add Stream::copied

[recursion128]

ref #129, Implement Steam::copied.

References

• https://doc.rust-lang.org/src/core/iter/adapters/mod.rs.html#136-171
• https://doc.rust-lang.org/src/core/iter/traits/iterator.rs.html#2298-2323

[ghost]

Perfect, thank you so much :)

[k-nasa]

This code con't run.

    async_std::task::block_on(async {
        use async_std::prelude::*;
        use std::collections::VecDeque;

        let v: VecDeque<_> = vec![&1, &2, &3].into_iter().collect();

        let mut v_copied  = v.copied();

        dbg!(v);
        dbg!(v_copied);
    });

I got this error message.

src/main.rs:11:14
   |
7  |         let v: VecDeque<_> = vec![&1, &2, &3].into_iter().collect();
   |             - move occurs because `v` has type `std::collections::VecDeque<&i32>`, which does not implement the `Copy` trait
8  |
9  |         let mut v_copied  = v.copied();
   |                             - value moved here
10 |
11 |         dbg!(v);
   |              ^ value used here after move

Is the copied method meaningful?

[ghost]

Oh no, I think I didn't look closely enough, my bad! :( I'll try to fix in a follow-up PR

[recursion128]

copied is the same as .map(|&x| x), consume an old stream and create a new stream that copy each element from &T to T.

This code:

use async_std::prelude::*;
use async_std::stream;
use std::collections::VecDeque;


fn main() {
    task::block_on(async {
        let v : VecDeque<_> = vec![1, 2, 3].into_iter().collect();

        let mut s = stream::from_iter(v.iter());

        let mut v_copied = s.copied();

        dbg!(v);

    });
}

output:

[examples/hello-world.rs:15] v = [
    1,
    2,
    3,