docs: Add list of excluded features

This commit adds a list of features that have been determined to be out
of scope for `futures-lite`. The intent of this list is to inform users
which features `futures-lite` does not implement, as well as potential
contributors which PRs will not be accepted.

cc #111

Signed-off-by: John Nunley <dev@notgull.net>

FEATURES.md

@@ -0,0 +1,275 @@
+# Intentional Occlusions from `futures-lite`
+
+[`futures-lite`] has an API that is deliberately smaller than the [`futures`]
+crate. This allows it to compile significantly faster and have fewer 
+dependencies.
+
+This fact does not mean that [`futures-lite`] is not open to new feature 
+requests. However it does mean that any proposed new features are subject to
+scrutiny to determine whether or not they are truly necessary for this crate.
+In many cases there are much simpler ways to implement these features, or they
+would be a much better fit for an external crate.
+
+This document aims to describe all intentional feature occlusions and provide
+suggestions for how these features can be used in the context of 
+[`futures-lite`]. If you have a feature request that you believe does not fall
+under any of the following occlusions, please open an issue on the
+[official `futures-lite` bug tracker](https://github.com/smol-rs/futures-lite/issues).
+
+## Simple Combinators 
+
+In general, anything that can be implemented in terms of `async`/`await` syntax
+is not implemented in [`futures-lite`]. This is done to encourage the use of
+modern `async`/`await` syntax rather than [`futures`] v1.0 combinator chaining.
+
+As an example, take the [`map`] method in [`futures`]. It takes a future and
+processes its output through a closure.
+
+```rust
+let my_future = async { 1 };
+
+// Add one to the result of `my_future`.
+let mapped_future = my_future.map(|x| x + 1);
+
+assert_eq!(mapped_future.await, 2);
+```
+
+However, this does not need to be implemented in the form of a combinator. With
+`async`/`await` syntax, you can simply `await` on `my_future` in an `async`
+block, then process its output. The following code is equivalent to the above,
+but doesn't use a combinator.
+
+```rust
+let my_future = async { 1 };
+
+// Add one to the result of `my_future`.
+let mapped_future = async move { my_future.await + 1 };
+
+assert_eq!(mapped_future.await, 2);
+```
+
+By not implementing combinators that can be implemented in terms of `async`,
+[`futures-lite`] has a significantly smaller API that still has roughly the
+same amount of power as [`futures`].
+
+As part of this policy, the [`TryFutureExt`] trait is not implemented. All of
+its methods can be implemented by just using `async`/`await` combined with
+other simpler future combinators. For instance, consider [`and_then`]:
+
+```rust
+let my_future = async { Ok(2) };
+
+let and_then = my_future.and_then(|x| async move {
+    Ok(x + 1)
+});
+
+assert_eq!(and_then.await.unwrap(), 3);
+```
+
+This can be implemented with an `async` block and the normal `and_then`
+combinator.
+
+```rust
+let my_future = async { Ok(2) };
+
+let and_then = async move {
+    let x = my_future.await;
+    x.and_then(|x| x + 1)
+};
+
+assert_eq!(and_then.await.unwrap(), 3);
+```
+
+One drawback of this approach is that `async` blocks are not named types. So
+if a trait (like [`Service`]) requires a named future type it cannot be
+returned.
+
+```rust
+impl Service for MyService {
+    type Future = /* ??? */;
+
+    fn call(&mut self) -> Self::Future {
+        async { 1 + 1 }
+    }    
+}
+```
+
+One possible solution is to box the future and return a dynamic dispatch 
+object, but in many cases this adds non trivial overhead.
+
+```rust
+impl Service for MyService {
+    type Future = Pin<Box<dyn Future<Output = i32>>>;
+
+    fn call(&mut self) -> Self::Future {
+        async { 1 + 1 }.boxed_local()
+    }    
+}
+```
+
+This problem is expected to be resolved in the future, thanks to
+[`async` fn in traits] and [TAIT]. At this point we would rather wait for these
+better solutions than significantly expand [`futures-lite`]'s API. If this is a
+deal breaker for you, [`futures`] is probably better for your use case.
