better force eval ergonomic

.gitignore

@@ -10,4 +10,4 @@ prof.json
 *.cpuprofile
 *.cpuprofile.gz
 *v8.log*
-callgrind.out.*
+callgrind.*

fix-vm/src/boxing.rs

@@ -160,6 +160,22 @@ enum TagVal {
 pub(crate) struct RawTag(TagVal);
 
 impl RawTag {
+    pub(crate) const P1: RawTag = RawTag(TagVal::_P1);
+    pub(crate) const P2: RawTag = RawTag(TagVal::_P2);
+    pub(crate) const P3: RawTag = RawTag(TagVal::_P3);
+    pub(crate) const P4: RawTag = RawTag(TagVal::_P4);
+    pub(crate) const P5: RawTag = RawTag(TagVal::_P5);
+    pub(crate) const P6: RawTag = RawTag(TagVal::_P6);
+    pub(crate) const P7: RawTag = RawTag(TagVal::_P7);
+
+    pub(crate) const N1: RawTag = RawTag(TagVal::_N1);
+    pub(crate) const N2: RawTag = RawTag(TagVal::_N2);
+    pub(crate) const N3: RawTag = RawTag(TagVal::_N3);
+    pub(crate) const N4: RawTag = RawTag(TagVal::_N4);
+    pub(crate) const N5: RawTag = RawTag(TagVal::_N5);
+    pub(crate) const N6: RawTag = RawTag(TagVal::_N6);
+    pub(crate) const N7: RawTag = RawTag(TagVal::_N7);
+
     #[inline]
     #[must_use]
     pub(crate) fn new(neg: bool, val: NonZeroU8) -> RawTag {
@@ -195,7 +211,7 @@ impl RawTag {
     /// `val` must be in the range `1..8`
     #[inline]
     #[must_use]
-    pub(crate) unsafe fn new_unchecked(neg: bool, val: u8) -> RawTag {
+    pub(crate) const unsafe fn new_unchecked(neg: bool, val: u8) -> RawTag {
         RawTag(match (neg, val) {
             (false, 1) => TagVal::_P1,
             (false, 2) => TagVal::_P2,
@@ -244,7 +260,7 @@ impl RawTag {
 
     #[inline]
     #[must_use]
-    pub(crate) fn neg_val(self) -> (bool, u8) {
+    pub(crate) const fn neg_val(self) -> (bool, u8) {
         match self.0 {
             TagVal::_P1 => (false, 1),
             TagVal::_P2 => (false, 2),
@@ -285,17 +301,17 @@ impl Header {
     }
 
     #[inline]
-    fn tag(self) -> RawTag {
+    const fn tag(self) -> RawTag {
         unsafe { RawTag::new_unchecked(self.get_sign(), self.get_tag()) }
     }
 
     #[inline]
-    fn get_sign(self) -> bool {
+    const fn get_sign(self) -> bool {
         self.0 & 0x8000 != 0
     }
 
     #[inline]
-    fn get_tag(self) -> u8 {
+    const fn get_tag(self) -> u8 {
         (self.0 & 0x0007) as u8
     }
 
@@ -341,7 +357,7 @@ impl Value {
 
     #[inline]
     #[must_use]
-    pub(crate) fn tag(&self) -> RawTag {
+    pub(crate) const fn tag(&self) -> RawTag {
         self.header.tag()
     }
 
@@ -417,7 +433,7 @@ impl RawBox {
 
     #[inline]
     #[must_use]
-    pub(crate) fn tag(&self) -> Option<RawTag> {
+    pub(crate) const fn tag(&self) -> Option<RawTag> {
         if self.is_value() {
             Some(unsafe { self.value.tag() })
         } else {
@@ -433,7 +449,7 @@ impl RawBox {
 
     #[inline]
     #[must_use]
-    pub(crate) fn is_value(&self) -> bool {
+    pub(crate) const fn is_value(&self) -> bool {
         (unsafe { self.float.is_nan() } && unsafe { self.bits & SIGN_MASK != QUIET_NAN })
     }
 

fix-vm/src/dispatch_tailcall.rs

@@ -2,11 +2,10 @@
 
 use gc_arena::Mutation;
 
-use crate::{BytecodeReader, ForceInfo, StepResult, Vm, VmContext};
+use crate::{Break, BytecodeReader, Step, Vm, VmContext};
 
-pub(crate) enum TailResult<'gc> {
+pub(crate) enum TailResult {
     YieldFuel(u32),
-    ForceThunk(ForceInfo<'gc>),
     Done,
 }
 
@@ -18,7 +17,7 @@ pub(crate) type OpFn<'gc, C> = extern "rust-preserve-none" fn(
     &DispatchTable<'gc, C>,
     u32,
     u32,
-) -> TailResult<'gc>;
+) -> TailResult;
 
 pub(crate) struct DispatchTable<'gc, C: VmContext>(pub(crate) [OpFn<'gc, C>; 256]);
 
@@ -30,16 +29,15 @@ extern "rust-preserve-none" fn op_illegal<'gc, C: VmContext>(
     _table: &DispatchTable<'gc, C>,
     pc: u32,
     _fuel: u32,
-) -> TailResult<'gc> {
+) -> TailResult {
     panic!("illegal opcode at pc = {pc}");
 }
 
 macro_rules! tail_dispatch_after {
     ($result:expr, $new_pc:expr, $vm:ident, $mc:ident, $ctx:ident, $bc:ident, $table:ident, $fuel:ident) => {{
         match $result {
-            StepResult::Continue => {}
-            StepResult::ForceThunk(info) => return TailResult::ForceThunk(info),
-            StepResult::Done => return TailResult::Done,
+            Step::Continue(()) | Step::Break(Break::Force) => {}
+            Step::Break(Break::Done) => return TailResult::Done,
         }
         let new_pc: u32 = $new_pc;
         if $fuel == 0 {
@@ -60,7 +58,7 @@ macro_rules! tail_fn {
             table: &DispatchTable<'gc, C>,
             pc: u32,
             fuel: u32,
-        ) -> TailResult<'gc> {
+        ) -> TailResult {
             let result = vm.$name();
             tail_dispatch_after!(result, pc + 1, vm, mc, ctx, bc, table, fuel)
         }
@@ -74,7 +72,7 @@ macro_rules! tail_fn {
             table: &DispatchTable<'gc, C>,
             pc: u32,
             fuel: u32,
-        ) -> TailResult<'gc> {
+        ) -> TailResult {
             let mut reader = BytecodeReader::from_after_op(bc, pc as usize);
             let result = vm.$name(&mut reader);
             tail_dispatch_after!(result, reader.pc() as u32, vm, mc, ctx, bc, table, fuel)
@@ -89,7 +87,7 @@ macro_rules! tail_fn {
             table: &DispatchTable<'gc, C>,
             pc: u32,
             fuel: u32,
-        ) -> TailResult<'gc> {
+        ) -> TailResult {
             let mut reader = BytecodeReader::from_after_op(bc, pc as usize);
             let result = vm.$name(&mut reader, mc);
             tail_dispatch_after!(result, reader.pc() as u32, vm, mc, ctx, bc, table, fuel)
@@ -104,26 +102,12 @@ macro_rules! tail_fn {
             table: &DispatchTable<'gc, C>,
             pc: u32,
             fuel: u32,
-        ) -> TailResult<'gc> {
+        ) -> TailResult {
             let mut reader = BytecodeReader::from_after_op(bc, pc as usize);
             let result = vm.$name(ctx, &mut reader, mc);
             tail_dispatch_after!(result, reader.pc() as u32, vm, mc, ctx, bc, table, fuel)
         }
     };
-    ($name:ident, (mc, inst_start_pc)) => {
-        extern "rust-preserve-none" fn $name<'gc, C: VmContext>(
-            vm: &mut Vm<'gc>,
-            mc: &Mutation<'gc>,
-            ctx: &mut C,
-            bc: &[u8],
-            table: &DispatchTable<'gc, C>,
-            pc: u32,
-            fuel: u32,
-        ) -> TailResult<'gc> {
-            let result = vm.$name(mc, pc as usize);
-            tail_dispatch_after!(result, pc + 1, vm, mc, ctx, bc, table, fuel)
-        }
-    };
     ($name:ident, (ctx)) => {
         extern "rust-preserve-none" fn $name<'gc, C: VmContext>(
             vm: &mut Vm<'gc>,
@@ -133,7 +117,7 @@ macro_rules! tail_fn {
             table: &DispatchTable<'gc, C>,
             pc: u32,
             fuel: u32,
-        ) -> TailResult<'gc> {
+        ) -> TailResult {
             let result = vm.$name(ctx);
             tail_dispatch_after!(result, pc + 1, vm, mc, ctx, bc, table, fuel)
         }
@@ -181,7 +165,7 @@ tail_fn!(op_gt, (ctx, reader, mc));
 tail_fn!(op_leq, (ctx, reader, mc));
 tail_fn!(op_geq, (ctx, reader, mc));
 tail_fn!(op_concat, (reader, mc));
-tail_fn!(op_update, (mc, inst_start_pc));
+tail_fn!(op_update, (reader, mc));
 
