This PR implements four key side-effect analysis relaxations in oxc to improve tree-shaking capabilities and match rolldown's behavior for better compatibility.

Background

Currently, oxc's side-effect analysis is overly conservative in several areas, preventing the removal of code that could safely be eliminated. This leads to larger bundle sizes compared to rolldown and other bundlers that have more sophisticated analysis.

Changes

1. Side-effect-free global constructors

Global constructors are now considered side-effect-free in specific argument shapes:

// These are now removable when unused:
new Map()
new Map([[key, value], ...])  // when all elements are arrays
new Map(null)
new Map(undefined)

new Set()
new Set([1, 2, 3])  // any array expression
new Set(null)
new Set(undefined)

new WeakMap()
new WeakSet()
new WeakMap(null)    // or undefined or []
new WeakSet([])

new Date()
new Date('2023-01-01')  // primitives only
new Date(123)
new Date(null)
new Date(undefined)

The constructors must be global references, and arguments are still evaluated for side effects.

2. Improved static class block analysis

Static class blocks now examine their content instead of assuming all non-empty blocks have side effects:

// Before: always considered side-effect-full
// After: can be removed when unused
class A {
  static {
    const x = 1;
    const y = 'hello';
  }
}

// Still considered side-effect-full (conservative)
class B {
  static {
    let x;        // TDZ effects
    this.prop = 1;  // assignment
  }
}

3. Known side-effect-free global member reads

Specific global property accesses are now whitelisted as side-effect-free:

// These reads are now side-effect-free:
Object.assign
Object.create
Object.prototype
Object.prototype.propertyIsEnumerable
JSON.stringify
Reflect.apply
Math.E
Symbol.asyncDispose

// Other reads still considered potentially side-effect-full:
Object.someOtherProp
Math.random

4. Symbol.iterator computed property keys

Computed property keys using Symbol.iterator are now side-effect-free:

// These are now side-effect-free:
const obj = { [Symbol.iterator]: function* () {} }
class MyClass { [Symbol.iterator]() {} }

5. Expanded global identifier whitelist

Added String and Proxy to the list of side-effect-free global identifier reads, joining NaN, Infinity, and undefined.

Testing

Added comprehensive test coverage for all new functionality:

• 25+ test cases covering constructor relaxations
• Tests for static block content analysis
• Tests for global member read whitelisting
• Tests for Symbol.iterator property keys
• Tests for expanded identifier whitelist

All existing tests continue to pass, ensuring no regressions.

Impact

These changes will result in:

• Better tree-shaking and smaller bundle sizes
• Improved compatibility with rolldown behavior
• More aggressive dead code elimination for common JavaScript patterns

The implementation maintains all existing configurability options and respects user preferences for side-effect analysis strictness.

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