+
+## Asynchronous Closures
+
+As a pattern, most combinators in [`futures-lite`] take regular closures rather
+than `async` closures. For example:
+
+```rust
+// In `futures`, the `all` combinator takes a closure returning a future.
+my_stream.all(|x| async move { x > 5 }).await;
+
+// In `futures-lite`, the `all` combinator just takes a closure.
+my_stream.all(|x| x > 5).await;
+```
+
+This strategy is taken for two primary reasons. 
+
+First of all, it is significantly simpler to implement. Since we don't need to 
+keep track of whether we are currently `poll`ing a future or not it makes the 
+combinators an order of magnitude easier to write.
+
+Second of all it avoids the common [`futures`] wart of needing to pass trivial
+values into `async move { ... }` or `future::ready(...)` for the vast
+majority of operations.
+
+For futures, combinators that would normally require `async` closures can 
+usually be implemented in terms of `async`/`await`. See the above section for
+more information on that. For streams, the [`then`] combinator is one of the 
+few that actually takes an `async` closure, and can therefore be used to
+implement operations that would normally need `async` closures.
+
+```rust
+// In `futures`.
+my_stream.all(|x| my_async_fn(x)).await;
+
+// In `futures-lite`, use `then` and pass the result to `all`.
+my_stream.then(|x| my_async_fn(x)).all(|pass| pass).await;
+```
+
+## Higher-Order Concurrency
+
+[`futures`] provides a number of primitives and combinators that allow for
+polling a significant number of futures at once. Examples of this include
+[`for_each_concurrent`] and [`FuturesUnordered`].
+
+[`futures-lite`] provides simple primitives like [`race`] and [`zip`]. However
+these don't really scale to handling more than two futures at once. It has
+been proposed in the past to add deeper concurrency primitives to
+[`futures-lite`]. However our current stance is that such primitives would
+represent a significant uptick in complexity and thus is better suited to
+other crates.
+
+[`futures-concurrency`] provides a number of simple APIs for dealing with
+fixed numbers of futures. For example, here is an example for waiting on
+multiple futures to complete.
+
+```rust
+let (a, b, c) = /* assume these are all futures */;
+
+// futures
+let (x, y, z) = join!(a, b, c);
+
+// futures-concurrency
+use futures_concurrency::prelude::*;
+let (x, y, z) = (a, b, c).join().await;
+```
+
+For large or variable numbers of futures it is recommended to use an executor
+instead. [`smol`] provides both an [`Executor`] and a [`LocalExecutor`] 
+depending on the flavor of your program.
+
+@notgull has a [blog post](https://notgull.net/futures-concurrency-in-smol/) 
+describing this in greater detail.
+
+To explicitly answer a frequently asked question, the popular [`select`] macro
+can be implemented by using simple `async`/`await` and a race combinator.
+
+```rust
+let (a, b, c) = /* assume these are all futures */;
+
+// futures
+let x = select! {
+   a_res = a => a_res + 1,
+   _ = b => 0,
+   c_res = c => c_res + 3,
+};
+
+// futures-concurrency
+let x = (
+    async move { a.await + 1 },
+    async move { b.await; 0 },
+    async move { c.await + 3 }
+).race().await;
+```
+
+## Sink Trait
+
+[`futures`] offers a [`Sink`] trait that is in many ways the opposite of the
+[`Stream`] trait. Rather than asynchronously producing values, the point of the
+[`Sink`] is to asynchronously receive values.
+
+[`futures-lite`] and the rest of [`smol`] intentionally does not support the
+[`Sink`] trait. [`Sink`] is a relic from the old [`futures`] v0.1 days where
+I/O was tied directly into the API. The `Error` subtype is wholly unnecessary
+and makes the API significantly harder to use. In addition the multi-call
+requirement makes the API harder to both use and implement. It increases the
+complexity of any futures that use it significantly, and its API necessitates
+that implementors have an internal buffer for objects.
+
+In short, the ideal [`Sink`] API would be if it was replaced with this trait.
+
+*Sidenote: [`Stream`], [`AsyncRead`] and [`AsyncWrite`] suffer from this same