 tail_fn!(op_neg, ());
 tail_fn!(op_not, ());
@@ -303,9 +287,8 @@ pub(crate) fn run_tailcall<'gc, C: VmContext>(
     ctx: &mut C,
     bc: &[u8],
     pc: u32,
-) -> TailResult<'gc> {
-    const FUEL: u32 = 1024;
+) -> TailResult {
     let table = &DispatchTable::<'gc, C>::NEW;
     let op = bc[pc as usize] as usize;
-    table.0[op](vm, mc, ctx, bc, table, pc, FUEL)
+    table.0[op](vm, mc, ctx, bc, table, pc, Vm::DEFAULT_FUEL_AMOUNT)
 }

fix-vm/src/forced.rs

@@ -0,0 +1,255 @@
+use fix_common::StringId;
+use gc_arena::{Gc, Mutation};
+
+use crate::value::*;
+use crate::{Break, BytecodeReader, NixNum, Step, Vm};
+
+pub(crate) 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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: 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(vm: &mut Vm<'gc>) -> Self;
+}
+
+impl<'gc> Forced<'gc> for StrictValue<'gc> {
+    const WIDTH: usize = 1;
+
+    #[inline(always)]
+    fn force_and_check(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+    ) -> Step {
+        vm.force_slot(base_depth, reader, mc)
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        vm.pop_forced()
+    }
+}
+
+macro_rules! impl_forced_inline {
+    ($($ty:ty => $nix_ty:expr),* $(,)?) => {
+        $(
+        impl<'gc> Forced<'gc> for $ty {
+            const WIDTH: usize = 1;
+
+            #[inline(always)]
+            fn force_and_check(
+                vm: &mut Vm<'gc>,
+                reader: &mut BytecodeReader<'_>,
+                mc: &Mutation<'gc>,
+                base_depth: usize,
+            ) -> Step {
+                vm.force_slot(base_depth, reader, mc)?;
+                let v = vm.peek_forced(base_depth);
+                if v.as_inline::<$ty>().is_none() {
+                    let _: Step = vm.finish_type_err($nix_ty, v.ty());
+                    return Step::Break(Break::Done);
+                }
+                Step::Continue(())
+            }
+
+            #[inline(always)]
+            fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+                vm.pop_forced()
+                    .as_inline::<$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, $ty> {
+            const WIDTH: usize = 1;
+
+            #[inline(always)]
+            fn force_and_check(
+                vm: &mut Vm<'gc>,
+                reader: &mut BytecodeReader<'_>,
+                mc: &Mutation<'gc>,
+                base_depth: usize,
+            ) -> Step {
+                vm.force_slot(base_depth, reader, mc)?;
+                let v = vm.peek_forced(base_depth);
+                if v.as_gc::<$ty>().is_none() {
+                    let _: Step = vm.finish_type_err($nix_ty, v.ty());
+                    return Step::Break(Break::Done);
+                }
+                Step::Continue(())
+            }
+
+            #[inline(always)]
+            fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+                vm.pop_forced()
+                    .as_gc::<$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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+    ) -> Step {
+        vm.force_slot(base_depth, reader, mc)?;
+        let v = vm.peek_forced(base_depth);
+        if v.as_num().is_none() {
+            let _: Step = vm.finish_type_err(NixType::Int, v.ty());
+            return Step::Break(Break::Done);
+        }
+        Step::Continue(())
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        vm.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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base_depth: usize,
+    ) -> Step {
+        vm.force_slot(base_depth, reader, mc)?;
+        let v = vm.peek_forced(base_depth);
+        if v.as_float().is_none() {
+            let _: Step = vm.finish_type_err(NixType::Float, v.ty());
+            return Step::Break(Break::Done);
+        }
+        Step::Continue(())
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        vm.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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+    ) -> Step {
+        A::force_and_check(vm, reader, mc, base + B::WIDTH)?;
+        B::force_and_check(vm, reader, mc, base)
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        let b = B::pop_converted(vm);
+        let a = A::pop_converted(vm);
+        (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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+    ) -> Step {
+        A::force_and_check(vm, reader, mc, base + B::WIDTH + C::WIDTH)?;
+        B::force_and_check(vm, reader, mc, base + C::WIDTH)?;
+        C::force_and_check(vm, reader, mc, base)
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        let c = C::pop_converted(vm);
+        let b = B::pop_converted(vm);
+        let a = A::pop_converted(vm);
+        (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(
+        vm: &mut Vm<'gc>,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+        base: usize,
+    ) -> Step {
+        A::force_and_check(vm, reader, mc, base + B::WIDTH + C::WIDTH + D::WIDTH)?;
+        B::force_and_check(vm, reader, mc, base + C::WIDTH + D::WIDTH)?;
+        C::force_and_check(vm, reader, mc, base + D::WIDTH)?;
+        D::force_and_check(vm, reader, mc, base)
+    }
+
+    #[inline(always)]
+    fn pop_converted(vm: &mut Vm<'gc>) -> Self {
+        let d = D::pop_converted(vm);
+        let c = C::pop_converted(vm);
+        let b = B::pop_converted(vm);
+        let a = A::pop_converted(vm);
+        (a, b, c, d)
+    }
+}

fix-vm/src/instructions/arithmetic.rs

@@ -2,7 +2,8 @@ use std::cmp::Ordering;
 
 use gc_arena::{Gc, Mutation};
 
-use crate::{BytecodeReader, NixNum, StepResult, StrictValue, Value, VmContextExt, VmError};
+use crate::value::*;
+use crate::{BytecodeReader, NixNum, Step, VmContextExt, VmError};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -11,28 +12,21 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
         if let (Some(ls), Some(rs)) = (
             VmContextExt::get_string(ctx, lhs),
             VmContextExt::get_string(ctx, rhs),
         ) {
             let ns = Gc::new(mc, crate::NixString::new(format!("{ls}{rs}")));
-            self.push_stack(Value::new_gc(ns));
-            return StepResult::Continue;
+            self.push(Value::new_gc(ns));
+            return Step::Continue(());
         }
         let res = numeric_binop(lhs, rhs, mc, i64::wrapping_add, |a, b| a + b);
         match res {
             Ok(val) => {
-                self.push_stack(val);
-                StepResult::Continue
+                self.push(val);
+                Step::Continue(())
             }
             Err(e) => self.finish_vm_err(e),
         }
@@ -43,8 +37,8 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        self.op_arith(mc, i64::wrapping_sub, |a, b| a - b, reader.inst_start_pc())
+    ) -> Step {
+        self.op_arith(reader, mc, i64::wrapping_sub, |a, b| a - b)
     }
 
     #[inline(always)]
@@ -52,31 +46,24 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        self.op_arith(mc, i64::wrapping_mul, |a, b| a * b, reader.inst_start_pc())
+    ) -> Step {
+        self.op_arith(reader, mc, i64::wrapping_mul, |a, b| a * b)
     }
 
