feat: add stream::from_fn

[nanoqsh]

This adds an analogue of std::iter::from_fn for streams.

Also creates a new feature async-closure that allows to use newer compiler version 1.85 or later to use async closures. Crates for older versions may use this function without the feature but it'll be more limited.

[taiki-e]

Thanks for the PR. This feature is currently not tested in CI, so could you add cargo test --all-features to the following?

futures-lite/.github/workflows/ci.yml

Lines 56 to 58 in 5c196b9

	- run: cargo test
	- run: cargo test --no-default-features
	- run: cargo test --no-default-features --features alloc

[nanoqsh]

Thanks for the PR. This feature is currently not tested in CI, so could you add cargo test --all-features to the following?

futures-lite/.github/workflows/ci.yml

Lines 56 to 58 in 5c196b9

	- run: cargo test
	- run: cargo test --no-default-features
	- run: cargo test --no-default-features --features alloc

Initially I added ignore to the example with async closure to be able to generate documentation and run tests on Rust earlier than 1.85. The syntax of async closure prevents its use from being hidden behind cfg(feature = "async-closure") attribute, it still won't compile.

[notgull]

My main concern is the use of the feature flag. Is there a case where I can write a crate using futures-lite with this feature flag turned off, that can then fail to compile if another crate in the dependency tree turns it on?

[nanoqsh]

Is there a case where I can write a crate using futures-lite with this feature flag turned off, that can then fail to compile if another crate in the dependency tree turns it on?

Yes, this code will compile without the feature and will not with it:

fn a<F, U>(f: F) -> impl Stream<Item = u8>
where
    F: FnMut() -> U,
    U: Future<Output = u8>,
{
    stream::from_fn(f)
    // the trait `AsyncFnMut()` is not implemented for `F`
}

I expected it to work, but at the moment this is a problem