feat(minifier): only apply arguments copy loop transformation in functions (#13952)

Made the arguments copy loop transformation to only apply inside functions to reduce the assumptions held.

Author: sapphi-red

crates/oxc_minifier/docs/ASSUMPTIONS.md

@@ -98,13 +98,13 @@ eval('var x = 1');
 new Function('return x');
 ```
 
-### No access to a variable named `arguments` outside functions
+### No side effects from accessing to a global variable named `arguments`
 
-`arguments` variables are only accessed inside functions. We intend to change this assumption to optional in the future.
+Accessing a global variable named `arguments` does not have a side effect. We intend to change this assumption to optional in the future.
 
 ```javascript
 // The minifier assumes this never happens:
-console.log(arguments);
+console.log(arguments); // ReferenceError: arguments is not defined
 ```
 
 ## Optional Assumptions

crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs

@@ -535,9 +535,6 @@ impl<'a> PeepholeOptimizations {
             return;
         }
 
-        // FIXME: this function treats `arguments` not inside a function scope as if they are inside it
-        //        we should check in a different way than `ctx.is_global_reference`
-
         // Parse statement: `r[a - offset] = arguments[a];`
         let body_assign_expr = {
             let assign = match &mut for_stmt.body {
@@ -661,6 +658,12 @@ impl<'a> PeepholeOptimizations {
             return;
         }
 
+        // make sure `arguments` points to the arguments object
+        // this is checked after the structure checks above because this check is slower than the structure checks
+        if ctx.ancestor_scopes().all(|s| !ctx.scoping().scope_flags(s).is_function()) {
+            return;
+        }
+
         let mut idx = 0usize;
 
         // Check `e = arguments.length`
@@ -2271,96 +2274,109 @@ mod test {
     #[test]
     fn test_rewrite_arguments_copy_loop() {
         test(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; } console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; } console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a] } console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a] } console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = new Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = new Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a]; console.log(r)",
-            "var r = [...arguments].slice(1); console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments].slice(1); console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e > 2 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a]; console.log(r)",
-            "var r = [...arguments].slice(2); console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e > 2 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments].slice(2); console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = [], a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = [], a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; console.log(r)",
-            "var r = [...arguments]; console.log(r)",
+            "function _() { for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments]; console.log(r) }",
         );
         test(
-            "for (var r = [], a = 1; a < arguments.length; a++) r[a - 1] = arguments[a]; console.log(r)",
-            "var r = [...arguments].slice(1); console.log(r)",
+            "function _() { for (var r = [], a = 1; a < arguments.length; a++) r[a - 1] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments].slice(1); console.log(r) }",
         );
         test(
-            "for (var r = [], a = 2; a < arguments.length; a++) r[a - 2] = arguments[a]; console.log(r)",
-            "var r = [...arguments].slice(2); console.log(r)",
+            "function _() { for (var r = [], a = 2; a < arguments.length; a++) r[a - 2] = arguments[a]; console.log(r) }",
+            "function _() { var r = [...arguments].slice(2); console.log(r) }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
-            "",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; }",
+            "function _() {}",
         );
         test(
-            "for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a]",
-            "",
+            "function _() { for (var e = arguments.length, r = Array(e > 1 ? e - 1 : 0), a = 1; a < e; a++) r[a - 1] = arguments[a] }",
+            "function _() {}",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) console.log(r[a]);",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) console.log(r[a]); }",
         );
         test(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; console.log(r); }",
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) (r[a] = arguments[a], console.log(r))",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) { r[a] = arguments[a]; console.log(r); } }",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) (r[a] = arguments[a], console.log(r)) }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] += arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] += arguments[a]; }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a + 1] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a + 1] = arguments[a]; }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a - 0.5] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a - 0.5] = arguments[a]; }",
         );
         test(
-            "var arguments; for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
-            "for (var arguments, e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a];",
+            "function _() { var arguments; for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; }",
+            "function _() { for (var arguments, e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; }",
+        );
+        test_same(
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = foo[a]; }",
         );
-        test_same("for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = foo[a];");
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; e--) r[a] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; e--) r[a] = arguments[a]; }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; r++) r[a] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; r++) r[a] = arguments[a]; }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < r; r++) r[a] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < r; r++) r[a] = arguments[a]; }",
         );
         test(
-            "var arguments; for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a];",
-            "for (var arguments, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a];",
+            "function _() { var arguments; for (var r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; }",
+            "function _() { for (var arguments, r = [], a = 0; a < arguments.length; a++) r[a] = arguments[a]; }",
         );
         test_same(
-            "for (var e = arguments.length, r = Array(e > 1 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a];",
+            "function _() { for (var e = arguments.length, r = Array(e > 1 ? e - 2 : 0), a = 2; a < e; a++) r[a - 2] = arguments[a]; }",
         );
 
         test_same_options_source_type(
-            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
+            "function _() { for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) }",
             SourceType::cjs(),
             &default_options(),
         );
+
+        test_same(
+            "for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r)",
+        );
+        test_same(
+            "{ let _; for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) }",
+        );
+        test(
+            "function _() { { let _; for (var e = arguments.length, r = Array(e), a = 0; a < e; a++) r[a] = arguments[a]; console.log(r) } }",
+            "function _() { { let _; var r = [...arguments]; console.log(r) } }",
+        );
     }
 
     #[test]