     #[inline(always)]
     fn op_arith(
         &mut self,
+        reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
         int_op: fn(i64, i64) -> i64,
         float_op: fn(f64, f64) -> f64,
-        inst_start_pc: usize,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, inst_start_pc, mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, inst_start_pc, mc) {
-            return step;
-        }
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
         let res = numeric_binop(lhs, rhs, mc, int_op, float_op);
         match res {
             Ok(val) => {
-                self.push_stack(val);
-                StepResult::Continue
+                self.push(val);
+                Step::Continue(())
             }
             Err(e) => self.finish_vm_err(e),
         }
@@ -87,15 +74,8 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
         match (get_num(rhs), get_num(lhs)) {
             (_, Some(NixNum::Int(0))) | (_, Some(NixNum::Float(0.))) => {
                 return self.finish_vm_err(VmError::Uncatchable(fix_error::Error::eval_error(
@@ -107,8 +87,8 @@ impl<'gc> crate::Vm<'gc> {
         let res = numeric_binop(lhs, rhs, mc, |a, b| a / b, |a, b| a / b);
         match res {
             Ok(val) => {
-                self.push_stack(val);
-                StepResult::Continue
+                self.push(val);
+                Step::Continue(())
             }
             Err(e) => self.finish_vm_err(e),
         }
@@ -120,19 +100,14 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let eq = match self.values_equal(ctx) {
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
+        let eq = match self.values_equal(ctx, lhs, rhs) {
             Ok(eq) => eq,
             Err(e) => return self.finish_vm_err(e),
         };
-        self.push_stack(Value::new_inline(eq));
-        StepResult::Continue
+        self.push(Value::new_inline(eq));
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -141,19 +116,14 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let eq = match self.values_equal(ctx) {
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
+        let eq = match self.values_equal(ctx, lhs, rhs) {
             Ok(eq) => eq,
             Err(e) => return self.finish_vm_err(e),
         };
-        self.push_stack(Value::new_inline(!eq));
-        StepResult::Continue
+        self.push(Value::new_inline(!eq));
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -162,7 +132,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         self.compare_values(ctx, reader, mc, Ordering::is_lt)
     }
 
@@ -172,7 +142,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         self.compare_values(ctx, reader, mc, Ordering::is_gt)
     }
 
@@ -182,7 +152,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         self.compare_values(ctx, reader, mc, Ordering::is_le)
     }
 
@@ -192,7 +162,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         self.compare_values(ctx, reader, mc, Ordering::is_ge)
     }
 
@@ -202,15 +172,10 @@ impl<'gc> crate::Vm<'gc> {
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
         pred: fn(Ordering) -> bool,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        match self.compare_values_inner(ctx, pred) {
-            Ok(()) => StepResult::Continue,
+    ) -> Step {
+        let (lhs, rhs) = self.try_force::<(StrictValue, StrictValue)>(reader, mc)?;
+        match self.compare_values_inner(ctx, pred, lhs, rhs) {
+            Ok(()) => Step::Continue(()),
             Err(e) => self.finish_vm_err(e),
         }
     }
@@ -220,74 +185,42 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
-        let Some(l) = lhs.as_gc::<crate::List>() else {
-            return self.finish_err(fix_error::Error::eval_error(
-                "cannot concatenate: left operand is not a list",
-            ));
-        };
-        let Some(r) = rhs.as_gc::<crate::List>() else {
-            return self.finish_err(fix_error::Error::eval_error(
-                "cannot concatenate: right operand is not a list",
-            ));
-        };
+    ) -> Step {
+        let (l, r) = self.try_force::<(Gc<List>, Gc<List>)>(reader, mc)?;
         let mut items = smallvec::SmallVec::new();
         items.extend_from_slice(&l);
         items.extend_from_slice(&r);
-        self.push_stack(Value::new_gc(Gc::new(mc, crate::List { inner: items })));
-        StepResult::Continue
+        self.push(Value::new_gc(Gc::new(mc, crate::List { inner: items })));
+        Step::Continue(())
     }
 
     #[inline(always)]
     pub(crate) fn op_update(
         &mut self,
+        reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-        inst_start_pc: usize,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(1, inst_start_pc, mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(0, inst_start_pc, mc) {
-            return step;
-        }
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
-        let Some(l) = lhs.as_gc::<crate::AttrSet>() else {
-            return self.finish_err(fix_error::Error::eval_error(
-                "cannot update: left operand is not a set",
-            ));
-        };
-        let Some(r) = rhs.as_gc::<crate::AttrSet>() else {
-            return self.finish_err(fix_error::Error::eval_error(
-                "cannot update: right operand is not a set",
-            ));
-        };
-        self.push_stack(Value::new_gc(l.merge(&r, mc)));
-        StepResult::Continue
+    ) -> Step {
+        let (l, r) = self.try_force::<(Gc<AttrSet>, Gc<AttrSet>)>(reader, mc)?;
+        self.push(Value::new_gc(l.merge(&r, mc)));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_neg(&mut self) -> StepResult<'gc> {
+    pub(crate) fn op_neg(&mut self) -> Step {
         todo!("implement unary operation");
     }
 
     #[inline(always)]
-    pub(crate) fn op_not(&mut self) -> StepResult<'gc> {
+    pub(crate) fn op_not(&mut self) -> Step {
         todo!("implement unary operation");
     }
 
-    pub(crate) fn values_equal(&mut self, ctx: &impl crate::VmContext) -> crate::VmResult<bool> {
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
-
+    pub(crate) fn values_equal(
+        &mut self,
+        ctx: &impl crate::VmContext,
+        lhs: StrictValue<'gc>,
+        rhs: StrictValue<'gc>,
+    ) -> crate::VmResult<bool> {
         if let (Some(a), Some(b)) = (get_num(lhs), get_num(rhs)) {
             return Ok(match (a, b) {
                 (NixNum::Int(a), NixNum::Int(b)) => a == b,
@@ -312,16 +245,10 @@ impl<'gc> crate::Vm<'gc> {
             if a.inner.len() != b.inner.len() {
                 return Ok(false);
             }
-            let len = a.inner.len();
-            for (x, y) in a.inner.iter().zip(b.inner.iter()).rev() {
-                self.push_stack(*x);
-                self.push_stack(*y);
-            }
-            for i in 0..len {
-                let eq = self.values_equal(ctx)?;
-                if !eq {
-                    let rem = len - 1 - i;
-                    self.stack.truncate(self.stack.len() - rem * 2);
+            for (x, y) in a.inner.iter().zip(b.inner.iter()) {
+                let lx = x.restrict().expect("forced");
+                let ly = y.restrict().expect("forced");
+                if !self.values_equal(ctx, lx, ly)? {
                     return Ok(false);
                 }
             }
@@ -331,19 +258,13 @@ impl<'gc> crate::Vm<'gc> {
             if a.len() != b.len() {
                 return Ok(false);
             }
-            let len = a.len();
-            for ((k1, v1), (k2, v2)) in a.iter().zip(b.iter()).rev() {
+            for ((k1, v1), (k2, v2)) in a.iter().zip(b.iter()) {
                 if k1 != k2 {
                     return Ok(false);
                 }
-                self.push_stack(*v1);
-                self.push_stack(*v2);
-            }
-            for i in 0..len {
-                let eq = self.values_equal(ctx)?;
-                if !eq {
-                    let rem = len - 1 - i;
-                    self.stack.truncate(self.stack.len() - rem * 2);
+                let lv1 = v1.restrict().expect("forced");
+                let lv2 = v2.restrict().expect("forced");
+                if !self.values_equal(ctx, lv1, lv2)? {
                     return Ok(false);
                 }
             }
@@ -356,10 +277,9 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         ctx: &impl crate::VmContext,
         pred: fn(Ordering) -> bool,
+        lhs: StrictValue<'gc>,
+        rhs: StrictValue<'gc>,
     ) -> crate::VmResult<()> {
-        let rhs = self.pop_stack_forced();
-        let lhs = self.pop_stack_forced();
-
         if let (Some(a), Some(b)) = (get_num(lhs), get_num(rhs)) {
             let ord = match (a, b) {
                 (NixNum::Int(a), NixNum::Int(b)) => a.cmp(&b),
@@ -371,16 +291,17 @@ impl<'gc> crate::Vm<'gc> {
                     a.partial_cmp(&(b as f64)).unwrap_or(Ordering::Less)
                 }
             };
-            self.push_stack(Value::new_inline(pred(ord)));
+            self.push(Value::new_inline(pred(ord)));
             return Ok(());
         }
         if let (Some(a), Some(b)) = (
             VmContextExt::get_string(ctx, lhs),
             VmContextExt::get_string(ctx, rhs),
         ) {
-            self.push_stack(Value::new_inline(pred(a.cmp(b))));
+            self.push(Value::new_inline(pred(a.cmp(b))));
             return Ok(());
         }
+        // TODO: compare other types
         Err(crate::vm_err("cannot compare these types"))
     }
 }

fix-vm/src/instructions/builtins_misc.rs

@@ -1,28 +1,28 @@
 use fix_builtins::BuiltinId;
 use num_enum::TryFromPrimitive;
 
