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8faf50e0 XL |
1 | //! Asynchronous values. |
2 | ||
3 | use core::cell::Cell; | |
4 | use core::marker::Unpin; | |
0bf4aa26 | 5 | use core::pin::Pin; |
8faf50e0 XL |
6 | use core::option::Option; |
7 | use core::ptr::NonNull; | |
532ac7d7 | 8 | use core::task::{Context, Poll}; |
8faf50e0 XL |
9 | use core::ops::{Drop, Generator, GeneratorState}; |
10 | ||
11 | #[doc(inline)] | |
48663c56 | 12 | #[stable(feature = "futures_api", since = "1.36.0")] |
8faf50e0 XL |
13 | pub use core::future::*; |
14 | ||
9fa01778 | 15 | /// Wrap a generator in a future. |
8faf50e0 XL |
16 | /// |
17 | /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give | |
18 | /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`). | |
dc9dc135 | 19 | #[doc(hidden)] |
8faf50e0 XL |
20 | #[unstable(feature = "gen_future", issue = "50547")] |
21 | pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> { | |
22 | GenFuture(x) | |
23 | } | |
24 | ||
25 | /// A wrapper around generators used to implement `Future` for `async`/`await` code. | |
dc9dc135 | 26 | #[doc(hidden)] |
8faf50e0 XL |
27 | #[unstable(feature = "gen_future", issue = "50547")] |
28 | #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)] | |
29 | struct GenFuture<T: Generator<Yield = ()>>(T); | |
30 | ||
31 | // We rely on the fact that async/await futures are immovable in order to create | |
32 | // self-referential borrows in the underlying generator. | |
33 | impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {} | |
34 | ||
dc9dc135 | 35 | #[doc(hidden)] |
8faf50e0 XL |
36 | #[unstable(feature = "gen_future", issue = "50547")] |
37 | impl<T: Generator<Yield = ()>> Future for GenFuture<T> { | |
38 | type Output = T::Return; | |
532ac7d7 | 39 | fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { |
9fa01778 XL |
40 | // Safe because we're !Unpin + !Drop mapping to a ?Unpin value |
41 | let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) }; | |
532ac7d7 | 42 | set_task_context(cx, || match gen.resume() { |
8faf50e0 XL |
43 | GeneratorState::Yielded(()) => Poll::Pending, |
44 | GeneratorState::Complete(x) => Poll::Ready(x), | |
45 | }) | |
46 | } | |
47 | } | |
48 | ||
49 | thread_local! { | |
532ac7d7 | 50 | static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None); |
8faf50e0 XL |
51 | } |
52 | ||
532ac7d7 | 53 | struct SetOnDrop(Option<NonNull<Context<'static>>>); |
8faf50e0 XL |
54 | |
55 | impl Drop for SetOnDrop { | |
56 | fn drop(&mut self) { | |
532ac7d7 XL |
57 | TLS_CX.with(|tls_cx| { |
58 | tls_cx.set(self.0.take()); | |
8faf50e0 XL |
59 | }); |
60 | } | |
61 | } | |
62 | ||
dc9dc135 | 63 | #[doc(hidden)] |
8faf50e0 XL |
64 | #[unstable(feature = "gen_future", issue = "50547")] |
65 | /// Sets the thread-local task context used by async/await futures. | |
532ac7d7 | 66 | pub fn set_task_context<F, R>(cx: &mut Context<'_>, f: F) -> R |
8faf50e0 XL |
67 | where |
68 | F: FnOnce() -> R | |
69 | { | |
532ac7d7 XL |
70 | // transmute the context's lifetime to 'static so we can store it. |
71 | let cx = unsafe { | |
72 | core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx) | |
73 | }; | |
74 | let old_cx = TLS_CX.with(|tls_cx| { | |
75 | tls_cx.replace(Some(NonNull::from(cx))) | |
8faf50e0 | 76 | }); |
532ac7d7 | 77 | let _reset = SetOnDrop(old_cx); |
8faf50e0 XL |
78 | f() |
79 | } | |
80 | ||
dc9dc135 | 81 | #[doc(hidden)] |
8faf50e0 | 82 | #[unstable(feature = "gen_future", issue = "50547")] |
532ac7d7 | 83 | /// Retrieves the thread-local task context used by async/await futures. |
8faf50e0 | 84 | /// |
532ac7d7 | 85 | /// This function acquires exclusive access to the task context. |
8faf50e0 | 86 | /// |
532ac7d7 XL |
87 | /// Panics if no context has been set or if the context has already been |
88 | /// retrieved by a surrounding call to get_task_context. | |
89 | pub fn get_task_context<F, R>(f: F) -> R | |
8faf50e0 | 90 | where |
532ac7d7 | 91 | F: FnOnce(&mut Context<'_>) -> R |
8faf50e0 | 92 | { |
532ac7d7 | 93 | let cx_ptr = TLS_CX.with(|tls_cx| { |
0bf4aa26 | 94 | // Clear the entry so that nested `get_task_waker` calls |
8faf50e0 | 95 | // will fail or set their own value. |
532ac7d7 | 96 | tls_cx.replace(None) |
8faf50e0 | 97 | }); |
532ac7d7 | 98 | let _reset = SetOnDrop(cx_ptr); |
8faf50e0 | 99 | |
532ac7d7 XL |
100 | let mut cx_ptr = cx_ptr.expect( |
101 | "TLS Context not set. This is a rustc bug. \ | |
8faf50e0 | 102 | Please file an issue on https://github.com/rust-lang/rust."); |
532ac7d7 XL |
103 | |
104 | // Safety: we've ensured exclusive access to the context by | |
105 | // removing the pointer from TLS, only to be replaced once | |
106 | // we're done with it. | |
107 | // | |
108 | // The pointer that was inserted came from an `&mut Context<'_>`, | |
109 | // so it is safe to treat as mutable. | |
110 | unsafe { f(cx_ptr.as_mut()) } | |
8faf50e0 XL |
111 | } |
112 | ||
dc9dc135 | 113 | #[doc(hidden)] |
8faf50e0 | 114 | #[unstable(feature = "gen_future", issue = "50547")] |
0bf4aa26 | 115 | /// Polls a future in the current thread-local task waker. |
532ac7d7 | 116 | pub fn poll_with_tls_context<F>(f: Pin<&mut F>) -> Poll<F::Output> |
8faf50e0 XL |
117 | where |
118 | F: Future | |
119 | { | |
532ac7d7 | 120 | get_task_context(|cx| F::poll(f, cx)) |
8faf50e0 | 121 | } |