Rust:预期类型 [X]，但发现类型 [X]

Question

当 Rust 提示两个相同的类型不匹配时，这是什么意思？

以下错误似乎是在比较类型...

<for<'_> fn(&u32) -> impl futures::Future<Output = u32> {f} as FnOnce<(&u32,)>>::Output

...对它自己。

error[E0308]: mismatched types
  --> src/main.rs:31:18
   |
31 |     let output = map(f, input);
   |                  ^^^ lifetime mismatch
   |
   = note: expected associated type `<for<'_> fn(&u32) -> impl futures::Future<Output = u32> {f} as FnOnce<(&u32,)>>::Output`
              found associated type `<for<'_> fn(&u32) -> impl futures::Future<Output = u32> {f} as FnOnce<(&u32,)>>::Output`
   = note: the required lifetime does not necessarily outlive the empty lifetime
note: the lifetime requirement is introduced here
  --> src/main.rs:6:39
   |
6  | pub fn map<U, V, W>(f: impl Fn(&U) -> W, items: Vec<U>) -> impl futures::Stream<Item = V>
   |                                       ^

我已将其简化为以下最小示例:

use futures::stream::{FuturesUnordered, StreamExt};
use async_stream::stream;

pub fn map<U, V, W>(f: impl Fn(&U) -> W, items: Vec<U>) -> impl futures::Stream<Item = V>
where V: Send, W: futures::Future<Output = V> + Send
{
    stream! {
        let mut futures = FuturesUnordered::new();
        let mut i = 2;
        if 2 <= items.len() {
            futures.push(tokio::spawn(f(&items[0])));
            futures.push(tokio::spawn(f(&items[1])));
            while let Some(result) = futures.next().await {
                let y = result.unwrap();
                yield y;
                futures.push(tokio::spawn(f(&items[i])));
                i += 1
            }
        }
    }
}

#[tokio::main]
async fn main() {
    async fn f(x: &u32) -> u32 {
        x + 1
    }
    let input = vec![1, 2, 3];
    let output = map(f, input);
    futures::pin_mut!(output);
    while let Some(x) = output.next().await {
        println!("{:?}", x);
    }
}

Answer 1

这意味着它们不相等，只是这样显示，rustc 省略了一些重要的细节。

在这种情况下，省略的重要信息是一辈子的，我来注释一下:

expected associated type `<for<'_> fn(&u32) -> impl futures::Future<Output = u32> {f} as FnOnce<(&u32,)>>::Output`
   found associated type `<for<'_> fn(&u32) -> impl futures::Future<Output = u32> + '_ {f} as FnOnce<(&u32,)>>::Output + '_`

或者用名字，

expected associated type `<for<'a> fn(&'a u32) -> impl futures::Future<Output = u32> {f} as FnOnce<(&'a u32,)>>::Output`
   found associated type `<for<'a> fn(&'a u32) -> impl futures::Future<Output = u32> + 'a {f} as FnOnce<(&'a u32,)>>::Output + 'a`

这是因为 async fn f() 被脱糖成:

fn f<'a>(x: &'a u32) -> impl futures::Future<Output = u32> + 'a {
    async move { x + 1 }
}

也就是说，最终的 future 取决于参数的生命周期(因为它捕获了它)。然而，map() 期望它不会，因为它是一个独立的通用参数 (W)。

有关更多和潜在的解决方案，请参阅 Lifetime of a reference passed to async callback .