isEmpty

  /// A Boolean value that indicates whether the set is empty.
  @inlinable
  public var isEmpty: Bool {
    return count == 0
  }

count

  /// The number of elements in the set.
  ///
  /// - Complexity: O(1).
  @inlinable
  public var count: Int {
    return _variant.count
  }

最后会返回__RawSetStorage里的count

  /// The current number of occupied entries in this set.
  @usableFromInline
  @nonobjc
  internal final var _count: Int

在修改Set的时候，会不断调整这个值。

  internal mutating func _delete(at bucket: Bucket) {
    ...
    _storage._count -= 1
    ...
  }

  internal func _unsafeInsertNew(_ element: __owned Element) {
    ...
    _storage._count &+= 1
  }

capacity

  /// The total number of elements that the set can contain without
  /// allocating new storage.
  @inlinable
  public var capacity: Int {
    return _variant.capacity
  }

注意不是实际占用内存的大小，因为要预留部分空闲空间，防止性能下降。

    storage._capacity = _HashTable.capacity(forScale: scale)