Rust: expected type [X], but found type [X]

What does it mean when Rust complains that two equal types don't match?

The following error appears to be comparing the type...

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

...to itself.

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>
   |                                       ^

I've reduced it to the following minimal example:

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);
    }
}