feat(tvix/eval): deduplicate overlap between Closure and Thunk r/5159

This commit deduplicates the Thunk-like functionality from Closure
and unifies it with Thunk.

Specifically, we now have one and only one way of breaking reference
cycles in the Value-graph: Thunk.  No other variant contains a
RefCell.  This should make it easier to reason about the behavior of
the VM.  InnerClosure and UpvaluesCarrier are no longer necessary.

This refactoring allowed an improvement in code generation:
`Rc<RefCell<>>`s are now created only for closures which do not have
self-references or deferred upvalues, instead of for all closures.
OpClosure has been split into two separate opcodes:

- OpClosure creates non-recursive closures with no deferred
  upvalues.  The VM will not create an `Rc<RefCell<>>` when executing
  this instruction.

- OpThunkClosure is used for closures with self-references or
  deferred upvalues.  The VM will create a Thunk when executing this
  opcode, but the Thunk will start out already in the
  `ThunkRepr::Evaluated` state, rather than in the
  `ThunkRepr::Suspeneded` state.

To avoid confusion, OpThunk has been renamed OpThunkSuspended.

Thanks to @sterni for suggesting that all this could be done without
adding an additional variant to ThunkRepr.  This does however mean
that there will be mutating accesses to `ThunkRepr::Evaluated`,
which was not previously the case.  The field `is_finalised:bool`
has been added to `Closure` to ensure that these mutating accesses
are performed only on finalised Closures.  Both the check and the
field are present only if `#[cfg(debug_assertions)]`.

Change-Id: I04131501029772f30e28da8281d864427685097f
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7019
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

9 files changed, 203 insertions, 149 deletions

diff --git a/tvix/eval/src/builtins/impure.rs b/tvix/eval/src/builtins/impure.rs
index 3d666528e2..5edd81999a 100644
--- a/tvix/eval/src/builtins/impure.rs
+++ b/tvix/eval/src/builtins/impure.rs
@@ -151,7 +151,10 @@ pub fn builtins_import(
 
             // Compilation succeeded, we can construct a thunk from whatever it spat
             // out and return that.
-            Ok(Value::Thunk(Thunk::new(result.lambda, vm.current_span())))
+            Ok(Value::Thunk(Thunk::new_suspended(
+                result.lambda,
+                vm.current_span(),
+            )))
         },
     )
 }
diff --git a/tvix/eval/src/chunk.rs b/tvix/eval/src/chunk.rs
index 5026cdb736..9f2fdf54dc 100644
--- a/tvix/eval/src/chunk.rs
+++ b/tvix/eval/src/chunk.rs
@@ -1,5 +1,5 @@
 use std::io::Write;
-use std::ops::Index;
+use std::ops::{Index, IndexMut};
 
 use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
 use crate::value::Value;
@@ -51,6 +51,12 @@ impl Index<CodeIdx> for Chunk {
     }
 }
 
+impl IndexMut<CodeIdx> for Chunk {
+    fn index_mut(&mut self, index: CodeIdx) -> &mut Self::Output {
+        &mut self.code[index.0]
+    }
+}
+
 impl Chunk {
     pub fn push_op(&mut self, data: OpCode, span: codemap::Span) -> CodeIdx {
         let idx = self.code.len();
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 9394dce484..819a37ec71 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -927,7 +927,7 @@ impl Compiler<'_> {
         if lambda.upvalue_count == 0 {
             self.emit_constant(
                 if is_thunk {
-                    Value::Thunk(Thunk::new(lambda, span))
+                    Value::Thunk(Thunk::new_suspended(lambda, span))
                 } else {
                     Value::Closure(Closure::new(lambda))
                 },
@@ -942,11 +942,11 @@ impl Compiler<'_> {
         // which the result can be constructed.
         let blueprint_idx = self.chunk().push_constant(Value::Blueprint(lambda));
 
