feat(minifier): remove unused variables in for init (#13508)

Remove unused variables in for initializer.

Author: sapphi-red

crates/oxc_minifier/src/peephole/minimize_statements.rs

@@ -817,6 +817,20 @@ impl<'a> PeepholeOptimizations {
 
         ctx: &mut Ctx<'a, '_>,
     ) {
+        if let Some(ForStatementInit::VariableDeclaration(var_decl)) = &mut for_stmt.init {
+            let old_len = var_decl.declarations.len();
+            var_decl.declarations.retain(|decl| {
+                !Self::should_remove_unused_declarator(decl, ctx)
+                    || decl.init.as_ref().is_some_and(|init| init.may_have_side_effects(ctx))
+            });
+            if old_len != var_decl.declarations.len() {
+                if var_decl.declarations.is_empty() {
+                    for_stmt.init = None;
+                }
+                ctx.state.changed = true;
+            }
+        }
+
         if ctx.options().sequences {
             match result.last_mut() {
                 Some(Statement::ExpressionStatement(prev_expr_stmt)) => {

crates/oxc_minifier/src/peephole/remove_unused_declaration.rs

@@ -103,6 +103,10 @@ mod test {
         test_same_options("export var x", &options);
         test_same_options("using x = foo", &options);
         test_same_options("await using x = foo", &options);
+
+        test_options("for (var x; ; );", "for (; ;);", &options);
+        test_options("for (var x = 1; ; );", "for (; ;);", &options);
+        test_same_options("for (var x = foo; ; );", &options); // can be improved
     }
 
     #[test]

crates/oxc_minifier/src/peephole/remove_unused_expression.rs

@@ -1076,7 +1076,7 @@ mod test {
         test_same_options("function foo(t) { return t = x(); } foo();", &options);
 
         // For loops
-        test_options("for (let i;;) i = 0", "for (let i;;);", &options);
+        test_options("for (let i;;) i = 0", "for (;;);", &options);
         test_same_options("for (let i;;) foo(i)", &options);
         test_same_options("for (let i;;) i = 0, foo(i)", &options);
         test_same_options("for (let i in []) foo(i)", &options);

tasks/minsize/minsize.snap

@@ -23,5 +23,5 @@ Original   | minified   | minified   | gzip       | gzip       | Iterations | Fi
 
 6.69 MB    | 2.22 MB    | 2.31 MB    | 459.28 kB  | 488.28 kB  |          4 | antd.js    
 
-10.95 MB   | 3.34 MB    | 3.49 MB    | 855.34 kB  | 915.50 kB  |          4 | typescript.js 
+10.95 MB   | 3.34 MB    | 3.49 MB    | 855.24 kB  | 915.50 kB  |          4 | typescript.js 
 

tasks/track_memory_allocations/allocs_minifier.snap

@@ -4,5 +4,5 @@ RadixUIAdoptionSection.jsx | 2.52 kB        ||             75 |              4 |
 
 pdf.mjs                    | 567.30 kB      ||          19577 |           2898 ||          47403 |           7782 | 1.624 MB      
 
-antd.js                    | 6.69 MB        ||          99856 |          13518 ||         331725 |          70354 | 17.408 MB     
+antd.js                    | 6.69 MB        ||          99854 |          13518 ||         331725 |          70354 | 17.408 MB