop.id as usize].0;
+            Value::PrimOp(name.strip_prefix("__").unwrap_or(name))
+        } else if let Some(app) = val.as_gc::<PrimOpApp>() {
+            let name = BUILTINS[app.primop.id as usize].0;
+            Value::PrimOpApp(name.strip_prefix("__").unwrap_or(name))
+        } else {
+            Value::Null
+        }
+    }
+}

fix-runtime/src/lib.rs

@@ -0,0 +1,19 @@
+mod boxing;
+mod forced;
+mod host;
+mod machine;
+mod path_util;
+mod resolve;
+mod state;
+mod string_context;
+mod value;
+
+pub use fix_bytecode::{BytecodeReader, OperandData};
+pub use forced::*;
+pub use host::*;
+pub use machine::*;
+pub use path_util::*;
+pub use resolve::*;
+pub use state::*;
+pub use string_context::*;
+pub use value::*;

fix-runtime/src/machine.rs

@@ -0,0 +1,174 @@
+use std::ops::ControlFlow;
+use std::path::{Path, PathBuf};
+
+use fix_error::Error;
+use fix_lang::{self, StringId};
+use gc_arena::Mutation;
+
+use crate::{
+    Break, BytecodeReader, CallFrame, ForceMode, Forced, GcEnv, NixType, PendingLoad, Step,
+    StrictValue, Value, VmError,
+};
+
+/// Abstract VM-side operations consumed by instruction handlers and primops.
+///
+/// Implementors maintain a value stack, a call stack, an environment chain,
+/// pending result/error state, and a set of GC-allocated globals. Methods
+/// fall into a few groups:
+///
+/// - Stack ops (`push` / `pop` / `peek` / `replace` / `pop_forced` / ...)
+/// - Forcing primitives (`force_slot` / `force_slot_to_pc`)
+/// - Calling (`call` / `return_from_primop`)
+/// - Call-frame management (`push_call_frame` / `pop_call_frame` / call-depth)
+/// - Environment access (`env` / `set_env` / `local`)
+/// - Result finalization (`finish_ok` / `finish_err` / ...)
+/// - Global lookup (`builtins` / `empty_list` / `empty_attrs` / ...)
+/// - Imports and scope slots (`import_cache_*` / `scope_slot*` / `set_pending_load`)
+pub trait Machine<'gc> {
+    fn push(&mut self, val: Value<'gc>);
+    fn pop(&mut self) -> Value<'gc>;
+    fn peek(&self, depth: usize) -> Value<'gc>;
+    fn peek_forced(&self, depth: usize) -> StrictValue<'gc>;
+    fn pop_forced(&mut self) -> StrictValue<'gc>;
+    fn replace(&mut self, depth: usize, val: Value<'gc>);
+    fn stack_len(&self) -> usize;
+
+    fn force_slot_to_pc(
+        &mut self,
+        depth: usize,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        resume_pc: usize,
+    ) -> Step;
+
+    #[inline(always)]
+    fn force_slot(
+        &mut self,
+        depth: usize,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+    ) -> Step {
+        let pc = reader.inst_start_pc();
+        self.force_slot_to_pc(depth, reader, mc, pc)
+    }
+
+    fn call(
+        &mut self,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        arg: Value<'gc>,
+        resume_pc: usize,
+    ) -> Step;
+
+    #[inline(always)]
+    fn return_from_primop(&mut self, val: Value<'gc>, reader: &mut BytecodeReader<'_>) -> Step {
+        self.push(val);
+        let Some(CallFrame {
+            pc: ret_pc,
+            thunk: _,
+            env,
+        }) = self.pop_call_frame()
+        else {
+            unreachable!()
+        };
+        reader.set_pc(ret_pc);
+        self.dec_call_depth();
+        self.set_env(env);
+        Step::Continue(())
+    }
+
+    fn push_call_frame(&mut self, frame: CallFrame<'gc>);
+    fn pop_call_frame(&mut self) -> Option<CallFrame<'gc>>;
+    fn call_depth(&self) -> usize;
+    fn inc_call_depth(&mut self);
+    fn dec_call_depth(&mut self);
+
+    fn env(&self) -> GcEnv<'gc>;
+    fn set_env(&mut self, env: GcEnv<'gc>);
+
+    #[inline(always)]
+    fn local(&self, layer: u8, idx: u32) -> Value<'gc> {
+        let mut cur = self.env();
+        for _ in 0..layer {
+            let prev = cur.borrow().prev.expect("env chain too short");
+            cur = prev;
+        }
+        cur.borrow().locals[idx as usize]
+    }
+
+    fn finish_ok(&mut self, val: fix_lang::Value) -> Step;
+    fn finish_err(&mut self, err: Box<Error>) -> Step;
+    fn finish_type_err(&mut self, expected: NixType, got: NixType) -> Step;
+
+    #[inline(always)]
+    fn finish_vm_err(&mut self, err: VmError) -> Step {
+        self.finish_err(err.into_error())
+    }
+
+    fn builtins(&self) -> Value<'gc>;
+    fn functor_sym(&self) -> StringId;
+    fn empty_list(&self) -> Value<'gc>;
+    fn empty_attrs(&self) -> Value<'gc>;
+    fn force_mode(&self) -> ForceMode;
+
+    fn import_cache_get(&self, path: &Path) -> Option<Value<'gc>>;
+    fn import_cache_insert(&mut self, path: PathBuf, val: Value<'gc>);
+    fn scope_slot(&self, idx: u32) -> Value<'gc>;
+    fn scope_slots_push(&mut self, val: Value<'gc>) -> u32;
+    fn set_pending_load(&mut self, load: PendingLoad);
+}
+
+/// Extension trait with convenience helpers built on top of [`Machine`].