-        self.push_op(
+        let code_idx = self.push_op(
             if is_thunk {
-                OpCode::OpThunk(blueprint_idx)
+                OpCode::OpThunkSuspended(blueprint_idx)
             } else {
-                OpCode::OpClosure(blueprint_idx)
+                OpCode::OpThunkClosure(blueprint_idx)
             },
             node,
         );
@@ -957,6 +957,23 @@ impl Compiler<'_> {
             compiled.scope.upvalues,
             compiled.captures_with_stack,
         );
+
+        if !is_thunk && !self.scope()[outer_slot].needs_finaliser {
+            if !self.scope()[outer_slot].must_thunk {
+                // The closure has upvalues, but is not recursive.  Therefore no thunk is required,
+                // which saves us the overhead of Rc<RefCell<>>
+                self.chunk()[code_idx] = OpCode::OpClosure(blueprint_idx);
+            } else {
+                // This case occurs when a closure has upvalue-references to itself but does not need a
+                // finaliser.  Since no OpFinalise will be emitted later on we synthesize one here.
+                // It is needed here only to set [`Closure::is_finalised`] which is used for sanity checks.
+                #[cfg(debug_assertions)]
+                self.push_op(
+                    OpCode::OpFinalise(self.scope().stack_index(outer_slot)),
+                    &self.span_for(node),
+                );
+            }
+        }
     }
 
     fn compile_apply(&mut self, slot: LocalIdx, node: &ast::Apply) {
@@ -996,6 +1013,10 @@ impl Compiler<'_> {
                         self.push_op(OpCode::DataDeferredLocal(stack_idx), &upvalue.span);
                         self.scope_mut().mark_needs_finaliser(slot);
                     } else {
+                        // a self-reference
+                        if this_depth == target_depth && this_stack_slot == stack_idx {
+                            self.scope_mut().mark_must_thunk(slot);
+                        }
                         self.push_op(OpCode::DataLocalIdx(stack_idx), &upvalue.span);
                     }
                 }
diff --git a/tvix/eval/src/compiler/scope.rs b/tvix/eval/src/compiler/scope.rs
index 88df218ebd..9269a3c44c 100644
--- a/tvix/eval/src/compiler/scope.rs
+++ b/tvix/eval/src/compiler/scope.rs
@@ -50,6 +50,9 @@ pub struct Local {
     /// Does this local need to be finalised after the enclosing scope
     /// is completely constructed?
     pub needs_finaliser: bool,
+
+    /// Does this local's upvalues contain a reference to itself?
+    pub must_thunk: bool,
 }
 
 impl Local {
@@ -228,6 +231,7 @@ impl Scope {
             name: LocalName::Phantom,
             depth: self.scope_depth,
             needs_finaliser: false,
+            must_thunk: false,
             used: true,
         });
 
@@ -243,6 +247,7 @@ impl Scope {
             depth: self.scope_depth,
             initialised: false,
             needs_finaliser: false,
+            must_thunk: false,
             used: false,
         });
 
@@ -259,6 +264,12 @@ impl Scope {
         self.locals[idx.0].needs_finaliser = true;
     }
 
+    /// Mark local as must be wrapped in a thunk.  This happens if
+    /// the local has a reference to itself in its upvalues.
+    pub fn mark_must_thunk(&mut self, idx: LocalIdx) {
+        self.locals[idx.0].must_thunk = true;
+    }
+
     /// Compute the runtime stack index for a given local by
     /// accounting for uninitialised variables at scopes below this
     /// one.
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 6b2783716c..382a857ef9 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -144,25 +144,27 @@ pub enum OpCode {
     OpCall,
     OpTailCall,
     OpGetUpvalue(UpvalueIdx),
+    // A Closure which has upvalues but no self-references
     OpClosure(ConstantIdx),
-
-    // Thunks
-    OpThunk(ConstantIdx),
+    // A Closure which has self-references (direct or via upvalues)
+    OpThunkClosure(ConstantIdx),
+    // A suspended thunk, used to ensure laziness
+    OpThunkSuspended(ConstantIdx),
     OpForce,
 
     /// Finalise initialisation of the upvalues of the value in the
     /// given stack index after the scope is fully bound.
     OpFinalise(StackIdx),
 
