From 4349d4a66f218308a05858bc7fcdd61fbf416ff4 Mon Sep 17 00:00:00 2001
From: Jeong YunWon <jeong@youknowone.org>
Date: Thu, 20 Aug 2026 08:18:15 +0900
Subject: [PATCH] compiler-core: implement DerefMut and IndexMut for Constants

`Constants<C>` exposed `Deref` and `Index<ConstIdx>` but neither mutable
counterpart, so an owner of a `CodeObject` could read a constant but not
replace one in place and had to rebuild the whole boxed slice.

`[T]` already carries both halves of the `VarNum` pair in this file.

Assisted-by: Claude
---
 crates/compiler-core/src/bytecode.rs | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/crates/compiler-core/src/bytecode.rs b/crates/compiler-core/src/bytecode.rs
index ba1639170a7..0272270526b 100644
--- a/crates/compiler-core/src/bytecode.rs
+++ b/crates/compiler-core/src/bytecode.rs
@@ -11,7 +11,7 @@ use bitflags::bitflags;
 use core::{
     cell::UnsafeCell,
     hash, mem,
-    ops::{Deref, Index, IndexMut},
+    ops::{Deref, DerefMut, Index, IndexMut},
     sync::atomic::{AtomicU8, AtomicU16, AtomicUsize, Ordering},
 };
 use itertools::Itertools;
@@ -383,6 +383,12 @@ impl<C: Constant> Deref for Constants<C> {
     }
 }
 
+impl<C: Constant> DerefMut for Constants<C> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
 impl<C: Constant> Index<oparg::ConstIdx> for Constants<C> {
     type Output = C;
 
@@ -391,6 +397,12 @@ impl<C: Constant> Index<oparg::ConstIdx> for Constants<C> {
     }
 }
 
+impl<C: Constant> IndexMut<oparg::ConstIdx> for Constants<C> {
+    fn index_mut(&mut self, consti: oparg::ConstIdx) -> &mut Self::Output {
+        &mut self.0[consti.as_usize()]
+    }
+}
+
 impl<C: Constant> FromIterator<C> for Constants<C> {
     fn from_iter<T: IntoIterator<Item = C>>(iter: T) -> Self {
         Self(iter.into_iter().collect())