Bug in Chain-based future's implementations of FusedFuture

[Diggsey]

futures-rs/futures-util/src/future/future/chain.rs

Line 48 in 2f8943d

return unsafe { Pin::new_unchecked(fut2) }.poll(cx);

As you can see, once the second future returns ready it will return immediately without first transitioning to the Empty state, which is what the is_terminated function looks at:

futures-rs/futures-util/src/future/future/chain.rs

Lines 16 to 18 in 2f8943d

	pub(crate)fn is_terminated(&self) -> bool {
	if let Chain::Empty = *self { true } else { false }
	}

[taiki-e]

I think this is fixed in #2128.

[Diggsey]

Yep.