-    // [`OpClosure`] and [`OpThunk`] have a variable number of
-    // arguments to the instruction, which is represented here by
-    // making their data part of the opcodes.  Each of these two
-    // opcodes has a `ConstantIdx`, which must reference a
-    // `Value::Blueprint(Lambda)`.  The `upvalue_count` field in
-    // that `Lambda` indicates the number of arguments it takes, and
-    // the `OpClosure` or `OpThunk` must be followed by exactly this
-    // number of `Data*` opcodes.  The VM skips over these by
-    // advancing the instruction pointer.
+    // [`OpClosure`], [`OpThunkSuspended`], and [`OpThunkClosure`] have a
+    // variable number of arguments to the instruction, which is
+    // represented here by making their data part of the opcodes.
+    // Each of these two opcodes has a `ConstantIdx`, which must
+    // reference a `Value::Blueprint(Lambda)`.  The `upvalue_count`
+    // field in that `Lambda` indicates the number of arguments it
+    // takes, and the opcode must be followed by exactly this number
+    // of `Data*` opcodes.  The VM skips over these by advancing the
+    // instruction pointer.
     //
     // It is illegal for a `Data*` opcode to appear anywhere else.
     DataLocalIdx(StackIdx),
diff --git a/tvix/eval/src/upvalues.rs b/tvix/eval/src/upvalues.rs
index 6da9bcf7cc..450ea130ff 100644
--- a/tvix/eval/src/upvalues.rs
+++ b/tvix/eval/src/upvalues.rs
@@ -7,10 +7,7 @@
 //! in order to resolve each free variable in the scope to a value.
 //! "Upvalue" is a term taken from Lua.
 
-use std::{
-    cell::{Ref, RefMut},
-    ops::Index,
-};
+use std::ops::Index;
 
 use crate::{opcode::UpvalueIdx, Value};
 
@@ -68,6 +65,16 @@ impl Upvalues {
             Some(stack) => stack.len(),
         }
     }
+
+    /// Resolve deferred upvalues from the provided stack slice,
+    /// mutating them in the internal upvalue slots.
+    pub fn resolve_deferred_upvalues(&mut self, stack: &[Value]) {
+        for upvalue in self.static_upvalues.iter_mut() {
+            if let Value::DeferredUpvalue(update_from_idx) = upvalue {
+                *upvalue = stack[update_from_idx.0].clone();
+            }
+        }
+    }
 }
 
 impl Index<UpvalueIdx> for Upvalues {
@@ -77,29 +84,3 @@ impl Index<UpvalueIdx> for Upvalues {
         &self.static_upvalues[index.0]
     }
 }
-
-/// `UpvalueCarrier` is implemented by all types that carry upvalues.
-pub trait UpvalueCarrier {
-    fn upvalue_count(&self) -> usize;
-
-    /// Read-only accessor for the stored upvalues.
-    fn upvalues(&self) -> Ref<'_, Upvalues>;
-
-    /// Mutable accessor for stored upvalues.
-    fn upvalues_mut(&self) -> RefMut<'_, Upvalues>;
-
-    /// Read an upvalue at the given index.
-    fn upvalue(&self, idx: UpvalueIdx) -> Ref<'_, Value> {
-        Ref::map(self.upvalues(), |v| &v.static_upvalues[idx.0])
-    }
-
-    /// Resolve deferred upvalues from the provided stack slice,
-    /// mutating them in the internal upvalue slots.
-    fn resolve_deferred_upvalues(&self, stack: &[Value]) {
-        for upvalue in self.upvalues_mut().static_upvalues.iter_mut() {
-            if let Value::DeferredUpvalue(idx) = upvalue {
-                *upvalue = stack[idx.0].clone();
-            }
-        }
-    }
-}
diff --git a/tvix/eval/src/value/function.rs b/tvix/eval/src/value/function.rs
index a3aa9325f5..bb33e2962a 100644
--- a/tvix/eval/src/value/function.rs
+++ b/tvix/eval/src/value/function.rs
@@ -1,17 +1,9 @@
 //! This module implements the runtime representation of functions.
-use std::{
-    cell::{Ref, RefCell, RefMut},
-    collections::HashMap,
-    hash::Hash,
-    rc::Rc,
-};
+use std::{collections::HashMap, hash::Hash, rc::Rc};
 
 use codemap::Span;
 
