Minor improvements to future::join!'s implementation

This is a follow-up from #91645, regarding [some remarks I made](https://rust-lang.zulipchat.com/#narrow/stream/187312-wg-async-foundations/topic/join!/near/264293660).

Mainly:
  - it hides the recursive munching through a private `macro`, to avoid leaking such details (a corollary is getting rid of the need to use `@` to disambiguate);
  - it uses a `match` binding, _outside_ the `async move` block, to better match the semantics from function-like syntax;
  - it pre-pins the future before calling into `poll_fn`, since `poll_fn`, alone, cannot guarantee that its capture does not move;
  - it uses `.ready()?` since it's such a neat pattern;
  - it renames `Took` to `Taken` for consistency with `Done`.

library/core/src/future/join.rs

@@ -45,59 +45,102 @@ use crate::task::{Context, Poll};
 /// # };
 /// ```
 #[unstable(feature = "future_join", issue = "91642")]
-pub macro join {
-    ( $($fut:expr),* $(,)?) => {
-        join! { @count: (), @futures: {}, @rest: ($($fut,)*) }
-    },
-    // Recurse until we have the position of each future in the tuple
+pub macro join( $($fut:expr),+ $(,)? ) {
+    // Funnel through an internal macro not to leak implementation details.
+    join_internal! {
+        current_position[]
+        futures_and_positions[]
+        munching[ $($fut)+ ]
+    }
+}
+
+/// To be able to *name* the i-th future in the tuple (say we want the .4-th),
+/// the following trick will be used: `let (_, _, _, _, it, ..) = tuple;`
+/// In order to do that, we need to generate a `i`-long repetition of `_`,
+/// for each i-th fut. Hence the recursive muncher approach.
+macro join_internal {
+    // Recursion step: map each future with its "position" (underscore count).
     (
-        // A token for each future that has been expanded: "_ _ _"
-        @count: ($($count:tt)*),
-        // Futures and their positions in the tuple: "{ a => (_), b => (_ _)) }"
-        @futures: { $($fut:tt)* },
-        // Take a future from @rest to expand
-        @rest: ($current:expr, $($rest:tt)*)
-    ) => {
-        join! {
-            @count: ($($count)* _),
-            @futures: { $($fut)* $current => ($($count)*), },
-            @rest: ($($rest)*)
+        // Accumulate a token for each future that has been expanded: "_ _ _".
+        current_position[
+            $($underscores:tt)*
+        ]
+        // Accumulate Futures and their positions in the tuple: `_0th ()   _1st ( _ ) …`.
+        futures_and_positions[
+            $($acc:tt)*
+        ]
+        // Munch one future.
+        munching[
+            $current:tt
+            $($rest:tt)*
+        ]
+    ) => (
+        join_internal! {
+            current_position[
+                $($underscores)*
+                _
+            ]
+            futures_and_positions[
+                $($acc)*
+                $current ( $($underscores)* )
+            ]
+            munching[
+                $($rest)*
+            ]
         }
-    },
-    // Now generate the output future
-    (
-        @count: ($($count:tt)*),
-        @futures: {
-            $( $(@$f:tt)? $fut:expr => ( $($pos:tt)* ), )*
-        },
-        @rest: ()
-    ) => {
-        async move {
-            let mut futures = ( $( MaybeDone::Future($fut), )* );
+    ),
 
+    // End of recursion: generate the output future.
+    (
+        current_position $_:tt
+        futures_and_positions[
+            $(
+                $fut_expr:tt ( $($pos:tt)* )
+            )*
+        ]
+        // Nothing left to munch.
+        munching[]
+    ) => (
+        match ( $( MaybeDone::Future($fut_expr), )* ) { futures => async {
+            let mut futures = futures;
+            // SAFETY: this is `pin_mut!`.
+            let futures = unsafe { Pin::new_unchecked(&mut futures) };
             poll_fn(move |cx| {
                 let mut done = true;
-
+                // For each `fut`, pin-project to it, and poll it.
                 $(
-                    let ( $($pos,)* fut, .. ) = &mut futures;
-
-                    // SAFETY: The futures are never moved
-                    done &= unsafe { Pin::new_unchecked(fut).poll(cx).is_ready() };
+                    // SAFETY: pinning projection
+                    let fut = unsafe {
+                        futures.as_mut().map_unchecked_mut(|it| {
+                            let ( $($pos,)* fut, .. ) = it;
+                            fut
+                        })
+                    };
+                    // Despite how tempting it may be to `let () = fut.poll(cx).ready()?;`
+                    // doing so would defeat the point of `join!`: to start polling eagerly all
+                    // of the futures, to allow parallelizing the waits.
+                    done &= fut.poll(cx).is_ready();
                 )*
-
-                if done {
-                    // Extract all the outputs
-                    Poll::Ready(($({
-                        let ( $($pos,)* fut, .. ) = &mut futures;
-
-                        fut.take_output().unwrap()
-                    }),*))
-                } else {
-                    Poll::Pending
+                if !done {
+                    return Poll::Pending;
                 }
+                // All ready; time to extract all the outputs.
+
+                // SAFETY: `.take_output()` does not break the `Pin` invariants for that `fut`.
+                let futures = unsafe {
+                    futures.as_mut().get_unchecked_mut()
+                };
+                Poll::Ready(
+                    ($(
+                        {
+                            let ( $($pos,)* fut, .. ) = &mut *futures;
+                            fut.take_output().unwrap()
+                        }
+                    ),*) // <- no trailing comma since we don't want 1-tuples.
+                )
             }).await
-        }
-    }
+        }}
+    ),
 }
 
 /// Future used by `join!` that stores it's output to
@@ -109,14 +152,14 @@ pub macro join {
 pub enum MaybeDone<F: Future> {
     Future(F),
     Done(F::Output),
-    Took,
+    Taken,
 }
 
 #[unstable(feature = "future_join", issue = "91642")]
 impl<F: Future> MaybeDone<F> {
     pub fn take_output(&mut self) -> Option<F::Output> {
-        match &*self {
-            MaybeDone::Done(_) => match mem::replace(self, Self::Took) {
+        match *self {
+            MaybeDone::Done(_) => match mem::replace(self, Self::Taken) {
                 MaybeDone::Done(val) => Some(val),
                 _ => unreachable!(),
             },
@@ -132,13 +175,14 @@ impl<F: Future> Future for MaybeDone<F> {
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         // SAFETY: pinning in structural for `f`
         unsafe {
-            match self.as_mut().get_unchecked_mut() {
-                MaybeDone::Future(f) => match Pin::new_unchecked(f).poll(cx) {
-                    Poll::Ready(val) => self.set(Self::Done(val)),
-                    Poll::Pending => return Poll::Pending,
-                },
+            // Do not mix match ergonomics with unsafe.
+            match *self.as_mut().get_unchecked_mut() {
+                MaybeDone::Future(ref mut f) => {
+                    let val = Pin::new_unchecked(f).poll(cx).ready()?;
+                    self.set(Self::Done(val));
+                }
                 MaybeDone::Done(_) => {}
-                MaybeDone::Took => unreachable!(),
+                MaybeDone::Taken => unreachable!(),
             }
         }