+///
+/// Auto-implemented for every `Machine<'gc>` so callers just need to bring
+/// `MachineExt` (or `Machine`) into scope.
+pub trait MachineExt<'gc>: Machine<'gc> {
+    /// Force the top `T::WIDTH` stack slots and return them as `T`.
+    ///
+    /// If any slot holds a pending thunk, this method pushes a call frame
+    /// whose resume PC is the **start of the current instruction**
+    /// (`reader.inst_start_pc()`), enters the thunk, and returns
+    /// `Break::Force`. When the thunk eventually returns, the VM will
+    /// **re-execute the entire opcode handler from the beginning**.
+    ///
+    /// # Invariants
+    ///
+    /// * **Do not call this method more than once in a single handler.**
+    ///   If you need to force multiple values, use a tuple type such as
+    ///   `(StrictValue, StrictValue)` so they are forced and popped in one
+    ///   atomic operation.
+    /// * The stack layout at the call site must be **identical** every time
+    ///   the handler is re-entered.
+    /// * Propagate the return value with `?` so `Break::Force` correctly
+    ///   unwinds to the dispatch loop.
+    #[inline(always)]
+    fn force_and_retry<T: Forced<'gc>>(
+        &mut self,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+    ) -> ControlFlow<Break, T>
+    where
+        Self: Sized,
+    {
+        let pc = reader.inst_start_pc();
+        self.force_and_retry_pc(reader, mc, pc)
+    }
+
+    /// Same as [`force_and_retry`](Self::force_and_retry) but allows
+    /// specifying a custom resume PC.
+    #[inline(always)]
+    fn force_and_retry_pc<T: Forced<'gc>>(
+        &mut self,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        resume_pc: usize,
+    ) -> ControlFlow<Break, T>
+    where
+        Self: Sized,
+    {
+        T::force_and_check(self, reader, mc, 0, resume_pc)?;
+        ControlFlow::Continue(T::pop_converted(self))
+    }
+}
+
+impl<'gc, M: Machine<'gc>> MachineExt<'gc> for M {}

fix-runtime/src/path_util.rs

@@ -0,0 +1,18 @@
+use std::path::{Component, PathBuf};
+
+pub fn canon_path_str(path: impl AsRef<std::path::Path>) -> String {
+    let p = path.as_ref();
+    let mut normalized = PathBuf::new();
+    for component in p.components() {
+        match component {
+            Component::Prefix(p) => normalized.push(p.as_os_str()),
+            Component::RootDir => normalized.push("/"),
+            Component::CurDir => {}
+            Component::ParentDir => {
+                normalized.pop();
+            }
+            Component::Normal(c) => normalized.push(c),
+        }
+    }
+    normalized.to_string_lossy().into_owned()
+}

fix-runtime/src/resolve.rs

@@ -0,0 +1,35 @@
+use fix_bytecode::OperandData;
+use gc_arena::{Gc, Mutation};
+
+use crate::{AttrSet, Machine, Value, VmRuntimeCtx};
+
+/// Resolve a decoded operand into a runtime [`Value`].
+///
+/// The operand decoder ([`crate::BytecodeReader::read_operand_data`])
+/// produces a static enum; this function materializes it against the
+/// running [`Machine`] (env chain, builtins, scope slots, ...).