-use crate::{
-    chunk::Chunk,
-    upvalues::{UpvalueCarrier, Upvalues},
-};
+use crate::{chunk::Chunk, upvalues::Upvalues};
 
 use super::NixString;
 
@@ -42,8 +34,8 @@ impl Formals {
 }
 
 /// The opcodes for a thunk or closure, plus the number of
-/// non-executable opcodes which are allowed after an OpClosure or
-/// OpThunk referencing it.  At runtime `Lambda` is usually wrapped
+/// non-executable opcodes which are allowed after an OpThunkClosure or
+/// OpThunkSuspended referencing it.  At runtime `Lambda` is usually wrapped
 /// in `Rc` to avoid copying the `Chunk` it holds (which can be
 /// quite large).
 #[derive(Debug, PartialEq)]
@@ -73,42 +65,37 @@ impl Lambda {
 }
 
 #[derive(Clone, Debug, PartialEq)]
-pub struct InnerClosure {
+pub struct Closure {
     pub lambda: Rc<Lambda>,
-    upvalues: Upvalues,
+    pub upvalues: Upvalues,
+    /// true if all upvalues have been realised
+    #[cfg(debug_assertions)]
+    pub is_finalised: bool,
 }
 
-#[repr(transparent)]
-#[derive(Clone, Debug, PartialEq)]
-pub struct Closure(Rc<RefCell<InnerClosure>>);
-
 impl Closure {
     pub fn new(lambda: Rc<Lambda>) -> Self {
-        Closure(Rc::new(RefCell::new(InnerClosure {
-            upvalues: Upvalues::with_capacity(lambda.upvalue_count),
-            lambda,
-        })))
-    }
-
-    pub fn chunk(&self) -> Ref<'_, Chunk> {
-        Ref::map(self.0.borrow(), |c| &c.lambda.chunk)
+        Self::new_with_upvalues(Upvalues::with_capacity(lambda.upvalue_count), lambda)
     }
 
-    pub fn lambda(&self) -> Rc<Lambda> {
-        self.0.borrow().lambda.clone()
+    pub fn new_with_upvalues(upvalues: Upvalues, lambda: Rc<Lambda>) -> Self {
+        Closure {
+            upvalues,
+            lambda,
+            #[cfg(debug_assertions)]
+            is_finalised: true,
+        }
     }
-}
 
-impl UpvalueCarrier for Closure {
-    fn upvalue_count(&self) -> usize {
-        self.0.borrow().lambda.upvalue_count
+    pub fn chunk(&self) -> &Chunk {
+        &self.lambda.chunk
     }
 
-    fn upvalues(&self) -> Ref<'_, Upvalues> {
-        Ref::map(self.0.borrow(), |c| &c.upvalues)
+    pub fn lambda(&self) -> Rc<Lambda> {
+        self.lambda.clone()
     }
 
-    fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
-        RefMut::map(self.0.borrow_mut(), |c| &mut c.upvalues)
+    pub fn upvalues(&self) -> &Upvalues {
+        &self.upvalues
     }
 }
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
index 887c297a53..818ec0f58a 100644
--- a/tvix/eval/src/value/thunk.rs
+++ b/tvix/eval/src/value/thunk.rs
@@ -28,7 +28,8 @@ use codemap::Span;
 
 use crate::{
     errors::{Error, ErrorKind},
-    upvalues::{UpvalueCarrier, Upvalues},
+    upvalues::Upvalues,
+    value::Closure,
     vm::VM,
     Value,
 };
@@ -36,6 +37,10 @@ use crate::{
 use super::Lambda;
 
 /// Internal representation of the different states of a thunk.
+///
+/// Upvalues must be finalised before leaving the initial state
+/// (Suspended or RecursiveClosure).  The [`value()`] function may
+/// not be called until the thunk is in the final state (Evaluated).
 #[derive(Clone, Debug, PartialEq)]
 enum ThunkRepr {
     /// Thunk is closed over some values, suspended and awaiting
@@ -43,6 +48,7 @@ enum ThunkRepr {
     Suspended {
         lambda: Rc<Lambda>,
         upvalues: Upvalues,
+        span: Span,
     },
 