+problem to an extent. I think they could also be fixed by transforming their
+`fn poll_[X]` functions into `async fn [X]` functions. However their APIs are
+not broken to the point that [`Sink`]'s is.*
+
+In order to avoid relying on a broken API, [`futures-lite`] does not import
+[`Sink`] or expose any APIs that build upon [`Sink`]. Unfortunately some crates
+make their only accessible API the [`Sink`] call. Ideally instead they would
+just have an `async fn send()` function.
+
+## Out-of-scope modules
+
+[`futures`] provides several sets of tools that are out of scope for
+[`futures-lite`]. Usually these are implemented in external crates, some of
+which depend on [`futures-lite`] themselves. Here are examples of these
+primitives:
+
+- **Channels:** [`async-channel`] provides an asynchronous MPMC channel, while
+  [`oneshot`] provides an asynchronous oneshot channel.
+- **Mutex:** [`async-lock`] provides asynchronous mutexes, alongside other
+  locking primitives.
+- **Atomic Wakers:** [`atomic-waker`] provides standalone atomic wakers.
+- **Executors:** [`async-executor`] provides [`Executor`] to replace
+  `ThreadPool` and [`LocalExecutor`] to replace `LocalPool`.
+
+[`smol`]: https://crates.io/crates/smol
+[`futures-lite`]: https://crates.io/crates/futures-lite
+[`futures`]: https://crates.io/crates/futures
+[`map`]: https://docs.rs/futures/latest/futures/future/trait.FutureExt.html#method.map
+[`TryFutureExt`]: https://docs.rs/futures/latest/futures/future/trait.TryFutureExt.html
+[`and_then`]: https://docs.rs/futures/latest/futures/future/trait.TryFutureExt.html#method.and_then
+[`Service`]: https://docs.rs/tower-service/latest/tower_service/trait.Service.html
+[`async` fn in traits]: https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html
+[TAIT]: https://rust-lang.github.io/impl-trait-initiative/explainer/tait.html
+[`then`]: https://docs.rs/futures-lite/latest/futures_lite/stream/trait.StreamExt.html#method.then
+[`FuturesUnordered`]: https://docs.rs/futures/latest/futures/stream/struct.FuturesUnordered.html
+[`for_each_concurrent`]: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.for_each_concurrent
+[`race`]: https://docs.rs/futures-lite/latest/futures_lite/future/fn.race.html
+[`zip`]: https://docs.rs/futures-lite/latest/futures_lite/future/fn.zip.html 
+[`futures-concurrency`]: https://docs.rs/futures-concurrency/latest/futures_concurrency/
+[`Executor`]: https://docs.rs/async-executor/latest/async_executor/struct.Executor.html
+[`LocalExecutor`]: https://docs.rs/async-executor/latest/async_executor/struct.LocalExecutor.html
+[`select`]: https://docs.rs/futures/latest/futures/macro.select.html
+[`Sink`]: https://docs.rs/futures/latest/futures/sink/trait.Sink.html
+[`Stream`]: https://docs.rs/futures-core/latest/futures_core/stream/trait.Stream.html
+[`AsyncRead`]: https://docs.rs/futures-io/latest/futures_io/trait.AsyncRead.html
+[`AsyncWrite`]: https://docs.rs/futures-io/latest/futures_io/trait.AsyncWrite.html
+[`async-channel`]: https://crates.io/crates/async-channel
+[`async-lock`]: https://crates.io/crates/async-lock
+[`async-executor`]: https://crates.io/crates/async-executor
+[`oneshot`]: https://crates.io/crates/oneshot
+[`atomic-waker`]: https://crates.io/crates/atomic-waker

README.md

@@ -17,7 +17,11 @@ warts in its API, fills in some obvious gaps, and removes almost all unsafe code
 In short, this crate aims to be more enjoyable than [futures] but still fully compatible with
 it.
 
+The API for this crate is intentionally constrained. Please consult the 
+[features list] for APIs that are occluded from this crate.
+
 [futures]: https://docs.rs/futures
+[features list]: https://github.com/smol-rs/futures-lite/blob/master/FEATURES.md
 
 ## Examples
 

src/lib.rs

@@ -6,7 +6,11 @@
 //! In short, this crate aims to be more enjoyable than [futures] but still fully compatible with
 //! it.
 //!
+//! The API for this crate is intentionally constrained. Please consult the [features list] for
+//! APIs that are occluded from this crate.
+//!
 //! [futures]: https://docs.rs/futures
+//! [features list]: https://github.com/smol-rs/futures-lite/blob/master/FEATURES.md
 //!
 //! # Examples
 //!