flat_map 与嵌套数据展开

  编译:成功
  运行(退出码 0):
    flat_map [1, 2, 3, 4, 5] / map+flatten [1, 2, 3, 4, 5] / flatten [1, 2, 3, 4, 5]
    按行拆词: ["to", "be", "or", "not", "to", "be"]
    字符: ['a', 'b', 'c', 'd']
    flat_map 把 Option 当作 0 或 1 个元素: [1, 3]
    一对多:每个数 n 展开成 1..=n: [1, 1, 2, 1, 2, 3]
    笛卡尔积: [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b')]

  源码:
      1 | fn main() {
      2 |     let v: Vec<(i32, i32)> = (1..=2).flat_map(|x| (1..=2).map(|y| (x, y))).collect();
      3 |     println!("{:?}", v);
      4 | }
  编译:失败
    error[E0373]: closure may outlive the current function, but it borrows `x`, which is owned by the current function
     --> <实验目录>/src/u65b.rs:2:63
      |
    2 |     let v: Vec<(i32, i32)> = (1..=2).flat_map(|x| (1..=2).map(|y| (x, y))).collect();
      |                                                               ^^^  - `x` is borrowed here
      |                                                               |
      |                                                               may outlive borrowed value `x`
      |
    note: closure is returned here
     --> <实验目录>/src/u65b.rs:2:51
      |
    2 |     let v: Vec<(i32, i32)> = (1..=2).flat_map(|x| (1..=2).map(|y| (x, y))).collect();
      |                                                   ^^^^^^^^^^^^^^^^^^^^^^^
    help: to force the closure to take ownership of `x` (and any other referenced variables), use the `move` keyword
      |
    2 |     let v: Vec<(i32, i32)> = (1..=2).flat_map(|x| (1..=2).map(move |y| (x, y))).collect();
      |                                                               ++++
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0373`.

  编译:成功
  运行(退出码 0):
    take(7) = [0, 0, 1, 0, 1, 2, 0]
    内层迭代器创建了 4 个(外层有 1000 个),内层元素共产出 7 个

  编译:成功
  运行(退出码 0):
    rev: [6, 5, 4, 3, 2, 1]
    size_hint: flatten (0, None) / 单层 iter (3, Some(3))
    flatten 在没开始消费时 size_hint 的下界 0 是真实元素数 6 吗? 否:展开前不知道内层长度
    Result 展开丢弃 Err: [1, 3]

  编译:成功
  运行(退出码 0):
    flat_map 链 0.154 ms  双重 for 0.156 ms  先对每行 sum 再 sum 0.153 ms  (结果 2570569 2570569 2570569)