如何修复 "Closure may outlive the current function"

Question

我有这个代码：

pub fn find_saddle_points(input: &[Vec<u64>]) -> Vec<(usize, usize)> {
  let mut row_max: Vec<HashSet<usize>> = vec![HashSet::new(); input.len()];
  let mut col_min: Vec<HashSet<usize>> = vec![HashSet::new(); input[0].len()];
  ...
  row_max.iter().enumerate()
    .flat_map(|(row, cols)| {
        cols.iter()
            .filter(|c| col_min[**c].contains(&row))
            .map(|c| (row, *c))
    })
    .collect()
}

编译失败。

error[E0373]: closure may outlive the current function, but it borrows `row`, which is owned by the current function
  --> src/lib.rs:48:21
   |
48 |             .filter(|c| col_min[**c].contains(&row))
   |                     ^^^                        --- `row` is borrowed here
   |                     |
   |                     may outlive borrowed value `row`
   |
note: closure is returned here
  --> src/lib.rs:47:9
   |
47 | /         cols.iter()
48 | |             .filter(|c| col_min[**c].contains(&row))
49 | |             .map(|c| (row, *c))
   | |_______________________________^
help: to force the closure to take ownership of `row` (and any other referenced variables), use the `move` keyword
   |
48 |             .filter(move |c| col_min[**c].contains(&row))
   |                     ++++

error[E0373]: closure may outlive the current function, but it borrows `row`, which is owned by the current function
  --> src/lib.rs:49:18
   |
49 |             .map(|c| (row, *c))
   |                  ^^^  --- `row` is borrowed here
   |                  |
   |                  may outlive borrowed value `row`
   |
note: closure is returned here
  --> src/lib.rs:47:9
   |
47 | /         cols.iter()
48 | |             .filter(|c| col_min[**c].contains(&row))
49 | |             .map(|c| (row, *c))
   | |_______________________________^
help: to force the closure to take ownership of `row` (and any other referenced variables), use the `move` keyword
   |
49 |             .map(move |c| (row, *c))
   |                  ++++

我可以在 map 中使用 move，因为没有人在它后面使用 row，但我不能在 filter 中使用 move因为 col_min 有类型 Vec<HashSet<usize>>，它没有实现 Copy 特征。如何解决这个问题？

Answer 1

要在 move 闭包中通过引用捕获某些内容，请在闭包外部创建一个引用，然后在闭包中使用该引用；引用将被移动到闭包中，而不是它引用的值。

  row_max.iter().enumerate()
    .flat_map(|(row, cols)| {
        // Shadow col_min with a reference to the outer variable.  The closure
        // will capture this reference instead of the col_min value above.
        let col_min = &col_min;
        cols.iter()
            .filter(move |c| col_min[**c].contains(&row))
            .map(move |c| (row, *c))
    })
    .collect()

(Playground)

如何修复 "Closure may outlive the current function"

How to fix "Closure may outlive the current function"

closures

lifetime

rust

borrow-checker