pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
    assert_stream::<A, _>(RepeatWith { repeater })
}
Creates a new stream that repeats elements of type `A` endlessly by
applying the provided closure, the repeater, `F: FnMut() -> A`.

The `repeat_with()` function calls the repeater over and over again.

Infinite stream like `repeat_with()` are often used with adapters like
[`stream.take()`], in order to make them finite.

If the element type of the stream you need implements [`Clone`], and
it is OK to keep the source element in memory, you should instead use
the [`stream.repeat()`] function.

# Examples

Basic usage:

```
# futures::executor::block_on(async {
use futures::stream::{self, StreamExt};

// let's assume we have some value of a type that is not `Clone`
// or which don't want to have in memory just yet because it is expensive:
#[derive(PartialEq, Debug)]
struct Expensive;

// a particular value forever:
let mut things = stream::repeat_with(|| Expensive);

assert_eq!(Some(Expensive), things.next().await);
assert_eq!(Some(Expensive), things.next().await);
assert_eq!(Some(Expensive), things.next().await);
# });
```

Using mutation and going finite:

```rust
# futures::executor::block_on(async {
use futures::stream::{self, StreamExt};

// From the zeroth to the third power of two:
let mut curr = 1;
let mut pow2 = stream::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
.take(4);

assert_eq!(Some(1), pow2.next().await);
assert_eq!(Some(2), pow2.next().await);
assert_eq!(Some(4), pow2.next().await);
assert_eq!(Some(8), pow2.next().await);

// ... and now we're done
assert_eq!(None, pow2.next().await);
# });
```
futures-util/src/stream/repeat_with.rs