     /// Thunk currently under-evaluation; encountering a blackhole
@@ -53,21 +59,31 @@ enum ThunkRepr {
     Evaluated(Value),
 }
 
+/// A thunk is created for any value which requires non-strict
+/// evaluation due to self-reference or lazy semantics (or both).
+/// Every reference cycle involving `Value`s will contain at least
+/// one `Thunk`.
 #[derive(Clone, Debug, PartialEq)]
-pub struct Thunk {
-    inner: Rc<RefCell<ThunkRepr>>,
-    span: Span,
-}
+pub struct Thunk(Rc<RefCell<ThunkRepr>>);
 
 impl Thunk {
-    pub fn new(lambda: Rc<Lambda>, span: Span) -> Self {
-        Thunk {
-            inner: Rc::new(RefCell::new(ThunkRepr::Suspended {
+    pub fn new_closure(lambda: Rc<Lambda>) -> Self {
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Evaluated(Value::Closure(
+            Closure {
                 upvalues: Upvalues::with_capacity(lambda.upvalue_count),
                 lambda: lambda.clone(),
-            })),
+                #[cfg(debug_assertions)]
+                is_finalised: false,
+            },
+        )))))
+    }
+
+    pub fn new_suspended(lambda: Rc<Lambda>, span: Span) -> Self {
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
+            upvalues: Upvalues::with_capacity(lambda.upvalue_count),
+            lambda: lambda.clone(),
             span,
-        }
+        })))
     }
 
     /// Evaluate the content of a thunk, potentially repeatedly, until
@@ -79,11 +95,11 @@ impl Thunk {
     /// are replaced.
     pub fn force(&self, vm: &mut VM) -> Result<(), ErrorKind> {
         loop {
-            let mut thunk_mut = self.inner.borrow_mut();
+            let mut thunk_mut = self.0.borrow_mut();
 
             match *thunk_mut {
                 ThunkRepr::Evaluated(Value::Thunk(ref inner_thunk)) => {
-                    let inner_repr = inner_thunk.inner.borrow().clone();
+                    let inner_repr = inner_thunk.0.borrow().clone();
                     *thunk_mut = inner_repr;
                 }
 
@@ -91,24 +107,33 @@ impl Thunk {
                 ThunkRepr::Blackhole => return Err(ErrorKind::InfiniteRecursion),
 
                 ThunkRepr::Suspended { .. } => {
-                    if let ThunkRepr::Suspended { lambda, upvalues } =
-                        std::mem::replace(&mut *thunk_mut, ThunkRepr::Blackhole)
+                    if let ThunkRepr::Suspended {
+                        lambda,
+                        upvalues,
+                        span,
+                    } = std::mem::replace(&mut *thunk_mut, ThunkRepr::Blackhole)
                     {
                         drop(thunk_mut);
-                        vm.enter_frame(lambda, upvalues, 0).map_err(|e| {
-                            ErrorKind::ThunkForce(Box::new(Error {
-                                span: self.span,
-                                ..e
-                            }))
-                        })?;
-                        let evaluated = ThunkRepr::Evaluated(vm.pop());
-                        (*self.inner.borrow_mut()) = evaluated;
+                        vm.enter_frame(lambda, upvalues, 0)
+                            .map_err(|e| ErrorKind::ThunkForce(Box::new(Error { span, ..e })))?;
+                        (*self.0.borrow_mut()) = ThunkRepr::Evaluated(vm.pop())
                     }
                 }
             }
         }
     }
 
