Refactor stack allocator to reuse freed slots

Introduce `free_slots` in `StackAllocator` to keep track of physical
slots that have been freed. When mapping a new virtual slot, the
allocator first attempts to reuse a slot from the `free_slots` vector
before allocating a new one.

This change also includes a new function `collect_scope_locals` that
identifies non-captured local variables defined within a block. After
these expressions are lowered, the allocator reclaims the physical slots
associated with these locals, making them available for reuse by
subsequent blocks.

A new test `test_slot_reuse_non_overlapping_scopes` is added to verify
that distinct scopes correctly reuse stack slots, ensuring optimal stack
usage.

src/ast/compiler/lowering.rs

@@ -8,6 +8,8 @@ use std::rc::Rc;
 
 struct StackAllocator {
     mapping: HashMap<u32, u32>,
+    /// Physical slots that have been freed and can be reused.
+    free_slots: Vec<u32>,
     next_slot: u32,
 }
 
@@ -15,15 +17,21 @@ impl StackAllocator {
     fn new() -> Self {
         Self {
             mapping: HashMap::new(),
+            free_slots: Vec::new(),
             next_slot: 0,
         }
     }
 
     fn map_slot(&mut self, slot: VirtualId) -> StackOffset {
         let entry = self.mapping.entry(slot.0).or_insert_with(|| {
+            // Reuse a freed slot before growing the frame.
+            if let Some(recycled) = self.free_slots.pop() {
+                recycled
+            } else {
                 let s = self.next_slot;
                 self.next_slot += 1;
                 s
+            }
         });
         StackOffset(*entry)
     }
@@ -35,6 +43,15 @@ impl StackAllocator {
             Address::Global(idx) => Address::Global(idx),
         }
     }
+
+    /// Releases the physical slot of `vid` back to the free-list.
+    /// If `vid` was never mapped (e.g. a dead def removed by the optimizer)
+    /// this is a no-op.
+    fn free_slot(&mut self, vid: VirtualId) {
+        if let Some(&physical) = self.mapping.get(&vid.0) {
+            self.free_slots.push(physical);
+        }
+    }
 }
 
 pub struct Lowering;
@@ -116,6 +133,12 @@ impl Lowering {
                 if exprs.is_empty() {
                     NodeKind::Block { exprs: vec![] }
                 } else {
+                    // Collect VirtualIds of non-captured locals before
+                    // transforming. After all exprs are lowered their
+                    // physical slots can be returned to the free-list,
+                    // allowing subsequent blocks to reuse them.
+                    let scope_locals = Self::collect_scope_locals(exprs);
+
                     let last_idx = exprs.len() - 1;
                     let mut new_exprs = Vec::with_capacity(exprs.len());
 
@@ -127,6 +150,13 @@ impl Lowering {
                             allocator,
                         )));
                     }
+
+                    // Release slots — variables defined here are no longer
+                    // live after this block ends.
+                    for vid in scope_locals {
+                        allocator.free_slot(vid);
+                    }
+
                     NodeKind::Block { exprs: new_exprs }
                 }
             }
@@ -268,4 +298,37 @@ impl Lowering {
             comments: node.comments.clone(),
         }
     }
+
+    /// Collects the `VirtualId`s of all non-captured locals defined at the
+    /// top level of `exprs`. These slots are safe to reclaim once the block
+    /// ends. Captured locals (upvalues) are excluded — closures that outlive
+    /// the block still hold live references to those physical slots.
+    fn collect_scope_locals(exprs: &[Rc<AnalyzedNode>]) -> Vec<VirtualId> {
+        let mut locals = Vec::new();
+        for expr in exprs {
+            if let NodeKind::Def { pattern, info, .. } = &expr.kind
+                && info.captured_by.is_empty()
+            {
+                Self::collect_pattern_vids(pattern, &mut locals);
+            }
+        }
+        locals
+    }
+
+    /// Recursively extracts `VirtualId`s from declaration identifiers in a
+    /// binding pattern. Handles flat (`x`) and destructuring (`[x y]`) forms.
+    fn collect_pattern_vids(pattern: &AnalyzedNode, out: &mut Vec<VirtualId>) {
+        match &pattern.kind {
+            NodeKind::Identifier {
+                binding: IdentifierBinding::Declaration { addr: Address::Local(vid), .. },
+                ..
+            } => out.push(*vid),
+            NodeKind::Tuple { elements } => {
+                for e in elements {
+                    Self::collect_pattern_vids(e, out);
+                }
+            }
+            _ => {}
+        }
+    }
 }

tests/optimizer.rs

@@ -180,3 +180,39 @@ fn test_reproduce_inlining_slot_clash_crash() {
     let res = env.run_script(source);
     assert!(res.is_ok(), "Inlining slot clash triggered: {:?}", res.err());
 }
+
+// ── Slot reuse ────────────────────────────────────────────────────────────────
+
+#[test]
+fn test_slot_reuse_non_overlapping_scopes() {
+    let mut env = Environment::new();
+    env.optimization = true;
+
+    // Two sequential inner blocks each bind a mutable local.
+    // assign prevents the optimizer from inlining the variables away,
+    // so both slots survive into the lowering pass.
+    // x lives only in the first block; once that block ends its slot is free.
+    // y is defined in the second block and must reuse x's slot.
+    // Expected result: the function returns 3 (y = 2+1).
+    let source = r#"
+    (fn []
+      (do
+        (do (def x 1) (assign x (+ x 1)) x)
+        (do (def y 2) (assign y (+ y 1)) y)))
+    "#;
+
+    // Correctness: calling the function must still produce the right value.
+    let result = env
+        .run_script("((fn [] (do (do (def x 1) (assign x (+ x 1)) x) (do (def y 2) (assign y (+ y 1)) y))))")
+        .unwrap();
+    assert_eq!(format!("{}", result), "3");
+
+    // Stack efficiency: with slot reuse the Lambda's stack_size must be 1,
+    // not 2. The dump format is "stack_size: N" in the Lambda metadata line.
+    let dump = env.dump_ast(source).unwrap();
+    assert!(
+        dump.contains("stack_size: 1"),
+        "Non-overlapping scopes must reuse the same slot (expected stack_size: 1). Dump:\n{}",
+        dump
+    );
+}