feat(minifier): merge single var declarations without init into for-in (#8812)

Compress `var a; for (a in b) c` into `for (var a in b) c`. This is possible because `var a` can be put in any place within the same scope. This is only possible with `var`.

Author: sapphi-red

crates/oxc_minifier/src/peephole/collapse_variable_declarations.rs

@@ -203,16 +203,16 @@ mod test {
 
         #[test]
         fn test_for_in() {
-            // test("var a; for(a in b) foo()", "for (var a in b) foo()");
+            test("var a; for(a in b) foo()", "for (var a in b) foo()");
             test("a = 0; for(a in b) foo()", "for (a in a = 0, b) foo();");
-            // test_same("var a = 0; for(a in b) foo()");
+            test_same("var a = 0; for(a in b) foo()");
 
             // We don't handle labels yet.
-            // test_same("var a; a:for(a in b) foo()");
-            // test_same("var a; a:b:for(a in b) foo()");
+            test_same("var a; a:for(a in b) foo()");
+            test_same("var a; a:b:for(a in b) foo()");
 
             // Verify FOR inside IFs.
-            // test("if(x){var a; for(a in b) foo()}", "if(x) for(var a in b) foo()");
+            test("if(x){var a; for(a in b) foo()}", "if(x) for(var a in b) foo()");
 
             // Any other expression.
             test("init(); for(a in b) foo()", "for (a in init(), b) foo();");
@@ -221,7 +221,7 @@ mod test {
             test_same("function f(){ for(a in b) foo() }");
 
             // We don't handle destructuring patterns yet.
-            // test("var a; var b; for ([a, b] in c) foo();", "var a, b; for ([a, b] in c) foo();");
+            test("var a; var b; for ([a, b] in c) foo();", "var a, b; for ([a, b] in c) foo();");
         }
 
         #[test]

crates/oxc_minifier/src/peephole/minimize_statements.rs

@@ -342,33 +342,68 @@ impl<'a> PeepholeOptimizations {
                 result.push(Statement::ForStatement(for_stmt));
             }
             Statement::ForInStatement(mut for_in_stmt) => {
-                // "a; for (var b in c) d" => "for (var b in a, c) d"
-                if let Some(Statement::ExpressionStatement(prev_expr_stmt)) = result.last_mut() {
-                    // Annex B.3.5 allows initializers in non-strict mode
-                    // <https://tc39.es/ecma262/multipage/additional-ecmascript-features-for-web-browsers.html#sec-initializers-in-forin-statement-heads>
-                    // If there's a side-effectful initializer, we should not move the previous statement inside.
-                    let has_side_effectful_initializer = {
-                        if let ForStatementLeft::VariableDeclaration(var_decl) = &for_in_stmt.left {
-                            if var_decl.declarations.len() == 1 {
-                                // only var can have a initializer
-                                var_decl.kind.is_var()
-                                    && var_decl.declarations[0].init.as_ref().is_some_and(|init| {
-                                        ctx.expression_may_have_side_effects(init)
-                                    })
+                match result.last_mut() {
+                    // "a; for (var b in c) d" => "for (var b in a, c) d"
+                    Some(Statement::ExpressionStatement(prev_expr_stmt)) => {
+                        // Annex B.3.5 allows initializers in non-strict mode
+                        // <https://tc39.es/ecma262/multipage/additional-ecmascript-features-for-web-browsers.html#sec-initializers-in-forin-statement-heads>
+                        // If there's a side-effectful initializer, we should not move the previous statement inside.
+                        let has_side_effectful_initializer = {
+                            if let ForStatementLeft::VariableDeclaration(var_decl) =
+                                &for_in_stmt.left
+                            {
+                                if var_decl.declarations.len() == 1 {
+                                    // only var can have a initializer
+                                    var_decl.kind.is_var()
+                                        && var_decl.declarations[0].init.as_ref().is_some_and(
+                                            |init| ctx.expression_may_have_side_effects(init),
+                                        )
+                                } else {
+                                    // the spec does not allow multiple declarations though
+                                    true
+                                }
                             } else {
-                                // the spec does not allow multiple declarations though
-                                true
+                                false
                             }
-                        } else {
-                            false
+                        };
+                        if !has_side_effectful_initializer {
+                            let a = &mut prev_expr_stmt.expression;
+                            for_in_stmt.right = Self::join_sequence(a, &mut for_in_stmt.right, ctx);
+                            result.pop();
+                            self.mark_current_function_as_changed();
                         }
-                    };
-                    if !has_side_effectful_initializer {
-                        let a = &mut prev_expr_stmt.expression;
-                        for_in_stmt.right = Self::join_sequence(a, &mut for_in_stmt.right, ctx);
-                        result.pop();
-                        self.mark_current_function_as_changed();
                     }
+                    // "var a; for (a in b) c" => "for (var a in b) c"
+                    Some(Statement::VariableDeclaration(prev_var_decl)) => {
+                        if let ForStatementLeft::AssignmentTargetIdentifier(id) = &for_in_stmt.left
+                        {
+                            let prev_var_decl_no_init_item = {
+                                if prev_var_decl.kind.is_var()
+                                    && prev_var_decl.declarations.len() == 1
+                                    && prev_var_decl.declarations[0].init.is_none()
+                                {
+                                    Some(&prev_var_decl.declarations[0])
+                                } else {
+                                    None
+                                }
+                            };
+                            if let Some(prev_var_decl_item) = prev_var_decl_no_init_item {
+                                if let BindingPatternKind::BindingIdentifier(decl_id) =
+                                    &prev_var_decl_item.id.kind
+                                {
+                                    if id.name == decl_id.name {
+                                        for_in_stmt.left =
+                                            ForStatementLeft::VariableDeclaration(ctx.ast.alloc(
+                                                ctx.ast.move_variable_declaration(prev_var_decl),
+                                            ));
+                                        result.pop();
+                                        self.mark_current_function_as_changed();
+                                    }
+                                }
+                            }
+                        }
+                    }
+                    _ => {}
                 }
                 result.push(Statement::ForInStatement(for_in_stmt));
             }