+    pub fn finalise(&self, stack: &[Value]) {
+        self.upvalues_mut().resolve_deferred_upvalues(stack);
+        #[cfg(debug_assertions)]
+        {
+            let inner: &mut ThunkRepr = &mut self.0.as_ref().borrow_mut();
+            if let ThunkRepr::Evaluated(Value::Closure(c)) = inner {
+                c.is_finalised = true;
+            }
+        }
+    }
+
     /// Returns a reference to the inner evaluated value of a thunk.
     /// It is an error to call this on a thunk that has not been
     /// forced, or is not otherwise known to be fully evaluated.
@@ -116,35 +141,48 @@ impl Thunk {
     // difficult to represent in the type system without impacting the
     // API too much.
     pub fn value(&self) -> Ref<Value> {
-        Ref::map(self.inner.borrow(), |thunk| {
+        Ref::map(self.0.borrow(), |thunk| {
             if let ThunkRepr::Evaluated(value) = thunk {
+                #[cfg(debug_assertions)]
+                if matches!(
+                    value,
+                    Value::Closure(Closure {
+                        is_finalised: false,
+                        ..
+                    })
+                ) {
+                    panic!("Thunk::value called on an unfinalised closure");
+                }
                 return value;
             }
 
             panic!("Thunk::value called on non-evaluated thunk");
         })
     }
-}
-
-impl UpvalueCarrier for Thunk {
-    fn upvalue_count(&self) -> usize {
-        if let ThunkRepr::Suspended { lambda, .. } = &*self.inner.borrow() {
-            return lambda.upvalue_count;
-        }
-
-        panic!("upvalues() on non-suspended thunk");
-    }
 
-    fn upvalues(&self) -> Ref<'_, Upvalues> {
-        Ref::map(self.inner.borrow(), |thunk| match thunk {
+    pub fn upvalues(&self) -> Ref<'_, Upvalues> {
+        Ref::map(self.0.borrow(), |thunk| match thunk {
             ThunkRepr::Suspended { upvalues, .. } => upvalues,
+            ThunkRepr::Evaluated(Value::Closure(Closure { upvalues, .. })) => upvalues,
             _ => panic!("upvalues() on non-suspended thunk"),
         })
     }
 
-    fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
-        RefMut::map(self.inner.borrow_mut(), |thunk| match thunk {
+    pub fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
+        RefMut::map(self.0.borrow_mut(), |thunk| match thunk {
             ThunkRepr::Suspended { upvalues, .. } => upvalues,
+            ThunkRepr::Evaluated(Value::Closure(Closure {
+                upvalues,
+                #[cfg(debug_assertions)]
+                is_finalised,
+                ..
+            })) => {
+                #[cfg(debug_assertions)]
+                if *is_finalised {
+                    panic!("Thunk::upvalues_mut() called on a finalised closure");
+                }
+                upvalues
+            }
             thunk => panic!("upvalues() on non-suspended thunk: {thunk:?}"),
         })
     }
@@ -152,7 +190,7 @@ impl UpvalueCarrier for Thunk {
 
 impl Display for Thunk {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self.inner.try_borrow() {
+        match self.0.try_borrow() {
             Ok(repr) => match &*repr {
                 ThunkRepr::Evaluated(v) => v.fmt(f),
                 _ => f.write_str("internal[thunk]"),
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 65662ed555..ff39cf1295 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -1,7 +1,7 @@
 //! This module implements the virtual (or abstract) machine that runs
 //! Tvix bytecode.
 
-use std::{cell::RefMut, path::PathBuf, rc::Rc};
+use std::{ops::DerefMut, path::PathBuf, rc::Rc};
 
 use crate::{
     chunk::Chunk,
@@ -9,7 +9,7 @@ use crate::{
     nix_search_path::NixSearchPath,
     observer::RuntimeObserver,
     opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
-    upvalues::{UpvalueCarrier, Upvalues},
+    upvalues::Upvalues,
     value::{Builtin, Closure, CoercionKind, Lambda, NixAttrs, NixList, Thunk, Value},
     warnings::{EvalWarning, WarningKind},
 };
@@ -675,30 +675,37 @@ impl<'o> VM<'o> {
                     upvalue_count > 0,
                     "OpClosure should not be called for plain lambdas"
                 );
-
-                let closure = Closure::new(blueprint);
-                let upvalues = closure.upvalues_mut();
-                self.push(Value::Closure(closure.clone()));
-
-                // From this point on we internally mutate the
-                // closure object's upvalues. The closure is
-                // already in its stack slot, which means that it
-                // can capture itself as an upvalue for
-                // self-recursion.
-                self.populate_upvalues(upvalue_count, upvalues)?;
+                let mut upvalues = Upvalues::with_capacity(blueprint.upvalue_count);
+                self.populate_upvalues(upvalue_count, &mut upvalues)?;
+                self.push(Value::Closure(Closure::new_with_upvalues(
+                    upvalues, blueprint,
+                )));
             }
 
