6 files changed, 33 insertions, 17 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 78fe76ca01a9..0d2ed66f5bf5 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -805,16 +805,17 @@ impl Compiler {
         // If the function is not a closure, just emit it directly and
         // move on.
         if compiled.lambda.upvalue_count == 0 {
-            self.emit_constant(Value::Closure(Closure::new(compiled.lambda)));
+            self.emit_constant(Value::Closure(Closure::new(Rc::new(compiled.lambda))));
             return;
         }
 
         // If the function is a closure, we need to emit the variable
         // number of operands that allow the runtime to close over the
-        // upvalues.
+        // upvalues and leave a blueprint in the constant index from
+        // which the runtime closure can be constructed.
         let closure_idx = self
             .chunk()
-            .push_constant(Value::Closure(Closure::new(compiled.lambda)));
+            .push_constant(Value::Blueprint(Rc::new(compiled.lambda)));
 
         self.chunk().push_op(OpCode::OpClosure(closure_idx));
         for upvalue in compiled.scope.upvalues {
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp
new file mode 100644
index 000000000000..8643cf6debac
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp
@@ -0,0 +1 @@
+89
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix
new file mode 100644
index 000000000000..9a22d85ac5f1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix
@@ -0,0 +1,7 @@
+let
+  fib' = i: n: m: if i == 0
+    then n
+    else fib' (i - 1) m (n + m);
+
+  fib = n: fib' n 1 1;
+in fib 10
diff --git a/tvix/eval/src/value/function.rs b/tvix/eval/src/value/function.rs
index d0209cc50725..e5db43d58ace 100644
--- a/tvix/eval/src/value/function.rs
+++ b/tvix/eval/src/value/function.rs
@@ -29,7 +29,7 @@ impl Lambda {
 
 #[derive(Clone, Debug)]
 pub struct InnerClosure {
-    pub lambda: Lambda,
+    pub lambda: Rc<Lambda>,
     pub upvalues: Vec<Value>,
 }
 
@@ -38,7 +38,7 @@ pub struct InnerClosure {
 pub struct Closure(Rc<RefCell<InnerClosure>>);
 
 impl Closure {
-    pub fn new(lambda: Lambda) -> Self {
+    pub fn new(lambda: Rc<Lambda>) -> Self {
         Closure(Rc::new(RefCell::new(InnerClosure {
             upvalues: Vec::with_capacity(lambda.upvalue_count),
             lambda,
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 54211e8ba313..911af9d6ae12 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -35,6 +35,7 @@ pub enum Value {
     AttrPath(Vec<NixString>),
     AttrNotFound,
     DynamicUpvalueMissing(NixString),
+    Blueprint(Rc<Lambda>),
 }
 
 impl Value {
@@ -55,9 +56,10 @@ impl Value {
             Value::Closure(_) | Value::Builtin(_) => "lambda",
 
             // Internal types
-            Value::AttrPath(_) | Value::AttrNotFound | Value::DynamicUpvalueMissing(_) => {
-                "internal"
-            }
+            Value::AttrPath(_)
+            | Value::AttrNotFound
+            | Value::DynamicUpvalueMissing(_)
+            | Value::Blueprint(_) => "internal",
         }
     }
 
@@ -166,6 +168,7 @@ impl Display for Value {
             // internal types
             Value::AttrPath(path) => write!(f, "internal[attrpath({})]", path.len()),
             Value::AttrNotFound => f.write_str("internal[not found]"),
+            Value::Blueprint(_) => f.write_str("internal[blueprint]"),
             Value::DynamicUpvalueMissing(name) => {
                 write!(f, "internal[no_dyn_upvalue({name})]")
             }
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 5d5cb57dbabf..7b3de6406549 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -385,15 +385,19 @@ impl VM {
                 }
 
                 OpCode::OpClosure(idx) => {
-                    let value = self.chunk().constant(idx).clone();
-                    self.push(value.clone());
+                    let blueprint = match self.chunk().constant(idx) {
+                        Value::Blueprint(lambda) => lambda.clone(),
+                        _ => panic!("compiler bug: non-blueprint in blueprint slot"),
+                    };
+
+                    let closure = Closure::new(blueprint);
+                    self.push(Value::Closure(closure.clone()));
 
-                    // This refers to the same Rc, and from this point
-                    // on internally mutates the closure objects
-                    // upvalues. The closure is already in its stack
-                    // slot, which means that it can capture itself as
-                    // an upvalue for self-recursion.
-                    let closure = value.to_closure()?;
+                    // 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.
 
                     debug_assert!(
                         closure.upvalue_count() > 0,
@@ -536,6 +540,6 @@ pub fn run_lambda(lambda: Lambda) -> EvalResult<Value> {
         with_stack: vec![],
     };
 
-    vm.call(Closure::new(lambda), 0);
+    vm.call(Closure::new(Rc::new(lambda)), 0);
     vm.run()
 }