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

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Merged
merged 1 commit into from
Sep 20, 2025
Merged

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

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6 changes: 3 additions & 3 deletions crates/oxc_minifier/docs/ASSUMPTIONS.md
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
102 changes: 59 additions & 43 deletions crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -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`
Expand Down Expand Up @@ -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]
Expand Down
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