-            OpCode::OpThunk(idx) => {
+            OpCode::OpThunkSuspended(idx) | OpCode::OpThunkClosure(idx) => {
                 let blueprint = match &self.chunk()[idx] {
                     Value::Blueprint(lambda) => lambda.clone(),
                     _ => panic!("compiler bug: non-blueprint in blueprint slot"),
                 };
 
                 let upvalue_count = blueprint.upvalue_count;
-                let thunk = Thunk::new(blueprint, self.current_span());
+                let thunk = if matches!(op, OpCode::OpThunkClosure(_)) {
+                    debug_assert!(
+                        upvalue_count > 0,
+                        "OpThunkClosure should not be called for plain lambdas"
+                    );
+                    Thunk::new_closure(blueprint)
+                } else {
+                    Thunk::new_suspended(blueprint, self.current_span())
+                };
                 let upvalues = thunk.upvalues_mut();
-
                 self.push(Value::Thunk(thunk.clone()));
+
+                // From this point on we internally mutate the
+                // upvalues. The closure (if `is_closure`) is
+                // already in its stack slot, which means that it
+                // can capture itself as an upvalue for
+                // self-recursion.
                 self.populate_upvalues(upvalue_count, upvalues)?;
             }
 
@@ -715,13 +722,9 @@ impl<'o> VM<'o> {
 
             OpCode::OpFinalise(StackIdx(idx)) => {
                 match &self.stack[self.frame().stack_offset + idx] {
-                    Value::Closure(closure) => {
-                        closure.resolve_deferred_upvalues(&self.stack[self.frame().stack_offset..])
-                    }
+                    Value::Closure(_) => panic!("attempted to finalise a closure"),
 
-                    Value::Thunk(thunk) => {
-                        thunk.resolve_deferred_upvalues(&self.stack[self.frame().stack_offset..])
-                    }
+                    Value::Thunk(thunk) => thunk.finalise(&self.stack[self.frame().stack_offset..]),
 
                     // In functions with "formals" attributes, it is
                     // possible for `OpFinalise` to be called on a
@@ -809,21 +812,23 @@ impl<'o> VM<'o> {
     fn populate_upvalues(
         &mut self,
         count: usize,
-        mut upvalues: RefMut<'_, Upvalues>,
+        mut upvalues: impl DerefMut<Target = Upvalues>,
     ) -> EvalResult<()> {
         for _ in 0..count {
             match self.inc_ip() {
                 OpCode::DataLocalIdx(StackIdx(local_idx)) => {
                     let idx = self.frame().stack_offset + local_idx;
-                    upvalues.push(self.stack[idx].clone());
+                    upvalues.deref_mut().push(self.stack[idx].clone());
                 }
 
                 OpCode::DataUpvalueIdx(upv_idx) => {
-                    upvalues.push(self.frame().upvalue(upv_idx).clone());
+                    upvalues
+                        .deref_mut()
+                        .push(self.frame().upvalue(upv_idx).clone());
                 }
 
                 OpCode::DataDeferredLocal(idx) => {
-                    upvalues.push(Value::DeferredUpvalue(idx));
+                    upvalues.deref_mut().push(Value::DeferredUpvalue(idx));
                 }
 
                 OpCode::DataCaptureWith => {
@@ -841,7 +846,7 @@ impl<'o> VM<'o> {
                         captured_with_stack.push(self.stack[*idx].clone());
                     }
 
-                    upvalues.set_with_stack(captured_with_stack);
+                    upvalues.deref_mut().set_with_stack(captured_with_stack);
                 }
 
                 _ => panic!("compiler error: missing closure operand"),