+#[inline]
+pub fn resolve_operand<'gc, M: Machine<'gc>>(
+    op: &OperandData,
+    mc: &Mutation<'gc>,
+    ctx: &impl VmRuntimeCtx,
+    m: &M,
+) -> Value<'gc> {
+    use OperandData::*;
+    match *op {
+        Const(id) => ctx.get_const(id).into(),
+        BigInt(val) => Value::new_gc(Gc::new(mc, val)),
+        Local { layer, idx } => m.local(layer, idx),
+        #[allow(clippy::unwrap_used)]
+        BuiltinConst(id) => m.builtins().as_gc::<AttrSet>().unwrap().lookup(id).unwrap(),
+        Builtins => m.builtins(),
+        ReplBinding(_id) => todo!(),
+        ScopedImportBinding { slot_id, name } => {
+            let scope = m.scope_slot(slot_id);
+            #[allow(clippy::unwrap_used)]
+            let attrs = scope.as_gc::<AttrSet>().expect("scope must be attrset");
+            #[allow(clippy::unwrap_used)]
+            attrs.lookup(name).expect("scoped binding not found")
+        }
+    }
+}

fix-runtime/src/state.rs

@@ -0,0 +1,89 @@
+use std::ops::ControlFlow;
+use std::path::PathBuf;
+
+use fix_error::Error;
+use fix_lang::StringId;
+use gc_arena::{Collect, Gc};
+use hashbrown::HashSet;
+
+use crate::{GcEnv, Thunk};
+
+#[allow(dead_code)]
+pub enum VmError {
+    Catchable(String),
+    Uncatchable(Box<Error>),
+}
+
+impl From<Box<Error>> for VmError {
+    fn from(e: Box<Error>) -> Self {
+        VmError::Uncatchable(e)
+    }
+}
+
+impl VmError {
+    pub fn into_error(self) -> Box<Error> {
+        match self {
+            VmError::Catchable(_) => todo!("Check for tryEval catch frames"),
+            VmError::Uncatchable(e) => e,
+        }
+    }
+}
+
+pub fn vm_err(msg: impl Into<String>) -> VmError {
+    VmError::Uncatchable(Error::eval_error(msg.into()))
+}
+
+#[derive(Collect, Clone, Copy, Debug, PartialEq, Eq, Default)]
+#[collect(require_static)]
+pub enum ForceMode {
+    #[default]
+    AsIs,
+    Shallow,
+    Deep,
+}
+
+#[repr(u8)]
+pub enum Break {
+    Force,
+    Done,
+    LoadFile,
+}
+
+pub type Step = ControlFlow<Break>;
+
+#[allow(dead_code)]
+pub struct ErrorFrame {
+    pub span_id: u32,
+    pub message: Option<String>,
+}
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub struct CallFrame<'gc> {
+    pub pc: usize,
+    pub thunk: Option<Gc<'gc, Thunk<'gc>>>,
+    pub env: GcEnv<'gc>,
+}
+
+#[derive(Debug)]
+pub struct PendingLoad {
+    pub path: PathBuf,
+    pub scope: Option<PendingScope>,
+}
+
+#[derive(Debug)]
+pub struct PendingScope {
+    pub keys: HashSet<StringId>,
+    pub slot_id: u32,
+}
+
+/// Extra scope passed to a re-entrant compile from inside a running VM.
+///
+/// Currently only `ScopedImport` is produced (by the `scopedImport` builtin),
+/// but the variant is kept open so REPL bindings could later land here too.
+pub enum ExtraScope {
+    ScopedImport {
+        keys: HashSet<StringId>,
+        slot_id: u32,
+    },
+}

fix-runtime/src/string_context.rs

@@ -0,0 +1,161 @@
+use std::cmp::Ordering;
+
+use smallvec::SmallVec;
+
+/// A string context element
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum StringContextElem {
+    // Plain store path reference
+    Opaque {
+        path: Box<str>,
+    },
+    // All outputs of a derivation
+    // encoded `=<drvPath>`
+    DrvDeep {
+        drv_path: Box<str>,
+    },
+    // A specific output of a derivation
+    // encoded `!<output>!<drvPath>`
+    Built {
+        drv_path: Box<str>,
+        output: Box<str>,
+    },
+}
+
+impl StringContextElem {
+    /// Decode the CppNix wire form (`!out!/p`, `=/p`, `/p`). Falls back to
+    /// `Opaque` for malformed `!`-prefixed inputs (matching nix-js).
