是否可以明确指定循环迭代的生命周期？

Question

以下代码利用生命周期省略进行编译：

fn g(a: &mut str, closure: impl Fn(&mut str, &str) -> ()) {
    let data = vec![];
    for d in data.iter() {
        closure(a, d);
    }
}

但是，假设我需要在闭包中明确指定变量的生命周期（在我的例子中，除非我这样做，否则调用点是不明确的）。如果我显式添加生命周期，编译器会抱怨 a 在之前的闭包迭代中被可变借用。

fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
    let data: Vec<&str> = vec![];
    'd: for d in data.iter() {
        closure(a, d);
    }
}

warning: label name `'d` shadows a lifetime name that is already in scope
 --> src/lib.rs:3:5
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |          -- first declared here
2 |     let data: Vec<&str> = vec![];
3 |     'd: for d in data.iter() {
  |     ^^ lifetime 'd already in scope

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
 --> src/lib.rs:4:17
  |
4 |         closure(a, d);
  |                 ^
  |
note: ...the reference is valid for the lifetime 'd as defined on the function body at 1:10...
 --> src/lib.rs:1:10
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |          ^^
note: ...but the borrowed content is only valid for the lifetime 'b as defined on the function body at 1:6
 --> src/lib.rs:1:6
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |      ^^

我认为那是因为编译器不会猜测我只打算 'd 在循环的每次迭代中持续。我搜索了一种引用生命周期的方法，但我所能找到的只是我有能力在 Rust 参考书中指定一个 "lifetime or label" ，这似乎只是所有意图和目的的标签。

有没有什么方法可以显式指定每个循环迭代的生命周期，或者显式写出编译器在第一个示例中省略的内容？

Answer 1

Is it possible to specify the lifetime of an iteration of a loop explicitly?

没有。这种东西根本不需要。

您没有提供与您的原始问题相对应的 MCVE，因此不可能提供解决该问题的解决方案。我的直觉和你的评论 "write out what the compiler has elided" 告诉我你应该看看：

长话短说：

fn g(a: &mut str, closure: impl for<'a, 'd> Fn(&'a mut str, &'d str) -> ())

是否可以明确指定循环迭代的生命周期？

Is it possible to specify the lifetime of an iteration of a loop explicitly?

loops

lifetime

rust