-use crate::{BytecodeReader, PrimOp, StepResult, Value};
+use crate::{BytecodeReader, PrimOp, Step, Value};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
-    pub(crate) fn op_load_builtins(&mut self) -> StepResult<'gc> {
-        self.push_stack(self.builtins);
-        StepResult::Continue
+    pub(crate) fn op_load_builtins(&mut self) -> Step {
+        self.push(self.builtins);
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_load_builtin(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_load_builtin(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let Ok(id) = BuiltinId::try_from_primitive(reader.read_u8())
             .map_err(|err| panic!("unknown builtin id: {}", err.number));
-        self.push_stack(Value::new_inline(PrimOp {
+        self.push(Value::new_inline(PrimOp {
             id,
             arity: fix_builtins::BUILTINS[id as usize].1,
         }));
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_mk_pos(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_mk_pos(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let _span_id = reader.read_u32();
         todo!("MkPos");
     }
@@ -31,7 +31,7 @@ impl<'gc> crate::Vm<'gc> {
     pub(crate) fn op_load_repl_binding(
         &mut self,
         reader: &mut BytecodeReader<'_>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let _name = reader.read_string_id();
         todo!("LoadReplBinding");
     }
@@ -40,7 +40,7 @@ impl<'gc> crate::Vm<'gc> {
     pub(crate) fn op_load_scoped_binding(
         &mut self,
         reader: &mut BytecodeReader<'_>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let _name = reader.read_string_id();
         todo!("LoadScopedBinding");
     }
@@ -51,7 +51,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         _mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let _parts_count = reader.read_u16() as usize;
         let _force_string = reader.read_u8() != 0;
         let mut _operands: smallvec::SmallVec<[crate::OperandData; 4]> =
@@ -63,7 +63,7 @@ impl<'gc> crate::Vm<'gc> {
     }
 
     #[inline(always)]
-    pub(crate) fn op_resolve_path(&mut self, _ctx: &mut impl crate::VmContext) -> StepResult<'gc> {
+    pub(crate) fn op_resolve_path(&mut self, _ctx: &mut impl crate::VmContext) -> Step {
         todo!("implement ResolvePath");
     }
 }

fix-vm/src/instructions/calls.rs

@@ -1,7 +1,8 @@
 use fix_error::Error;
 use gc_arena::{Gc, Mutation, RefLock};
 
-use crate::{BytecodeReader, CallFrame, Closure, Env, StepResult, ThunkState, VmContextExt};
+use crate::value::*;
+use crate::{BytecodeReader, CallFrame, Closure, Env, Step, ThunkState, VmContextExt};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -10,15 +11,12 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
+    ) -> Step {
+        let func = self.try_force::<StrictValue>(reader, mc)?;
         if self.call_depth > 10000 {
             return self.finish_err(Error::eval_error("stack overflow; max-call-depth exceeded"));
         }
         self.call_depth += 1;
-        let func = self.pop_stack();
         let arg = reader.read_operand_data(ctx).resolve(mc, self);
         if let Some(closure) = func.as_gc::<Closure>() {
             let ip = closure.ip;
@@ -41,7 +39,7 @@ impl<'gc> crate::Vm<'gc> {
         } else {
             todo!("call other types: {func:?}")
         }
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -50,7 +48,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         self.handle_return(reader, ctx, mc)
     }
 
@@ -59,7 +57,7 @@ impl<'gc> crate::Vm<'gc> {
         reader: &mut BytecodeReader<'_>,
         ctx: &C,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let ret_inst_pc = reader.pc() - 1;
         let Some(CallFrame {
             pc: ret_pc,
@@ -69,18 +67,18 @@ impl<'gc> crate::Vm<'gc> {
             with_env,
         }) = self.call_stack.pop()
         else {
-            let val = self.pop_stack();
+            let val = self.pop();
             return self.finish_ok(ctx.convert_value(val));
         };
         reader.set_pc(ret_pc);
         if let Some(outer_thunk) = thunk {
-            let val = self.pop_stack();
+            let val = self.pop();
             match val.restrict() {
                 Ok(val) => {
                     *outer_thunk.borrow_mut(mc) = ThunkState::Evaluated(val);
                     if reader.bytecode().get(ret_pc).copied() == Some(fix_codegen::Op::Return as u8)
                     {
-                        self.push_stack(val.relax());
+                        self.push(val.relax());
                     }
                 }
                 Err(inner_thunk) => {
@@ -109,14 +107,14 @@ impl<'gc> crate::Vm<'gc> {
                             reader.set_pc(inner_ip);
                             self.env = inner_env;
                             self.with_env = inner_with_env;
-                            return StepResult::Continue;
+                            return Step::Continue(());
                         }
                         ThunkState::Evaluated(val) => {
                             *outer_thunk.borrow_mut(mc) = ThunkState::Evaluated(val);
                             if reader.bytecode().get(ret_pc).copied()
                                 == Some(fix_codegen::Op::Return as u8)
                             {
-                                self.push_stack(val.relax());
+                                self.push(val.relax());
                             }
                         }
                         ThunkState::Apply { func: _, arg: _ } => todo!("force Apply thunk"),
@@ -132,6 +130,6 @@ impl<'gc> crate::Vm<'gc> {
         }
         self.env = env;
         self.with_env = with_env;
-        StepResult::Continue
+        Step::Continue(())
     }
 }

fix-vm/src/instructions/closures.rs

@@ -1,6 +1,6 @@
 use gc_arena::{Gc, Mutation, RefLock};
 
-use crate::{BytecodeReader, StepResult, ThunkState, Value};
+use crate::{BytecodeReader, Step, ThunkState, Value};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -8,7 +8,7 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let entry_point = reader.read_u32();
         let thunk = Gc::new(
             mc,
@@ -18,8 +18,8 @@ impl<'gc> crate::Vm<'gc> {
                 with_env: self.with_env,
             }),
         );
-        self.push_stack(Value::new_gc(thunk));
-        StepResult::Continue
+        self.push(Value::new_gc(thunk));
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -27,7 +27,7 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let entry_point = reader.read_u32();
         let n_locals = reader.read_u32();
         let closure = Gc::new(
@@ -39,8 +39,8 @@ impl<'gc> crate::Vm<'gc> {
                 pattern: None,
             },
         );
-        self.push_stack(Value::new_gc(closure));
-        StepResult::Continue
+        self.push(Value::new_gc(closure));
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -48,7 +48,7 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let entry_point = reader.read_u32();
         let n_locals = reader.read_u32();
         let req_count = reader.read_u16() as usize;
@@ -89,7 +89,7 @@ impl<'gc> crate::Vm<'gc> {
                 pattern: Some(pattern),
             },
         );
-        self.push_stack(Value::new_gc(closure));
-        StepResult::Continue
+        self.push(Value::new_gc(closure));
+        Step::Continue(())
     }
 }

fix-vm/src/instructions/collections.rs

@@ -3,7 +3,7 @@ use gc_arena::Gc;
 use smallvec::SmallVec;
 
 use crate::{
-    AttrKeyData, AttrSet, BytecodeReader, List, NixString, OperandData, StepResult, Value,
+    AttrKeyData, AttrSet, BytecodeReader, List, NixString, OperandData, Step, StrictValue, Value,
 };
 
 impl<'gc> crate::Vm<'gc> {
@@ -13,7 +13,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let count = reader.read_u32() as usize;
         let mut entries: SmallVec<[AttrEntry; 4]> = SmallVec::with_capacity(count);
         for _ in 0..count {
@@ -37,14 +37,14 @@ impl<'gc> crate::Vm<'gc> {
         }
         kv.sort_by_key(|(k, _)| *k);
         let attrs = Gc::new(mc, AttrSet::from_sorted_unchecked(kv));
-        self.push_stack(Value::new_gc(attrs));
-        StepResult::Continue
+        self.push(Value::new_gc(attrs));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_make_empty_attrs(&mut self) -> StepResult<'gc> {
-        self.push_stack(self.empty_attrs);
-        StepResult::Continue
+    pub(crate) fn op_make_empty_attrs(&mut self) -> Step {
+        self.push(self.empty_attrs);
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -53,24 +53,15 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let _span_id = reader.read_u32();
         let key = reader.read_string_id();
 
