From 16c555cf3eb5d483e4eea4b529cba2bb1812a2f8 Mon Sep 17 00:00:00 2001
From: yuzhe <53376445+bkmashiro@users.noreply.github.com>
Date: Mon, 6 Apr 2026 19:10:18 +0100
Subject: [PATCH] burn(bug): Fix unused retSlot variable in LIR lowering
 [9R14TP]

---
 src/__tests__/lir/lower.test.ts | 32 ++++++++++++++++++++++++++++++++
 src/lir/lower.ts                |  1 -
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/src/__tests__/lir/lower.test.ts b/src/__tests__/lir/lower.test.ts
index 7ed8ede7..560d99ab 100644
--- a/src/__tests__/lir/lower.test.ts
+++ b/src/__tests__/lir/lower.test.ts
@@ -519,6 +519,38 @@ describe('MIR→LIR lowering — return', () => {
     const main = lir.functions.find(f => f.name === 'main')!
     expect(main.instructions.find(i => i.kind === 'return_value')).toBeUndefined()
   })
+
+  test('return with constant → materialises const slot, not $ret slot', () => {
+    // Regression: retSlot ($ret) was declared but unused; return_value must
+    // carry the *source* slot, not the destination.
+    const mod = mkModule([
+      mkFn('main', [
+        mkBlock('entry', [], { kind: 'return', value: c(7) }),
+      ]),
+    ])
+    const lir = lowerToLIR(mod)
+    const main = lir.functions.find(f => f.name === 'main')!
+    const ret = main.instructions.find(i => i.kind === 'return_value') as any
+    expect(ret).toBeDefined()
+    // slot must be the materialised const slot, not $ret
+    expect(ret.slot).toEqual({ player: '$__const_7', obj: OBJ })
+    expect(ret.slot.player).not.toBe('$ret')
+  })
+
+  test('return_value slot is never the $ret player (regression guard)', () => {
+    // Ensures the removed retSlot was not accidentally used as the instruction slot.
+    const mod = mkModule([
+      mkFn('main', [
+        mkBlock('entry', [
+          { kind: 'const', dst: 't0', value: 99 },
+        ], { kind: 'return', value: t('t0') }),
+      ]),
+    ])
+    const lir = lowerToLIR(mod)
+    const main = lir.functions.find(f => f.name === 'main')!
+    const ret = main.instructions.find(i => i.kind === 'return_value') as any
+    expect(ret.slot.player).not.toBe('$ret')
+  })
 })
 
 // ---------------------------------------------------------------------------
diff --git a/src/lir/lower.ts b/src/lir/lower.ts
index d7ce6ef1..4a6f6b77 100644
--- a/src/lir/lower.ts
+++ b/src/lir/lower.ts
@@ -636,7 +636,6 @@ function lowerTerminatorInner(
   switch (term.kind) {
     case 'return': {
       if (term.value) {
-        const retSlot: Slot = { player: '$ret', obj: ctx.objective }
         const srcSlot = operandToSlot(term.value, ctx, instrs)
         instrs.push({ kind: 'return_value', slot: srcSlot })
       }