Iterator 的惰性执行

  源码:
      1 | fn main() {
      2 |     let v = vec![1, 2, 3];
      3 |     v.iter().map(|x| { println!("闭包执行了 x={}", x); x * 2 });
      4 |     println!("上一行之后,没有任何「闭包执行了」输出");
      5 | }
  编译:成功;警告:
    warning: unused `Map` that must be used
     --> <实验目录>/src/u62a.rs:3:5
      |
    3 |     v.iter().map(|x| { println!("闭包执行了 x={}", x); x * 2 });
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: iterators are lazy and do nothing unless consumed
      = note: `#[warn(unused_must_use)]` (part of `#[warn(unused)]`) on by default
    help: use `let _ = ...` to ignore the resulting value
      |
    3 |     let _ = v.iter().map(|x| { println!("闭包执行了 x={}", x); x * 2 });
      |     +++++++
    
    warning: 1 warning emitted
  运行(退出码 0):
    上一行之后,没有任何「闭包执行了」输出

  源码:
      1 | fn main() {
      2 |     let r: Vec<i32> = (1..=4)
      3 |         .inspect(|x| println!("  源      产出 {}", x))
      4 |         .filter(|x| { let keep = x % 2 == 0; println!("  filter  {} -> {}", x, keep); keep })
      5 |         .map(|x| { println!("  map     {} -> {}", x, x * 10); x * 10 })
      6 |         .collect();
      7 |     println!("结果 {:?}", r);
      8 | }
  编译:成功
  运行(退出码 0):
      源      产出 1
      filter  1 -> false
      源      产出 2
      filter  2 -> true
      map     2 -> 20
      源      产出 3
      filter  3 -> false
      源      产出 4
      filter  4 -> true
      map     4 -> 40
    结果 [20, 40]

  编译:成功
  运行(退出码 0):
    filter(%3==0).take(3) = [3, 6, 9]  源总共产出 9 个
    find(==7) = Some(7)  源总共产出 7 个
    any(>100) = true  源总共产出 101 个
    take(5).count() = 5  源总共产出 5 个
    take_while(<4).sum() = 6  源总共产出 4 个(多产出一个用来发现 4 不满足条件)

  编译:成功
  运行(退出码 0):
    无穷序列取前 5 个偶数的平方: [4, 16, 36, 64, 100]
    类型: map::Map<filter::Filter<Range<i32>, u62d::main::{{closure}}>, u62d::main::{{closure}}>
    size_of(这条链) = 12 字节(Range<i32> 本身 8 字节;闭包 filter 无捕获、map 捕获了 k)
    在 100 万元素 Vec 上建一条 map+filter 链: size_of = 16 字节(只存了迭代器状态,不存元素)

  源码:
      1 | fn main() {
      2 |     let it;
      3 |     {
      4 |         let v = vec![1, 2, 3];
      5 |         it = v.iter().map(|x| x * 2);
      6 |     }
      7 |     println!("{:?}", it.collect::<Vec<_>>());
      8 | }
  编译:失败
    error[E0597]: `v` does not live long enough
     --> <实验目录>/src/u62e.rs:5:14
      |
    4 |         let v = vec![1, 2, 3];
      |             - binding `v` declared here
    5 |         it = v.iter().map(|x| x * 2);
      |              ^ borrowed value does not live long enough
    6 |     }
      |     - `v` dropped here while still borrowed
    7 |     println!("{:?}", it.collect::<Vec<_>>());
      |                      -- borrow later used here
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0597`.
  编译:成功
  运行(退出码 0):
    建好链之后 log = (此时不能读,因为闭包还借着 log)
    消费之后 log = [1, 2, 3]