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-
-        let attrs = self.peek_stack(0).restrict().expect("forced");
-        let Some(attrset) = attrs.as_gc::<AttrSet>() else {
-            return self.finish_err(Error::eval_error(
-                "value is not a set while a set was expected",
-            ));
-        };
+        let attrset = self.try_force::<Gc<AttrSet>>(reader, mc)?;
 
         match attrset.lookup(key) {
             Some(v) => {
-                self.replace_stack(0, v);
+                self.push(v);
             }
             None => loop {
                 let byte = reader.bytecode()[reader.pc()];
@@ -80,7 +71,6 @@ impl<'gc> crate::Vm<'gc> {
                     reader.set_pc(reader.pc() + 1 + 4);
                 } else if byte == fix_codegen::Op::JumpIfSelectSucceeded as u8 {
                     reader.set_pc(reader.pc() + 1 + 4);
-                    let _ = self.pop_stack();
                     break;
                 } else {
                     let name = ctx.resolve_string(key);
@@ -89,7 +79,7 @@ impl<'gc> crate::Vm<'gc> {
                 }
             },
         }
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -98,19 +88,11 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let _span_id = reader.read_u32();
 
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        if let Some(step) = self.try_force_resolved(1, reader.inst_start_pc(), mc) {
-            return step;
-        }
+        let (attrset, key_val) = self.try_force::<(Gc<AttrSet>, StrictValue)>(reader, mc)?;
 
-        let key_val = self.stack[self.stack.len() - 1]
-            .restrict()
-            .expect("dynamic key must be forced");
         let key_sid = if let Some(sid) = key_val.as_inline::<crate::StringId>() {
             sid
         } else if let Some(ns) = key_val.as_gc::<NixString>() {
@@ -119,38 +101,30 @@ impl<'gc> crate::Vm<'gc> {
             return self.finish_err(Error::eval_error("dynamic select key must be a string"));
         };
 
-        let attrset_val = self.stack[self.stack.len() - 2].restrict().expect("forced");
-        let Some(attrset) = attrset_val.as_gc::<AttrSet>() else {
-            return self.finish_err(Error::eval_error(
-                "value is not a set while a set was expected",
-            ));
-        };
-
         match attrset.lookup(key_sid) {
             Some(v) => {
-                self.stack.truncate(self.stack.len() - 2);
-                self.push_stack(v);
+                self.push(v);
             }
             None => {
                 let name = ctx.resolve_string(key_sid);
                 return self.finish_err(Error::eval_error(format!("attribute '{name}' missing")));
             }
         }
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
     pub(crate) fn op_jump_if_select_succeeded(
         &mut self,
         reader: &mut BytecodeReader<'_>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let offset = reader.read_i32();
         reader.set_pc(((reader.pc() as isize) + (offset as isize)) as usize);
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_has_attr(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_has_attr(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let _n = reader.read_u16() as usize;
         todo!("HasAttr");
     }
@@ -161,21 +135,21 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let count = reader.read_u32() as usize;
         let mut items: SmallVec<[Value; 4]> = SmallVec::with_capacity(count);
         for _ in 0..count {
             items.push(reader.read_operand_data(ctx).resolve(mc, self));
         }
         let list = Gc::new(mc, List { inner: items });
-        self.push_stack(Value::new_gc(list));
-        StepResult::Continue
+        self.push(Value::new_gc(list));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_make_empty_list(&mut self) -> StepResult<'gc> {
-        self.push_stack(self.empty_list);
-        StepResult::Continue
+    pub(crate) fn op_make_empty_list(&mut self) -> Step {
+        self.push(self.empty_list);
+        Step::Continue(())
     }
 }
 

fix-vm/src/instructions/control.rs

@@ -1,4 +1,5 @@
-use crate::{BytecodeReader, StepResult};
+use crate::value::*;
+use crate::{BytecodeReader, Step};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -6,16 +7,13 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let offset = reader.read_i32();
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let cond = self.pop_stack();
+        let cond = self.try_force::<StrictValue>(reader, mc)?;
         if cond.as_inline::<bool>() == Some(false) {
             reader.set_pc(((reader.pc() as isize) + (offset as isize)) as usize);
         }
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -23,27 +21,24 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let offset = reader.read_i32();
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-        let cond = self.pop_stack();
+        let cond = self.try_force::<StrictValue>(reader, mc)?;
         if cond.as_inline::<bool>() == Some(true) {
             reader.set_pc(((reader.pc() as isize) + (offset as isize)) as usize);
         }
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_jump(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_jump(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let offset = reader.read_i32();
         reader.set_pc(((reader.pc() as isize) + (offset as isize)) as usize);
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_assert(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_assert(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let _raw_idx = reader.read_u32();
         let _span_id = reader.read_u32();
         todo!("implement Assert (force TOS)");

fix-vm/src/instructions/literals.rs

@@ -1,13 +1,13 @@
 use gc_arena::{Gc, Mutation};
 
-use crate::{BytecodeReader, StepResult, Value};
+use crate::{BytecodeReader, Step, Value};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
-    pub(crate) fn op_push_smi(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_push_smi(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let val = reader.read_i32();
-        self.push_stack(Value::new_inline(val));
-        StepResult::Continue
+        self.push(Value::new_inline(val));
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -15,41 +15,41 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let val = reader.read_i64();
-        self.push_stack(Value::new_gc(Gc::new(mc, val)));
-        StepResult::Continue
+        self.push(Value::new_gc(Gc::new(mc, val)));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_push_float(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_push_float(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let val = reader.read_f64();
-        self.push_stack(Value::new_float(val));
-        StepResult::Continue
+        self.push(Value::new_float(val));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_push_string(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_push_string(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let sid = reader.read_string_id();
-        self.push_stack(Value::new_inline(sid));
-        StepResult::Continue
+        self.push(Value::new_inline(sid));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_push_null(&mut self) -> StepResult<'gc> {
-        self.push_stack(Value::new_inline(crate::Null));
-        StepResult::Continue
+    pub(crate) fn op_push_null(&mut self) -> Step {
+        self.push(Value::new_inline(crate::Null));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_push_true(&mut self) -> StepResult<'gc> {
-        self.push_stack(Value::new_inline(true));
-        StepResult::Continue
+    pub(crate) fn op_push_true(&mut self) -> Step {
+        self.push(Value::new_inline(true));
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_push_false(&mut self) -> StepResult<'gc> {
-        self.push_stack(Value::new_inline(false));
-        StepResult::Continue
+    pub(crate) fn op_push_false(&mut self) -> Step {
+        self.push(Value::new_inline(false));
+        Step::Continue(())
     }
 }