+    pub fn decode(encoded: &str) -> Self {
+        if let Some(drv_path) = encoded.strip_prefix('=') {
+            Self::DrvDeep {
+                drv_path: drv_path.into(),
+            }
+        } else if let Some(rest) = encoded.strip_prefix('!') {
+            if let Some(second_bang) = rest.find('!') {
+                Self::Built {
+                    output: rest[..second_bang].into(),
+                    drv_path: rest[second_bang + 1..].into(),
+                }
+            } else {
+                Self::Opaque {
+                    path: encoded.into(),
+                }
+            }
+        } else {
+            Self::Opaque {
+                path: encoded.into(),
+            }
+        }
+    }
+
+    pub fn encode(&self) -> String {
+        match self {
+            Self::Opaque { path } => path.to_string(),
+            Self::DrvDeep { drv_path } => format!("={drv_path}"),
+            Self::Built { drv_path, output } => format!("!{output}!{drv_path}"),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct StringContext {
+    data: SmallVec<[StringContextElem; 1]>,
+}
+
+impl IntoIterator for StringContext {
+    type Item = StringContextElem;
+    type IntoIter = <SmallVec<[StringContextElem; 1]> as IntoIterator>::IntoIter;
+    fn into_iter(self) -> Self::IntoIter {
+        self.data.into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a StringContext {
+    type Item = &'a StringContextElem;
+    type IntoIter = <&'a SmallVec<[StringContextElem; 1]> as IntoIterator>::IntoIter;
+    fn into_iter(self) -> Self::IntoIter {
+        self.data.iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a mut StringContext {
+    type Item = &'a mut StringContextElem;
+    type IntoIter = <&'a mut SmallVec<[StringContextElem; 1]> as IntoIterator>::IntoIter;
+    fn into_iter(self) -> Self::IntoIter {
+        self.data.iter_mut()
+    }
+}
+
+impl FromIterator<StringContextElem> for StringContext {
+    fn from_iter<T: IntoIterator<Item = StringContextElem>>(iter: T) -> Self {
+        Self {
+            data: iter.into_iter().collect(),
+        }
+    }
+}
+
+impl StringContext {
+    pub fn empty() -> &'static Self {
+        static EMPTY: StringContext = StringContext {
+            data: SmallVec::new_const(),
+        };
+        &EMPTY
+    }
+
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    pub fn insert(&mut self, elem: StringContextElem) {
+        match self.data.binary_search(&elem) {
+            Ok(_) => {}
+            Err(pos) => self.data.insert(pos, elem),
+        }
+    }
+
+    pub fn merge(&self, other: &Self) -> Self {
+        if self.data.is_empty() {
+            return other.clone();
+        }
+        if other.data.is_empty() {
+            return self.clone();
+        }
+
+        let a = &self.data;
+        let b = &other.data;
+        let mut out = SmallVec::with_capacity(a.len() + b.len());
+        let (mut i, mut j) = (0, 0);
+        while i < a.len() && j < b.len() {
+            match a[i].cmp(&b[j]) {
+                Ordering::Less => {
+                    out.push(a[i].clone());
+                    i += 1;
+                }
+                Ordering::Greater => {
+                    out.push(b[j].clone());
+                    j += 1;
+                }
+                Ordering::Equal => {
+                    out.push(a[i].clone());
+                    i += 1;
+                    j += 1;
+                }
+            }
+        }
+        out.extend(a[i..].iter().cloned());
+        out.extend(b[j..].iter().cloned());
+        Self { data: out }
+    }
+
+    pub fn iter(&self) -> <&Self as IntoIterator>::IntoIter {
+        self.into_iter()
+    }
+
+    pub fn iter_mut(&mut self) -> <&mut Self as IntoIterator>::IntoIter {
+        self.into_iter()
+    }
+}

fix-runtime/src/value.rs

@@ -0,0 +1,727 @@
+#![allow(dead_code)]
+
+use std::cell::RefCell;
+use std::fmt;
+use std::marker::PhantomData;
+use std::mem::size_of;
+use std::ops::Deref;
+
+use fix_lang::*;
+use gc_arena::barrier::Unlock;
+use gc_arena::collect::Trace;
+use gc_arena::{Collect, Gc, GcRefLock, Mutation, RefLock};
+use num_enum::TryFromPrimitive;
+use smallvec::SmallVec;
+use string_interner::Symbol;
+use string_interner::symbol::SymbolU32;
+
+use crate::boxing::{RawBox, RawStore, RawTag, Value as RawValue};
+use crate::string_context::StringContext;
+
+mod private {
+    pub trait Cealed {}
+}
+
+/// # Safety
+///
+/// TAG must be unique among all implementors.
