feat(ecmascript): support integer index access for array and string in may_have_side_effects

Author: sapphi-red

crates/oxc_ecmascript/src/lib.rs

@@ -24,6 +24,9 @@ mod to_numeric;
 mod to_primitive;
 mod to_string;
 
+// other
+mod to_integer_index;
+
 pub mod constant_evaluation;
 pub mod is_global_reference;
 pub mod side_effects;
@@ -44,6 +47,7 @@ pub use self::{
     to_big_int::ToBigInt,
     to_boolean::ToBoolean,
     to_int_32::ToInt32,
+    to_integer_index::ToIntegerIndex,
     to_number::ToNumber,
     to_primitive::ToPrimitive,
     to_string::ToJsString,

crates/oxc_ecmascript/src/side_effects/may_have_side_effects.rs

@@ -1,8 +1,8 @@
 use oxc_ast::ast::*;
 
 use crate::{
-    constant_evaluation::DetermineValueType, is_global_reference::IsGlobalReference,
-    to_numeric::ToNumeric, to_primitive::ToPrimitive,
+    ToBigInt, ToIntegerIndex, constant_evaluation::DetermineValueType,
+    is_global_reference::IsGlobalReference, to_numeric::ToNumeric, to_primitive::ToPrimitive,
 };
 
 /// Returns true if subtree changes application state.
@@ -448,6 +448,27 @@ impl MayHaveSideEffects for ComputedMemberExpression<'_> {
                     is_global_reference,
                 )
             }
+            Expression::NumericLiteral(n) => !n.value.to_integer_index().is_some_and(|n| {
+                !integer_index_property_access_may_have_side_effects(
+                    &self.object,
+                    n,
+                    is_global_reference,
+                )
+            }),
+            Expression::BigIntLiteral(b) => {
+                if b.is_negative() {
+                    return true;
+                }
+                !b.to_big_int(is_global_reference)
+                    .and_then(ToIntegerIndex::to_integer_index)
+                    .is_some_and(|b| {
+                        !integer_index_property_access_may_have_side_effects(
+                            &self.object,
+                            b,
+                            is_global_reference,
+                        )
+                    })
+            }
             _ => true,
         }
     }
@@ -467,6 +488,35 @@ fn property_access_may_have_side_effects(
     }
 }
 
+fn integer_index_property_access_may_have_side_effects(
+    object: &Expression,
+    property: u32,
+    is_global_reference: &impl IsGlobalReference,
+) -> bool {
+    if object.may_have_side_effects(is_global_reference) {
+        return true;
+    }
+    match object {
+        Expression::StringLiteral(s) => property as usize >= s.value.encode_utf16().count(),
+        Expression::ArrayExpression(arr) => property as usize >= get_array_minimum_length(arr),
+        _ => true,
+    }
+}
+
+fn get_array_minimum_length(arr: &ArrayExpression) -> usize {
+    arr.elements
+        .iter()
+        .map(|e| match e {
+            ArrayExpressionElement::SpreadElement(spread) => match &spread.argument {
+                Expression::ArrayExpression(arr) => get_array_minimum_length(arr),
+                Expression::StringLiteral(str) => str.value.chars().count(),
+                _ => 0,
+            },
+            _ => 1,
+        })
+        .sum()
+}
+
 impl MayHaveSideEffects for CallExpression<'_> {
     fn may_have_side_effects(&self, is_global_reference: &impl IsGlobalReference) -> bool {
         if self.pure {

crates/oxc_ecmascript/src/to_integer_index.rs

@@ -0,0 +1,31 @@
+use num_bigint::BigInt;
+use num_traits::{ToPrimitive, Zero};
+
+/// Convert a value to u32 if the value can be an integer index and can be represented with u32.
+///
+/// Note that the max value of u32 is `2^32 - 1`, which is smaller than the max value of
+/// safe integer `2^53 - 1`.
+///
+/// If the value is a non-negative safe integer, it can be an integer index.
+/// <https://tc39.es/ecma262/multipage/ecmascript-data-types-and-values.html#sec-object-type>
+pub trait ToIntegerIndex {
+    fn to_integer_index(self) -> Option<u32>;
+}
+
+impl ToIntegerIndex for f64 {
+    fn to_integer_index(self) -> Option<u32> {
+        if self.fract() != 0.0 || self < 0.0 {
+            return None;
+        }
+        self.to_u32()
+    }
+}
+
+impl ToIntegerIndex for BigInt {
+    fn to_integer_index(self) -> Option<u32> {
+        if self < BigInt::zero() {
+            return None;
+        }
+        self.to_u32()
+    }
+}

crates/oxc_minifier/tests/ecmascript/may_have_side_effects.rs

@@ -641,6 +641,32 @@ fn test_property_access() {
     test("[][`length`]", false);
 
     test("a[0]", true);
+    test("''[-1]", true); // String.prototype[-1] may be overridden
+    test("''[0.3]", true); // String.prototype[0.3] may be overridden
+    test("''[0]", true); // String.prototype[0] may be overridden
+    test("'a'[0]", false);
+    test("'a'[0n]", false);
+    test("'a'[1]", true); // String.prototype[1] may be overridden
+    test("'あ'[0]", false);
+    test("'あ'[1]", true); // the length of "あ" is 1
+    test("'😀'[0]", false);
+    test("'😀'[1]", false);
+    test("'😀'[2]", true); // the length of "😀" is 2
+
+    test("[][-1]", true); // Array.prototype[-1] may be overridden
+    test("[][0.3]", true); // Array.prototype[0.3] may be overridden
+    test("[][0]", true); // Array.prototype[0] may be overridden
+    test("[1][0]", false);
+    test("[1][0n]", false);
+    test("[1][1]", true); // Array.prototype[1] may be overridden
+    test("[,][0]", false);
+    test("[...[], 1][0]", false);
+    test("[...[1]][0]", false);
+    test("[...'a'][0]", false);
+    test("[...'a'][1]", true);
+    test("[...'😀'][0]", false);
+    test("[...'😀'][1]", true);
+    test("[...a, 1][0]", true); // "...a" may have a sideeffect
 }
 
 #[test]