fix-vm/src/instructions/variables.rs

@@ -1,15 +1,15 @@
-use crate::{BytecodeReader, Mutation, StepResult, Value};
+use crate::{BytecodeReader, Mutation, Step, Value};
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
-    pub(crate) fn op_load_local(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_load_local(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let idx = reader.read_u32() as usize;
-        self.push_stack(self.env.borrow().locals[idx]);
-        StepResult::Continue
+        self.push(self.env.borrow().locals[idx]);
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_load_outer(&mut self, reader: &mut BytecodeReader<'_>) -> StepResult<'gc> {
+    pub(crate) fn op_load_outer(&mut self, reader: &mut BytecodeReader<'_>) -> Step {
         let layer = reader.read_u8();
         let idx = reader.read_u32() as usize;
         let mut cur = self.env;
@@ -18,8 +18,8 @@ impl<'gc> crate::Vm<'gc> {
             cur = prev;
         }
         let val = cur.borrow().locals[idx];
-        self.push_stack(val);
-        StepResult::Continue
+        self.push(val);
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -27,11 +27,11 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let idx = reader.read_u32() as usize;
-        let val = self.pop_stack();
+        let val = self.pop();
         self.env.borrow_mut(mc).locals[idx] = val;
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -39,12 +39,12 @@ impl<'gc> crate::Vm<'gc> {
         &mut self,
         reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let count = reader.read_u32() as usize;
         self.env
             .borrow_mut(mc)
             .locals
             .extend(std::iter::repeat_n(Value::default(), count));
-        StepResult::Continue
+        Step::Continue(())
     }
 }

fix-vm/src/instructions/with_scope.rs

@@ -2,7 +2,8 @@ use fix_common::Symbol;
 use fix_error::Error;
 use gc_arena::Gc;
 
-use crate::{BytecodeReader, CallFrame, StepResult, WithEnv};
+use crate::{BytecodeReader, CallFrame, Step, WithEnv};
+use crate::value::*;
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -11,7 +12,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let env = reader.read_operand_data(ctx).resolve(mc, self);
         let scope = Gc::new(
             mc,
@@ -21,20 +22,20 @@ impl<'gc> crate::Vm<'gc> {
             },
         );
         self.with_env = Some(scope);
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_pop_with(&mut self) -> StepResult<'gc> {
+    pub(crate) fn op_pop_with(&mut self) -> Step {
         let Some(scope) = self.with_env else {
             unreachable!("no with_scope to pop");
         };
         self.with_env = scope.prev;
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
-    pub(crate) fn op_prepare_with(&mut self) -> StepResult<'gc> {
+    pub(crate) fn op_prepare_with(&mut self) -> Step {
         self.call_stack.push(CallFrame {
             pc: usize::MAX,
             stack_depth: 0,
@@ -42,7 +43,7 @@ impl<'gc> crate::Vm<'gc> {
             env: self.env,
             with_env: self.with_env,
         });
-        StepResult::Continue
+        Step::Continue(())
     }
 
     #[inline(always)]
@@ -51,7 +52,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult<'gc> {
+    ) -> Step {
         let name = reader.read_string_id();
 
         let Some(&WithEnv { env, prev }) = self.with_env.as_deref() else {
@@ -64,23 +65,19 @@ impl<'gc> crate::Vm<'gc> {
                 Symbol::from(ctx.resolve_string(name))
             )));
         };
-        self.push_stack(env);
-        if let Some(step) = self.try_force_resolved(0, reader.inst_start_pc(), mc) {
-            return step;
-        }
-
-        let env = self.pop_stack().as_gc::<crate::AttrSet>().unwrap();
+        self.push(env);
+        let env = self.try_force::<Gc<AttrSet>>(reader, mc)?;
         let Some(val) = env.lookup(name) else {
             reader.set_pc(reader.inst_start_pc());
             self.with_env = prev;
-            return StepResult::Continue;
+            return Step::Continue(());
         };
 
-        self.push_stack(val);
+        self.push(val);
         let Some(CallFrame { with_env, .. }) = self.call_stack.pop() else {
             unreachable!()
         };
         self.with_env = with_env;
-        StepResult::Continue
+        Step::Continue(())
     }
 }

fix-vm/src/lib.rs

@@ -21,12 +21,14 @@ mod boxing;
 mod bytecode_reader;
 #[cfg(feature = "tailcall")]
 mod dispatch_tailcall;
+mod forced;
 mod value;
 pub use value::StaticValue;
 use value::*;
 mod helpers;
 pub(crate) mod instructions;
 pub(crate) use bytecode_reader::BytecodeReader;
+pub(crate) use forced::Forced;
 use helpers::*;
 
 type VmResult<T> = std::result::Result<T, VmError>;
@@ -131,20 +133,13 @@ impl<T: VmContext> VmContextExt for T {
     }
 }
 
-pub(crate) enum StepResult<'gc> {
-    Continue,
-    ForceThunk(ForceInfo<'gc>),
+#[repr(u8)]
+pub(crate) enum Break {
+    Force,
     Done,
 }
 
-pub(crate) struct ForceInfo<'gc> {
-    pub(crate) thunk: Gc<'gc, Thunk<'gc>>,
-    pub(crate) stack_depth: usize,
-    pub(crate) inst_start_pc: usize,
-    pub(crate) ip: usize,
-    pub(crate) env: GcEnv<'gc>,
-    pub(crate) with_env: Option<GcWithEnv<'gc>>,
-}
+pub(crate) type Step = std::ops::ControlFlow<Break>;
 
 #[derive(Collect)]
 #[collect(no_drop)]
@@ -152,6 +147,7 @@ pub struct Vm<'gc> {
     pub(crate) stack: Vec<Value<'gc>>,
     pub(crate) call_stack: Vec<CallFrame<'gc>>,
     pub(crate) call_depth: usize,
+    #[allow(dead_code)]
     #[collect(require_static)]
     pub(crate) error_context: Vec<ErrorFrame>,
 
@@ -279,36 +275,42 @@ impl<'gc> Vm<'gc> {
     }
 
     #[inline(always)]
-    pub(crate) fn finish_ok(&mut self, val: fix_common::Value) -> StepResult<'gc> {
+    pub(crate) fn finish_ok(&mut self, val: fix_common::Value) -> Step {
         self.result = Some(Ok(val));
-        StepResult::Done
+        Step::Break(Break::Done)
     }
 
     #[inline(always)]
-    pub(crate) fn finish_err(&mut self, err: Box<Error>) -> StepResult<'gc> {
+    pub(crate) fn finish_err(&mut self, err: Box<Error>) -> Step {
         self.result = Some(Err(err));
-        StepResult::Done
+        Step::Break(Break::Done)
     }
 
     #[inline(always)]
-    pub(crate) fn finish_vm_err(&mut self, err: VmError) -> StepResult<'gc> {
+    pub(crate) fn finish_type_err(&mut self, expected: NixType, got: NixType) -> Step {
+        self.result = Some(Err(Error::eval_error(format!("expected {expected}, got {got}"))));
+        Step::Break(Break::Done)
+    }
+
+    #[inline(always)]
+    pub(crate) fn finish_vm_err(&mut self, err: VmError) -> Step {
         self.finish_err(err.into_error())
     }
 
     #[inline(always)]
-    pub(crate) fn push_stack(&mut self, val: Value<'gc>) {
+    pub(crate) fn push(&mut self, val: Value<'gc>) {
         self.stack.push(val);
     }
 
     #[inline(always)]
     #[must_use]
-    pub(crate) fn pop_stack(&mut self) -> Value<'gc> {
+    pub(crate) fn pop(&mut self) -> Value<'gc> {
         self.stack.pop().expect("stack underflow")
     }
 
     #[inline(always)]
     #[must_use]
-    pub(crate) fn peek_stack(&mut self, depth: usize) -> Value<'gc> {
+    pub(crate) fn peek(&mut self, depth: usize) -> Value<'gc> {
         *self
             .stack
             .get(self.stack.len() - depth - 1)
@@ -316,7 +318,18 @@ impl<'gc> Vm<'gc> {
     }
 