+#[allow(private_interfaces)]
+pub unsafe trait Storable: private::Cealed {
+    const TAG: RawTag;
+}
+#[allow(private_bounds)]
+pub trait InlineStorable: Storable + RawStore {}
+pub trait GcStorable: Storable {}
+
+macro_rules! define_value_types {
+    (
+        inline { $($itype:ty => $itag:expr, $iname:literal;)* }
+        gc { $($gtype:ty => $gtag:expr, $gname:literal;)* }
+    ) => {
+        $(
+        #[allow(private_interfaces)]
+        unsafe impl Storable for $itype {
+            const TAG: RawTag = $itag;
+        }
+        impl InlineStorable for $itype {}
+        impl private::Cealed for $itype {}
+        )*
+        $(
+        #[allow(private_interfaces)]
+        unsafe impl Storable for $gtype {
+            const TAG: RawTag = $gtag;
+        }
+        impl GcStorable for $gtype {}
+        impl private::Cealed for $gtype {}
+        )*
+
+        const _: () = assert!(size_of::<Value<'static>>() == 8);
+        $(const _: () = assert!(size_of::<$itype>() <= 6);)*
+
+        const _: () = {
+            let tags: &[(bool, u8)] = &[$(RawTag::neg_val($itag)),*, $(RawTag::neg_val($gtag)),*];
+            let mut mask_false: u8 = 0;
+            let mut mask_true: u8 = 0;
+            let mut i = 0;
+            while i < tags.len() {
+                let (neg, val) = tags[i];
+                let bit = 1 << val;
+                if neg {
+                    assert!(mask_true & bit == 0, "duplicate true tag id");
+                    mask_true |= bit;
+                } else {
+                    assert!(mask_false & bit == 0, "duplicate false tag id");
+                    mask_false |= bit;
+                }
+                i += 1;
+            }
+        };
+
+        unsafe impl<'gc> Collect<'gc> for Value<'gc> {
+            const NEEDS_TRACE: bool = true;
+            fn trace<T: Trace<'gc>>(&self, cc: &mut T) {
+                let Some(tag) = self.raw.tag() else { return };
+                match tag {
+                    $(<$gtype as Storable>::TAG => unsafe {
+                        self.load_gc::<$gtype>().trace(cc)
+                    },)*
+                    $(<$itype as Storable>::TAG => (),)*
+                    _ => unreachable!("invalid value tag"),
+                }
+            }
+        }
+
+        impl fmt::Debug for Value<'_> {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                match self.tag() {
+                    None => write!(f, "Float({:?})", unsafe {
+                        self.raw.float().unwrap_unchecked()
+                    }),
+                    $(Some(<$itype as Storable>::TAG) => write!(f, "{}({:?})", $iname, unsafe {
+                        self.as_inline::<$itype>().unwrap_unchecked()
+                    }),)*
+                    $(Some(<$gtype as Storable>::TAG) =>
+                        write!(f, "{}(..)", $gname),)*
+                    _ => unreachable!("invalid value tag"),
+                }
+            }
+        }
+    };
+}
+
+define_value_types! {
+    inline {
+        i32 => RawTag::P1, "SmallInt";
+        bool => RawTag::P2, "Bool";
+        Null => RawTag::P3, "Null";
+        StringId => RawTag::P4, "SmallString";
+        PrimOp => RawTag::P5, "PrimOp";
+        Path => RawTag::N6, "Path";
+    }
+    gc {
+        i64 => RawTag::P6, "BigInt";
+        NixString => RawTag::P7, "String";
+        AttrSet<'_> => RawTag::N1, "AttrSet";
+        List<'_> => RawTag::N2, "List";
+        Thunk<'_> => RawTag::N3, "Thunk";
+        Closure<'_> => RawTag::N4, "Closure";
+        PrimOpApp<'_> => RawTag::N5, "PrimOpApp";
+    }
+}
+
+/// # Nix runtime value representation
+///
+/// NaN-boxed value fitting in 8 bytes.
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub struct Value<'gc> {
+    raw: RawBox,
+    _marker: PhantomData<Gc<'gc, ()>>,
+}
+
+impl Default for Value<'_> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self::new_inline(Null)
+    }
+}
+
+impl<'gc> Value<'gc> {
+    #[inline(always)]
+    fn from_raw_value(rv: RawValue) -> Self {
+        Self {
+            raw: RawBox::from_value(rv),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Load a GC pointer from a value with a negative tag.
+    ///
+    /// # Safety
+    ///
+    /// The value must actually store a `Gc<'gc, T>` with the matching type.
+    #[inline(always)]
+    unsafe fn load_gc<T: GcStorable>(self) -> Gc<'gc, T> {
+        unsafe {
+            let rv = self.raw.value().unwrap_unchecked();
+            let ptr: *const T = <*const T as RawStore>::from_val(rv);
+            Gc::from_ptr(ptr)
+        }
