Adding Elements

  internal func _unsafeInsertNew(_ element: __owned Element) {
    _internalInvariant(count + 1 <= capacity)
    let hashValue = self.hashValue(for: element)
    if _isDebugAssertConfiguration() {
      // In debug builds, perform a full lookup and trap if we detect duplicate
      // elements -- these imply that the Element type violates Hashable
      // requirements. This is generally more costly than a direct insertion,
      // because we'll need to compare elements in case of hash collisions.
      let (bucket, found) = find(element, hashValue: hashValue)
      guard !found else {
        ELEMENT_TYPE_OF_SET_VIOLATES_HASHABLE_REQUIREMENTS(Element.self)
      }
      hashTable.insert(bucket)
      uncheckedInitialize(at: bucket, to: element)
    } else {
      let bucket = hashTable.insertNew(hashValue: hashValue)
      uncheckedInitialize(at: bucket, to: element)
    }
    _storage._count &+= 1
  }

最终走到了_unsafeInsertNew 的 _unsafeInsertNew 方法里。
最后做了三件事：

1. 修改标记位，对应位置已经被占用了
2. 把插入的元素移动到指定的内存位置
3. count 增加 1

reserveCapacity

  @inline(__always)
  internal mutating func ensureUnique(isUnique: Bool, capacity: Int) -> Bool {
    if _fastPath(capacity <= self.capacity && isUnique) {
      return false
    }
    if isUnique {
      resize(capacity: capacity)
      return true
    }
    if capacity <= self.capacity {
      copy()
      return false
    }
    copyAndResize(capacity: capacity)
    return true
  }

  internal mutating func reserveCapacity(_ capacity: Int, isUnique: Bool) {
    _ = ensureUnique(isUnique: isUnique, capacity: capacity)
  }

这里有两个操作，copy和resize。
resize操作会把分配的内存变大，所有元素需要被重新插入一次，叫做rehashed。
被重新插入一次以后，位置会和原来的不一样。并不是说，resize之后，新增的内存部分是空的。

remove

最后会走到uncheckedRemove操作。

  @inlinable
  @inline(__always)
  internal mutating func uncheckedRemove(
    at bucket: Bucket,
    isUnique: Bool) -> Element {
    _internalInvariant(hashTable.isOccupied(bucket))
    let rehashed = ensureUnique(isUnique: isUnique, capacity: capacity)
    _internalInvariant(!rehashed)
    let old = (_elements + bucket.offset).move()
    _delete(at: bucket)
    return old
  }



不知道这里的 age 是什么操作。