     #[inline(always)]
-    pub(crate) fn replace_stack(&mut self, depth: usize, val: Value<'gc>) {
+    #[must_use]
+    pub(crate) fn peek_forced(&mut self, depth: usize) -> StrictValue<'gc> {
+        self
+            .stack
+            .get(self.stack.len() - depth - 1)
+            .expect("stack underflow")
+            .restrict()
+            .expect("forced")
+    }
+
+    #[inline(always)]
+    pub(crate) fn replace(&mut self, depth: usize, val: Value<'gc>) {
         let len = self.stack.len();
         *self
             .stack
@@ -326,7 +339,7 @@ impl<'gc> Vm<'gc> {
 
     #[inline(always)]
     #[cfg_attr(debug_assertions, track_caller)]
-    pub(crate) fn pop_stack_forced(&mut self) -> StrictValue<'gc> {
+    pub(crate) fn pop_forced(&mut self) -> StrictValue<'gc> {
         self.stack
             .pop()
             .expect("stack underflow")
@@ -335,67 +348,53 @@ impl<'gc> Vm<'gc> {
     }
 
     #[inline(always)]
-    pub(crate) fn try_force(
+    pub(crate) fn try_force<T: Forced<'gc>>(
+        &mut self,
+        reader: &mut BytecodeReader<'_>,
+        mc: &Mutation<'gc>,
+    ) -> std::ops::ControlFlow<Break, T> {
+        T::force_and_check(self, reader, mc, 0)?;
+        std::ops::ControlFlow::Continue(T::pop_converted(self))
+    }
+
+    #[inline(always)]
+    pub(crate) fn force_slot(
         &mut self,
         depth: usize,
-        inst_start_pc: usize,
+        reader: &mut BytecodeReader<'_>,
         mc: &Mutation<'gc>,
-    ) -> StepResult<'gc> {
-        let val = self.peek_stack(depth);
-        if let Some(thunk) = val.as_gc::<Thunk>() {
+    ) -> Step {
+        let Some(thunk) = self.peek(depth).as_gc::<Thunk>() else {
+            return Step::Continue(());
+        };
         let mut state = thunk.borrow_mut(mc);
         match *state {
             ThunkState::Pending { ip, env, with_env } => {
                 *state = ThunkState::Blackhole;
                 drop(state);
-                    StepResult::ForceThunk(ForceInfo {
-                        thunk,
-                        stack_depth: depth,
-                        inst_start_pc,
-                        ip,
-                        env,
-                        with_env,
-                    })
-                }
-                ThunkState::Evaluated(v) => {
-                    self.replace_stack(depth, v.relax());
-                    StepResult::Continue
-                }
-                ThunkState::Apply { .. } => todo!("force apply"),
-                ThunkState::Blackhole => {
-                    self.finish_err(Error::eval_error("infinite recursion encountered"))
-                }
-            }
-        } else {
-            StepResult::Continue
-        }
-    }
-
-    #[inline(always)]
-    pub(crate) fn try_force_resolved(
-        &mut self,
-        depth: usize,
-        inst_start_pc: usize,
-        mc: &Mutation<'gc>,
-    ) -> Option<StepResult<'gc>> {
-        match self.try_force(depth, inst_start_pc, mc) {
-            StepResult::Continue => None,
-            other => Some(other),
-        }
-    }
-
-    #[inline(always)]
-    pub(crate) fn apply_force_thunk(&mut self, info: ForceInfo<'gc>) -> usize {
                 self.call_stack.push(CallFrame {
-            thunk: Some(info.thunk),
-            stack_depth: info.stack_depth,
-            pc: info.inst_start_pc,
+                    thunk: Some(thunk),
+                    stack_depth: depth,
+                    pc: reader.inst_start_pc(),
                     env: self.env,
                     with_env: self.with_env,
                 });
-        self.env = info.env;
-        self.with_env = info.with_env;
-        info.ip
+                self.env = env;
+                self.with_env = with_env;
+                reader.set_pc(ip);
+                Step::Break(Break::Force)
+            }
+            ThunkState::Evaluated(v) => {
+                drop(state);
+                self.replace(depth, v.relax());
+                Step::Continue(())
+            }
+            ThunkState::Apply { .. } => todo!("force apply"),
+            ThunkState::Blackhole => {
+                drop(state);
+                self.finish_err(Error::eval_error("infinite recursion encountered"))
+            }
+        }
     }
 }
 
@@ -457,6 +456,8 @@ impl Vm<'_> {
 }
 
 impl<'gc> Vm<'gc> {
+    const DEFAULT_FUEL_AMOUNT: u32 = 1024;
+
     #[inline(always)]
     fn dispatch_batch<C: VmContext>(
         &mut self,
@@ -476,10 +477,6 @@ impl<'gc> Vm<'gc> {
                 TailResult::YieldFuel(new_pc) => Action::Continue {
                     pc: new_pc as usize,
                 },
-                TailResult::ForceThunk(info) => {
-                    let new_pc = self.apply_force_thunk(info);
-                    Action::Continue { pc: new_pc }
-                }
                 TailResult::Done => {
                     Action::Done(self.result.take().expect("TailResult::Done without result"))
                 }
@@ -497,10 +494,9 @@ impl<'gc> Vm<'gc> {
         mc: &Mutation<'gc>,
     ) -> Action {
         use fix_codegen::Op::*;
-        const DEFAULT_FUEL_AMOUNT: usize = 1024;
 
         let mut reader = BytecodeReader::new(bytecode, pc);
-        let mut fuel = DEFAULT_FUEL_AMOUNT;
+        let mut fuel = Self::DEFAULT_FUEL_AMOUNT;
 
         loop {
             if fuel == 0 {
@@ -552,7 +548,7 @@ impl<'gc> Vm<'gc> {
                 OpLeq => self.op_leq(ctx, &mut reader, mc),
                 OpGeq => self.op_geq(ctx, &mut reader, mc),
                 OpConcat => self.op_concat(&mut reader, mc),
-                OpUpdate => self.op_update(mc, reader.inst_start_pc()),
+                OpUpdate => self.op_update(&mut reader, mc),
 
                 OpNeg => self.op_neg(),
                 OpNot => self.op_not(),
@@ -582,14 +578,10 @@ impl<'gc> Vm<'gc> {
             };
 
             match result {
-                StepResult::Continue => {}
-                StepResult::ForceThunk(info) => {
-                    let new_pc = self.apply_force_thunk(info);
-                    reader.set_pc(new_pc);
-                }
-                StepResult::Done => {
+                Step::Continue(()) | Step::Break(Break::Force) => {}
+                Step::Break(Break::Done) => {
                     return Action::Done(
-                        self.result.take().expect("StepResult::Done without result"),
+                        self.result.take().expect("Break::Done without result"),
                     );
                 }
             }

fix-vm/src/value.rs

@@ -23,11 +23,12 @@ mod private {
 
 /// # Safety
 ///
-/// 1. TAG must be unique among all implementors.
-/// 2. TAG must be within 1..=7
+/// TAG must be unique among all implementors.
+#[allow(private_interfaces)]
 pub unsafe trait Storable: private::Cealed {
-    const TAG: (bool, u8);
+    const TAG: RawTag;
 }
+#[allow(private_bounds)]
 pub trait InlineStorable: Storable + RawStore {}
 pub trait GcStorable: Storable {}
 