+    }
+
+    #[inline(always)]
+    const fn tag(self) -> Option<RawTag> {
+        self.raw.tag()
+    }
+}
+
+impl<'gc> Value<'gc> {
+    #[inline]
+    pub fn new_float(val: f64) -> Self {
+        Self {
+            raw: RawBox::from_float(val),
+            _marker: PhantomData,
+        }
+    }
+
+    #[inline]
+    pub fn new_inline<T: InlineStorable>(val: T) -> Self {
+        Self::from_raw_value(RawValue::store(T::TAG, val))
+    }
+
+    #[inline]
+    pub fn new_gc<T: GcStorable>(gc: Gc<'gc, T>) -> Self {
+        let ptr = Gc::as_ptr(gc);
+        Self::from_raw_value(RawValue::store(T::TAG, ptr))
+    }
+
+    #[inline]
+    pub fn make_int(val: i64, mc: &Mutation<'gc>) -> Self {
+        if val >= i32::MIN as i64 && val <= i32::MAX as i64 {
+            Value::new_inline(val as i32)
+        } else {
+            Value::new_gc(Gc::new(mc, val))
+        }
+    }
+}
+
+impl<'gc> Value<'gc> {
+    #[inline]
+    pub fn is_float(self) -> bool {
+        self.raw.is_float()
+    }
+
+    #[inline]
+    pub fn is<T: Storable>(self) -> bool {
+        self.tag() == Some(T::TAG)
+    }
+}
+
+impl<'gc> Value<'gc> {
+    #[inline]
+    pub fn as_float(self) -> Option<f64> {
+        self.raw.float().copied()
+    }
+
+    #[inline]
+    pub fn as_inline<T: InlineStorable>(self) -> Option<T> {
+        if self.is::<T>() {
+            Some(unsafe {
+                let rv = self.raw.value().unwrap_unchecked();
+                T::from_val(rv)
+            })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    pub fn as_gc<T: GcStorable>(self) -> Option<Gc<'gc, T>> {
+        if self.is::<T>() {
+            Some(unsafe {
+                let rv = self.raw.value().unwrap_unchecked();
+                let ptr: *const T = <*const T as RawStore>::from_val(rv);
+                Gc::from_ptr(ptr)
+            })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    pub fn to_bits(self) -> u64 {
+        self.raw.to_bits()
+    }
+
+    #[inline]
+    pub fn as_num(self) -> Option<NixNum> {
+        if let Some(i) = self.as_inline::<i32>() {
+            Some(NixNum::Int(i as i64))
+        } else if let Some(gc_i) = self.as_gc::<i64>() {
+            Some(NixNum::Int(*gc_i))
+        } else {
+            self.as_float().map(NixNum::Float)
+        }
+    }
+
+    #[inline]
+    pub fn restrict(self) -> Result<StrictValue<'gc>, Gc<'gc, Thunk<'gc>>> {
+        if let Some(thunk) = self.as_gc::<Thunk<'gc>>() {
+            Err(thunk)
+        } else {
+            Ok(StrictValue(self))
+        }
+    }
+
+    #[inline]
+    pub fn ty(self) -> NixType {
+        if self.is_float() {
+            NixType::Float
+        } else if self.is::<i32>() || self.is::<i64>() {
+            NixType::Int
+        } else if self.is::<bool>() {
+            NixType::Bool
+        } else if self.is::<Null>() {
+            NixType::Null
+        } else if self.is::<StringId>() {
+            NixType::String
+        } else if self.is::<PrimOp>() {
+            NixType::PrimOp
+        } else if self.is::<NixString>() {
+            NixType::String
+        } else if self.is::<Path>() {
+            NixType::Path
+        } else if self.is::<AttrSet>() {
+            NixType::AttrSet
+        } else if self.is::<List>() {
+            NixType::List
+        } else if self.is::<Thunk>() {
+            NixType::Thunk
+        } else if self.is::<Closure>() {
+            NixType::Closure
+        } else if self.is::<PrimOpApp>() {
+            NixType::PrimOpApp
+        } else {
+            unreachable!("value has no recognized type tag")
+        }
+    }
+
+    #[inline]
+    pub fn expect_inline<T: InlineStorable>(self) -> Result<T, NixType> {
+        self.as_inline::<T>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub fn expect_gc<T: GcStorable>(self) -> Result<Gc<'gc, T>, NixType> {
+        self.as_gc::<T>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub fn expect_num(self) -> Result<NixNum, NixType> {
+        self.as_num().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub fn expect_bool(self) -> Result<bool, NixType> {
+        self.as_inline::<bool>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub fn expect_float(self) -> Result<f64, NixType> {
+        self.as_float().ok_or_else(|| self.ty())
+    }
+}
+
+#[derive(Copy, Clone, Default)]
+#[repr(transparent)]
+pub struct StaticValue(Value<'static>);
+
+impl<'gc> From<StaticValue> for Value<'gc> {
+    #[inline]
+    fn from(value: StaticValue) -> Self {
+        // SAFETY: StaticValue is guaranteed to not contain any `Gc`.
