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1 | // Copyright 2018 The Rust Project Developers. See the COPYRIGHT |
2 | // file at the top-level directory of this distribution and at | |
3 | // http://rust-lang.org/COPYRIGHT. | |
4 | // | |
5 | // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | |
6 | // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | |
7 | // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | |
8 | // option. This file may not be copied, modified, or distributed | |
9 | // except according to those terms. | |
10 | ||
11 | //! Asynchronous values. | |
12 | ||
13 | use core::cell::Cell; | |
14 | use core::marker::Unpin; | |
b7449926 | 15 | use core::pin::PinMut; |
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16 | use core::option::Option; |
17 | use core::ptr::NonNull; | |
18 | use core::task::{self, Poll}; | |
19 | use core::ops::{Drop, Generator, GeneratorState}; | |
20 | ||
21 | #[doc(inline)] | |
22 | pub use core::future::*; | |
23 | ||
24 | /// Wrap a future in a generator. | |
25 | /// | |
26 | /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give | |
27 | /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`). | |
28 | #[unstable(feature = "gen_future", issue = "50547")] | |
29 | pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> { | |
30 | GenFuture(x) | |
31 | } | |
32 | ||
33 | /// A wrapper around generators used to implement `Future` for `async`/`await` code. | |
34 | #[unstable(feature = "gen_future", issue = "50547")] | |
35 | #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)] | |
36 | struct GenFuture<T: Generator<Yield = ()>>(T); | |
37 | ||
38 | // We rely on the fact that async/await futures are immovable in order to create | |
39 | // self-referential borrows in the underlying generator. | |
40 | impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {} | |
41 | ||
42 | #[unstable(feature = "gen_future", issue = "50547")] | |
43 | impl<T: Generator<Yield = ()>> Future for GenFuture<T> { | |
44 | type Output = T::Return; | |
45 | fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> { | |
46 | set_task_cx(cx, || match unsafe { PinMut::get_mut_unchecked(self).0.resume() } { | |
47 | GeneratorState::Yielded(()) => Poll::Pending, | |
48 | GeneratorState::Complete(x) => Poll::Ready(x), | |
49 | }) | |
50 | } | |
51 | } | |
52 | ||
53 | thread_local! { | |
54 | static TLS_CX: Cell<Option<NonNull<task::Context<'static>>>> = Cell::new(None); | |
55 | } | |
56 | ||
57 | struct SetOnDrop(Option<NonNull<task::Context<'static>>>); | |
58 | ||
59 | impl Drop for SetOnDrop { | |
60 | fn drop(&mut self) { | |
61 | TLS_CX.with(|tls_cx| { | |
62 | tls_cx.set(self.0.take()); | |
63 | }); | |
64 | } | |
65 | } | |
66 | ||
67 | #[unstable(feature = "gen_future", issue = "50547")] | |
68 | /// Sets the thread-local task context used by async/await futures. | |
69 | pub fn set_task_cx<F, R>(cx: &mut task::Context, f: F) -> R | |
70 | where | |
71 | F: FnOnce() -> R | |
72 | { | |
73 | let old_cx = TLS_CX.with(|tls_cx| { | |
74 | tls_cx.replace(NonNull::new( | |
75 | cx | |
76 | as *mut task::Context | |
77 | as *mut () | |
78 | as *mut task::Context<'static> | |
79 | )) | |
80 | }); | |
81 | let _reset_cx = SetOnDrop(old_cx); | |
82 | f() | |
83 | } | |
84 | ||
85 | #[unstable(feature = "gen_future", issue = "50547")] | |
86 | /// Retrieves the thread-local task context used by async/await futures. | |
87 | /// | |
88 | /// This function acquires exclusive access to the task context. | |
89 | /// | |
90 | /// Panics if no task has been set or if the task context has already been | |
b7449926 | 91 | /// retrieved by a surrounding call to get_task_cx. |
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92 | pub fn get_task_cx<F, R>(f: F) -> R |
93 | where | |
94 | F: FnOnce(&mut task::Context) -> R | |
95 | { | |
96 | let cx_ptr = TLS_CX.with(|tls_cx| { | |
97 | // Clear the entry so that nested `with_get_cx` calls | |
98 | // will fail or set their own value. | |
99 | tls_cx.replace(None) | |
100 | }); | |
101 | let _reset_cx = SetOnDrop(cx_ptr); | |
102 | ||
103 | let mut cx_ptr = cx_ptr.expect( | |
104 | "TLS task::Context not set. This is a rustc bug. \ | |
105 | Please file an issue on https://github.com/rust-lang/rust."); | |
106 | unsafe { f(cx_ptr.as_mut()) } | |
107 | } | |
108 | ||
109 | #[unstable(feature = "gen_future", issue = "50547")] | |
110 | /// Polls a future in the current thread-local task context. | |
b7449926 | 111 | pub fn poll_in_task_cx<F>(f: PinMut<F>) -> Poll<F::Output> |
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112 | where |
113 | F: Future | |
114 | { | |
b7449926 | 115 | get_task_cx(|cx| f.poll(cx)) |
8faf50e0 | 116 | } |