fix(minifier): allow argument rewrite with with leading var assignments (#28056)

allow rewrite of `arguments` loop with leading empty var assignments.

```js
function _() { for (var keep, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }

function _() { var keep, r = [...arguments]; use(r, keep) }
```

crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs

@@ -855,7 +855,18 @@ impl<'a> PeepholeOptimizations {
 
         let Some(init) = &mut for_stmt.init else { return };
         let ForStatementInit::VariableDeclaration(var_init) = init else { return };
-        let Some(siblings) = var_init.declarations.get(init_decl_len..) else { return };
+        let leading_var_count = if var_init.kind.is_var() {
+            let Some(var_position) =
+                var_init.declarations.iter().position(|decl| decl.init.is_some())
+            else {
+                return;
+            };
+            var_position
+        } else {
+            0
+        };
+        let pattern_end = leading_var_count + init_decl_len;
+        let Some(siblings) = var_init.declarations.get(pattern_end..) else { return };
         // Sibling declarators may be in the source loop or folded into it by `sequences`.
         // Keep uninitialized `var` siblings, but don't move an initializer across the spread.
         if !siblings.is_empty()
@@ -873,7 +884,7 @@ impl<'a> PeepholeOptimizations {
             return;
         }
 
-        let mut idx = 0usize;
+        let mut idx = leading_var_count;
 
         // Check `e = arguments.length`
         if let Some((e_id_name, _)) = e_id_info {
@@ -1012,13 +1023,13 @@ impl<'a> PeepholeOptimizations {
             let new_decl = VariableDeclarator::new(SPAN, r_id_pat, None, Some(arr), false, ctx);
             // Drop only the copy-loop declarators. Siblings remain in the `var` statement.
             // Walk removed declarators so refs inside them reach `PassChanges`.
-            for decl in var_init.declarations.drain(..init_decl_len) {
+            for decl in var_init.declarations.drain(leading_var_count..pattern_end) {
                 ctx.drop_variable_declarator(&decl);
             }
-            var_init.declarations.insert(0, new_decl);
+            var_init.declarations.insert(leading_var_count, new_decl);
         } else {
             // The copied array is unused, but sibling declarations still need to survive.
-            for decl in var_init.declarations.drain(..init_decl_len) {
+            for decl in var_init.declarations.drain(leading_var_count..pattern_end) {
                 ctx.drop_variable_declarator(&decl);
             }
             // Avoid invalid `for (var; 0;)` when no siblings remain.

crates/oxc_minifier/tests/peephole/substitute_alternate_syntax.rs

@@ -1078,6 +1078,14 @@ fn test_rewrite_arguments_copy_loop() {
         "function _() { for (var r = [], a = 0, keep; a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }",
         "function _() { var r = [...arguments], keep; use(r, keep) }",
     );
+    test(
+        "function _() { for (var keep, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }",
+        "function _() { var keep, r = [...arguments]; use(r, keep) }",
+    );
+    test(
+        "function _() { for (var first, second, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; use(r, first, second) }",
+        "function _() { var first, second, r = [...arguments]; use(r, first, second) }",
+    );
     test(
         "function _() { var items = [], t = 0, i, u, r; for (; t < arguments.length; t++) items[t] = arguments[t]; if (this.list = [], items != null) for (i = 0, u = items; i < u.length; i++) r = u[i], this.list.push(r) }",
         "function _() { var items = [...arguments], i, u, r; if (this.list = [], items != null) for (i = 0, u = items; i < u.length; i++) r = u[i], this.list.push(r) }",
@@ -1089,6 +1097,12 @@ fn test_rewrite_arguments_copy_loop() {
     test_same(
         "function _() { for (var r = [], a = 0, keep = side(); a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }",
     );
+    test_same(
+        "function _() { for (var keep = side(), r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }",
+    );
+    test_same(
+        "function _() { for (var keep; a < arguments.length; a++) r[a] = arguments[a]; use(r, keep) }",
+    );
 }
 
 #[test]

tasks/minsize/minsize.snap

@@ -23,5 +23,5 @@ Original   | minified   | minified   | gzip       | gzip       | Iterations | Fi
 
 6.69 MB    | 2.12 MB    | 2.31 MB    | 453.41 kB  | 488.28 kB  |          4 | antd.js
 
-10.95 MB   | 3.31 MB    | 3.49 MB    | 849.81 kB  | 915.50 kB  |          4 | typescript.js
+10.95 MB   | 3.31 MB    | 3.49 MB    | 849.76 kB  | 915.50 kB  |          4 | typescript.js