+        unsafe { std::mem::transmute::<Value<'static>, Value<'gc>>(value.0) }
+    }
+}
+
+impl StaticValue {
+    #[inline]
+    pub fn new_float(val: f64) -> Self {
+        Self(Value::new_float(val))
+    }
+    #[inline]
+    pub fn new_inline<T: InlineStorable>(val: T) -> Self {
+        Self(Value::new_inline(val))
+    }
+    #[inline]
+    pub fn new_primop(id: BuiltinId, arity: u8, dispatch_ip: u32) -> Self {
+        Self(Value::new_inline(PrimOp {
+            id,
+            arity,
+            dispatch_ip,
+        }))
+    }
+    #[inline]
+    pub fn is_float(self) -> bool {
+        self.0.is_float()
+    }
+    #[inline]
+    pub fn is<T: InlineStorable>(self) -> bool {
+        self.0.is::<T>()
+    }
+    #[inline]
+    pub fn as_float(self) -> Option<f64> {
+        self.0.as_float()
+    }
+    #[inline]
+    pub fn as_inline<T: InlineStorable>(self) -> Option<T> {
+        self.0.as_inline::<T>()
+    }
+    #[inline]
+    pub fn to_bits(self) -> u64 {
+        self.0.raw.to_bits()
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct Null;
+impl RawStore for Null {
+    fn to_val(self, value: &mut RawValue) {
+        value.set_data([0; 6]);
+    }
+    fn from_val(_: &RawValue) -> Self {
+        Self
+    }
+}
+
+impl RawStore for StringId {
+    fn to_val(self, value: &mut RawValue) {
+        (self.0.to_usize() as u32).to_val(value);
+    }
+    fn from_val(value: &RawValue) -> Self {
+        Self(
+            SymbolU32::try_from_usize(u32::from_val(value) as usize)
+                .expect("failed to read StringId from Value"),
+        )
+    }
+}
+
+/// A canonicalized absolute path. Inline value carrying an interned
+/// `StringId` whose contents are the path's absolute, dot-resolved form.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Path(pub StringId);
+
+impl RawStore for Path {
+    fn to_val(self, value: &mut RawValue) {
+        self.0.to_val(value);
+    }
+    fn from_val(value: &RawValue) -> Self {
+        Self(StringId::from_val(value))
+    }
+}
+
+#[derive(Collect)]
+#[collect(require_static)]
+pub struct NixString {
+    data: Box<str>,
+    context: StringContext,
+}
+
+impl NixString {
+    pub fn new(s: impl Into<Box<str>>) -> Self {
+        Self {
+            data: s.into(),
+            context: StringContext::new(),
+        }
+    }
+
+    /// Construct a `NixString` whose `context` is already sorted+deduped.
+    /// The caller is responsible for invariant maintenance.
+    pub fn with_context(s: impl Into<Box<str>>, context: StringContext) -> Self {
+        Self {
+            data: s.into(),
+            context,
+        }
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.data
+    }
+
+    pub fn context(&self) -> &StringContext {
+        &self.context
+    }
+
+    pub fn has_context(&self) -> bool {
+        !self.context.is_empty()
+    }
+}
+
+impl fmt::Debug for NixString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.data, f)
+    }
+}
+
+#[derive(Collect, Debug, Default)]
+#[collect(no_drop)]
+pub struct AttrSet<'gc> {
+    pub entries: SmallVec<[(StringId, Value<'gc>); 4]>,
+}
+
+impl<'gc> AttrSet<'gc> {
+    pub fn from_sorted_unchecked(entries: SmallVec<[(StringId, Value<'gc>); 4]>) -> Self {
+        debug_assert!(entries.is_sorted_by_key(|(key, _)| *key));
+        Self { entries }
+    }
+
+    pub fn lookup(&self, key: StringId) -> Option<Value<'gc>> {
+        self.entries
+            .binary_search_by_key(&key, |(k, _)| *k)
+            .ok()
+            .map(|i| self.entries[i].1)
+    }
+
+    pub fn has(&self, key: StringId) -> bool {
+        self.entries.binary_search_by_key(&key, |(k, _)| *k).is_ok()
+    }
+
+    pub fn merge(&self, other: &Self, mc: &Mutation<'gc>) -> Gc<'gc, Self> {
+        use std::cmp::Ordering::*;
+
+        debug_assert!(self.entries.is_sorted_by_key(|(key, _)| *key));
+        debug_assert!(other.entries.is_sorted_by_key(|(key, _)| *key));
+
+        let mut entries = SmallVec::new();
+        let mut i = 0;
+        let mut j = 0;
+        while i < self.entries.len() && j < other.entries.len() {
+            match self.entries[i].0.cmp(&other.entries[j].0) {
+                Less => {
+                    entries.push(self.entries[i]);
+                    i += 1;
+                }
+                Greater => {
+                    entries.push(other.entries[j]);
+                    j += 1;
+                }
+                Equal => {
+                    entries.push(other.entries[j]);
+                    i += 1;
+                    j += 1;
+                }
+            }
+        }
+        entries.extend(other.entries[j..].iter().cloned());
+        entries.extend(self.entries[i..].iter().cloned());
+
+        debug_assert!(entries.is_sorted_by_key(|(key, _)| *key));
+