@@ -37,15 +38,17 @@ macro_rules! define_value_types {
         gc { $($gtype:ty => $gtag:expr, $gname:literal;)* }
     ) => {
         $(
+        #[allow(private_interfaces)]
         unsafe impl Storable for $itype {
-            const TAG: (bool, u8) = $itag;
+            const TAG: RawTag = $itag;
         }
         impl InlineStorable for $itype {}
         impl private::Cealed for $itype {}
         )*
         $(
+        #[allow(private_interfaces)]
         unsafe impl Storable for $gtype {
-            const TAG: (bool, u8) = $gtag;
+            const TAG: RawTag = $gtag;
         }
         impl GcStorable for $gtype {}
         impl private::Cealed for $gtype {}
@@ -53,11 +56,9 @@ macro_rules! define_value_types {
 
         const _: () = assert!(size_of::<Value<'static>>() == 8);
         $(const _: () = assert!(size_of::<$itype>() <= 6);)*
-        $(const _: () = { let (_, val) = $itag; assert!(val >= 1 && val <= 7); };)*
-        $(const _: () = { let (_, val) = $gtag; assert!(val >= 1 && val <= 7); };)*
 
         const _: () = {
-            let tags: &[(bool, u8)] = &[$($itag),*, $($gtag),*];
+            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;
@@ -79,12 +80,12 @@ macro_rules! define_value_types {
             const NEEDS_TRACE: bool = true;
             fn trace<T: Trace<'gc>>(&self, cc: &mut T) {
                 let Some(tag) = self.raw.tag() else { return };
-                match tag.neg_val() {
+                match tag {
                     $(<$gtype as Storable>::TAG => unsafe {
                         self.load_gc::<$gtype>().trace(cc)
                     },)*
                     $(<$itype as Storable>::TAG => (),)*
-                    (neg, val) => unreachable!("invalid tag: neg={neg}, val={val}"),
+                    _ => unreachable!("invalid value tag"),
                 }
             }
         }
@@ -100,7 +101,7 @@ macro_rules! define_value_types {
                     }),)*
                     $(Some(<$gtype as Storable>::TAG) =>
                         write!(f, "{}(..)", $gname),)*
-                    Some((neg, val)) => write!(f, "Unknown(neg={neg}, val={val})"),
+                    _ => unreachable!("invalid value tag"),
                 }
             }
         }
@@ -109,20 +110,20 @@ macro_rules! define_value_types {
 
 define_value_types! {
     inline {
-        i32 => (false, 1), "SmallInt";
-        bool => (false, 2), "Bool";
-        Null => (false, 3), "Null";
-        StringId => (false, 4), "SmallString";
-        PrimOp => (false, 5), "PrimOp";
+        i32 => RawTag::P1, "SmallInt";
+        bool => RawTag::P2, "Bool";
+        Null => RawTag::P3, "Null";
+        StringId => RawTag::P4, "SmallString";
+        PrimOp => RawTag::P5, "PrimOp";
     }
     gc {
-        i64 => (false, 6), "BigInt";
-        NixString => (false, 7), "String";
-        AttrSet<'_> => (true, 1), "AttrSet";
-        List<'_> => (true, 2), "List";
-        Thunk<'_> => (true, 3), "Thunk";
-        Closure<'_> => (true, 4), "Closure";
-        PrimOpApp<'_> => (true, 5), "PrimOpApp";
+        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";
     }
 }
 
@@ -131,7 +132,7 @@ define_value_types! {
 /// NaN-boxed value fitting in 8 bytes.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
-pub(crate) struct Value<'gc> {
+pub struct Value<'gc> {
     raw: RawBox,
     _marker: PhantomData<Gc<'gc, ()>>,
 }
@@ -166,16 +167,15 @@ impl<'gc> Value<'gc> {
         }
     }
 
-    /// Returns the `(negative, val)` tag, or `None` for a float.
     #[inline(always)]
-    fn tag(self) -> Option<(bool, u8)> {
-        self.raw.tag().map(|t| t.neg_val())
+    const fn tag(self) -> Option<RawTag> {
+        self.raw.tag()
     }
 }
 
 impl<'gc> Value<'gc> {
     #[inline]
-    pub(crate) fn new_float(val: f64) -> Self {
+    pub fn new_float(val: f64) -> Self {
         Self {
             raw: RawBox::from_float(val),
             _marker: PhantomData,
@@ -183,24 +183,24 @@ impl<'gc> Value<'gc> {
     }
 
     #[inline]
-    pub(crate) fn new_inline<T: InlineStorable>(val: T) -> Self {
+    pub fn new_inline<T: InlineStorable>(val: T) -> Self {
         Self::from_raw_value(RawValue::store(
-            unsafe { RawTag::new_unchecked(T::TAG.0, T::TAG.1) },
+            T::TAG,
             val,
         ))
     }
 
     #[inline]
-    pub(crate) fn new_gc<T: GcStorable>(gc: Gc<'gc, T>) -> Self {
+    pub fn new_gc<T: GcStorable>(gc: Gc<'gc, T>) -> Self {
         let ptr = Gc::as_ptr(gc);
         Self::from_raw_value(RawValue::store(
-            unsafe { RawTag::new_unchecked(T::TAG.0, T::TAG.1) },
+            T::TAG,
             ptr,
         ))
     }
 
     #[inline]
-    pub(crate) fn make_int(val: i64, mc: &Mutation<'gc>) -> Self {
+    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 {
@@ -211,24 +211,24 @@ impl<'gc> Value<'gc> {
 
 impl<'gc> Value<'gc> {
     #[inline]
-    pub(crate) fn is_float(self) -> bool {
+    pub fn is_float(self) -> bool {
         self.raw.is_float()
     }
 
     #[inline]
-    pub(crate) fn is<T: Storable>(self) -> bool {
+    pub fn is<T: Storable>(self) -> bool {
         self.tag() == Some(T::TAG)
     }
 }
 
 impl<'gc> Value<'gc> {
     #[inline]
-    pub(crate) fn as_float(self) -> Option<f64> {
+    pub fn as_float(self) -> Option<f64> {
         self.raw.float().copied()
     }
 
     #[inline]
-    pub(crate) fn as_inline<T: InlineStorable>(self) -> Option<T> {
+    pub fn as_inline<T: InlineStorable>(self) -> Option<T> {
         if self.is::<T>() {
             Some(unsafe {
                 let rv = self.raw.value().unwrap_unchecked();
@@ -240,7 +240,7 @@ impl<'gc> Value<'gc> {
     }
 
     #[inline]
-    pub(crate) fn as_gc<T: GcStorable>(self) -> Option<Gc<'gc, T>> {
+    pub fn as_gc<T: GcStorable>(self) -> Option<Gc<'gc, T>> {
         if self.is::<T>() {
             Some(unsafe {
                 let rv = self.raw.value().unwrap_unchecked();
@@ -253,7 +253,7 @@ impl<'gc> Value<'gc> {
     }
 
     #[inline]
-    pub(crate) fn as_num(self) -> Option<NixNum> {
+    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>() {
@@ -264,13 +264,69 @@ impl<'gc> Value<'gc> {
     }
 
     #[inline]
-    pub(crate) fn restrict(self) -> Result<StrictValue<'gc>, Gc<'gc, Thunk<'gc>>> {
+    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::<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(crate) fn expect_inline<T: InlineStorable>(self) -> Result<T, NixType> {
+        self.as_inline::<T>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub(crate) fn expect_gc<T: GcStorable>(self) -> Result<Gc<'gc, T>, NixType> {
+        self.as_gc::<T>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub(crate) fn expect_num(self) -> Result<NixNum, NixType> {
+        self.as_num().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub(crate) fn expect_bool(self) -> Result<bool, NixType> {
+        self.as_inline::<bool>().ok_or_else(|| self.ty())
+    }
+
+    #[inline]
+    pub(crate) fn expect_float(self) -> Result<f64, NixType> {
+        self.as_float().ok_or_else(|| self.ty())
+    }
 }
 
 #[derive(Copy, Clone, Default)]
@@ -549,8 +605,9 @@ pub(crate) struct PrimOpApp<'gc> {
     pub(crate) args: SmallVec<[Value<'gc>; 2]>,
 }
 
-#[derive(Copy, Clone, Default)]
+#[derive(Copy, Clone, Default, Collect)]
 #[repr(transparent)]
+#[collect(no_drop)]
 pub(crate) struct StrictValue<'gc>(Value<'gc>);
 
 impl<'gc> StrictValue<'gc> {
@@ -574,9 +631,43 @@ impl fmt::Debug for StrictValue<'_> {
     }
 }
 
-unsafe impl<'gc> Collect<'gc> for StrictValue<'gc> {
-    const NEEDS_TRACE: bool = true;
-    fn trace<T: gc_arena::collect::Trace<'gc>>(&self, cc: &mut T) {
-        self.0.trace(cc);
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Collect)]
+#[collect(require_static)]
+pub(crate) enum NixType {
+    Int,
+    Float,
+    Bool,
+    Null,
+    String,
+    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",
+            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())
     }
 }