Rust `map` 调用在这里有什么用吗

是的，前者返回实现Iterator<Item = (&usize, &T)> + '_的类型，而后者返回实现Iterator<Item = &(usize, T)> + '_的类型.

下面是一个最小的例子:

pub fn iter1(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
    v.iter().map(|(key, value)| (key, value))
}

pub fn iter2(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
    v.iter()
}

编译失败，并显示以下错误消息:

error[E0271]: expected `std::slice::Iter<'_, (usize, i32)>` to be an iterator that yields `(&usize, &i32)`, but it yields `&(usize, i32)`
 --> src/lib.rs:5:37
  |
5 | pub fn iter2(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
  |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `&(usize, i32)`, found tuple
6 |     v.iter()
  |     -------- return type was inferred to be `std::slice::Iter<'_, (usize, i32)>` here
  |
  = note: expected reference `&(usize, i32)`
                 found tuple `(&usize, &i32)`

For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground` due to previous error

Playground.个

前者之所以有效，是因为Rust的模式语法中内置了人体工程学，称为binding modes:

当参考值与非参考模式匹配时，它将被自动视为REF或REF MUT绑定.

所以它基本上是 v.iter().map(|&(ref key, ref value)| (key, value))的句法糖，这就更清楚地说明了为什么前者有效，而后者不行.