+        Gc::new(mc, AttrSet { entries })
+    }
+}
+
+#[derive(Collect, Debug, Default)]
+#[repr(transparent)]
+#[collect(no_drop)]
+pub struct List<'gc> {
+    pub inner: RefLock<SmallVec<[Value<'gc>; 4]>>,
+}
+
+impl<'gc> List<'gc> {
+    pub fn new(mc: &Mutation<'gc>, data: SmallVec<[Value<'gc>; 4]>) -> Gc<'gc, Self> {
+        Gc::new(
+            mc,
+            Self {
+                inner: RefLock::new(data),
+            },
+        )
+    }
+
+    pub fn new_gc(mc: &Mutation<'gc>) -> Gc<'gc, Self> {
+        Gc::new(mc, Self::default())
+    }
+}
+
+impl<'gc> Unlock for List<'gc> {
+    type Unlocked = RefCell<SmallVec<[Value<'gc>; 4]>>;
+    unsafe fn unlock_unchecked(&self) -> &Self::Unlocked {
+        unsafe { self.inner.unlock_unchecked() }
+    }
+}
+
+pub type Thunk<'gc> = RefLock<ThunkState<'gc>>;
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub enum ThunkState<'gc> {
+    Pending { ip: usize, env: GcEnv<'gc> },
+    Apply { func: Value<'gc>, arg: Value<'gc> },
+    Blackhole,
+    Evaluated(StrictValue<'gc>),
+}
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub struct Env<'gc> {
+    pub locals: SmallVec<[Value<'gc>; 4]>,
+    pub prev: Option<GcEnv<'gc>>,
+}
+pub type GcEnv<'gc> = GcRefLock<'gc, Env<'gc>>;
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub struct WithEnv<'gc> {
+    pub env: Value<'gc>,
+    pub prev: Option<GcWithEnv<'gc>>,
+}
+pub type GcWithEnv<'gc> = Gc<'gc, WithEnv<'gc>>;
+
+impl<'gc> Env<'gc> {
+    pub fn empty() -> Self {
+        Env {
+            locals: SmallVec::new(),
+            prev: None,
+        }
+    }
+
+    pub fn with_arg(arg: Value<'gc>, n_locals: u32, prev: Gc<'gc, RefLock<Env<'gc>>>) -> Self {
+        let mut locals = smallvec::smallvec![Value::default(); 1 + n_locals as usize];
+        locals[0] = arg;
+        Env {
+            locals,
+            prev: Some(prev),
+        }
+    }
+}
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub struct Closure<'gc> {
+    pub ip: u32,
+    pub n_locals: u32,
+    pub env: Gc<'gc, RefLock<Env<'gc>>>,
+    pub pattern: Option<Gc<'gc, PatternInfo>>,
+}
+
+#[derive(Collect, Debug)]
+#[collect(require_static)]
+pub struct PatternInfo {
+    pub required: SmallVec<[StringId; 4]>,
+    pub optional: SmallVec<[StringId; 4]>,
+    pub ellipsis: bool,
+    pub param_spans: Box<[(StringId, u32)]>,
+}
+
+#[repr(packed, Rust)]
+#[derive(Clone, Copy, Debug, Collect)]
+#[collect(require_static)]
+pub struct PrimOp {
+    pub id: BuiltinId,
+    pub arity: u8,
+    pub dispatch_ip: u32,
+}
+
+impl RawStore for PrimOp {
+    fn to_val(self, value: &mut RawValue) {
+        let bytes = self.dispatch_ip.to_le_bytes();
+        value.set_data([
+            self.id as u8,
+            self.arity,
+            bytes[0],
+            bytes[1],
+            bytes[2],
+            bytes[3],
+        ]);
+    }
+    fn from_val(value: &RawValue) -> Self {
+        let [id, arity, bytes @ ..] = *value.data();
+        Self {
+            id: BuiltinId::try_from_primitive(id).expect("invalid BuiltinId"),
+            arity,
+            dispatch_ip: u32::from_le_bytes(bytes),
+        }
+    }
+}
+
+#[derive(Collect, Debug)]
+#[collect(no_drop)]
+pub struct PrimOpApp<'gc> {
+    pub primop: PrimOp,
+    pub arity: u8,
+    pub args: [Value<'gc>; 3],
+}
+
+#[derive(Copy, Clone, Default, Collect)]
+#[repr(transparent)]
+#[collect(no_drop)]
+pub struct StrictValue<'gc>(Value<'gc>);
+
+impl<'gc> StrictValue<'gc> {
+    #[inline]
+    pub fn relax(self) -> Value<'gc> {
+        self.0
+    }
+}
+
+impl<'gc> Deref for StrictValue<'gc> {
+    type Target = Value<'gc>;
+    #[inline]
+    fn deref(&self) -> &Value<'gc> {
+        &self.0
+    }
+}
+
+impl fmt::Debug for StrictValue<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.0, f)
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Collect)]
+#[collect(require_static)]
+pub enum NixType {
+    Int,
+    Float,
+    Bool,
+    Null,
+    String,
+    Path,
+    AttrSet,
+    List,
+    Thunk,
+    Closure,
+    PrimOp,
+    PrimOpApp,
+}
+
+impl NixType {
+    pub fn display(self) -> &'static str {
+        use NixType::*;
+        match self {
+            Int => "an integer",
+            Float => "a float",
+            Bool => "a boolean",
+            Null => "null",
+            String => "a string",
+            Path => "a path",
+            AttrSet => "a set",
+            List => "a list",
+            Thunk => "a thunk",
+            Closure => "a function",
+            PrimOp => "a built-in function",
+            PrimOpApp => "a partially applied built-in function",
+        }
+    }
+}
+
+impl std::fmt::Display for NixType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.display())
+    }
+}
+
+pub enum NixNum {
+    Int(i64),
+    Float(f64),
+}