1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197
/*
* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
* SPDX-License-Identifier: Apache-2.0
*/
//! Provides the [`NowOrLater`] future with an explicit `Now` variant
//!
//! When a future is immediately, ready, this enables avoiding an unnecessary allocation.
//! This is intended to be used with `Pin<Box<dyn Future>>` or similar as the future variant. For
//! convenience, [`BoxFuture`] is provided for this use case.
//!
//! Typically, this is used when creating a manual async trait. In this case, it's critical that the
//! lifetime is captured to enable interop with the async-trait macro.
//!
//! # Examples
//!
//! ```rust
//! mod future {
//! use aws_smithy_async::future::now_or_later::{NowOrLater, BoxFuture};
//! use std::future::Future;
//! pub struct ProvideRegion<'a>(NowOrLater<Option<String>, BoxFuture<'a, Option<String>>>);
//! impl<'a> ProvideRegion<'a> {
//! pub fn new(f: impl Future<Output = Option<String>> + Send + 'a) -> Self {
//! Self(NowOrLater::new(Box::pin(f)))
//! }
//!
//! pub fn ready(region: Option<String>) -> Self {
//! Self(NowOrLater::ready(region))
//! }
//! }
//! }
//!
//! pub trait ProvideRegion {
//! fn provide_region<'a>(&'a self) -> future::ProvideRegion<'a> where Self: 'a;
//! }
//!
//! struct AsyncRegionProvider;
//! impl AsyncRegionProvider {
//! async fn region(&self) -> Option<String> {
//! todo!()
//! }
//! }
//!
//! impl ProvideRegion for AsyncRegionProvider {
//! fn provide_region<'a>(&'a self) -> future::ProvideRegion<'a> where Self: 'a {
//! future::ProvideRegion::new(self.region())
//! }
//! }
//! ```
use std::fmt;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use pin_project_lite::pin_project;
/// Boxed future type alias
pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
#[derive(Debug)]
/// Zero sized type for using NowOrLater when no future variant exists.
pub enum OnlyReady {}
pin_project! {
/// Future with an explicit `Now` variant
///
/// See the [module documentation](crate::future::now_or_later) for more information.
pub struct NowOrLater<T, F> {
#[pin]
inner: Inner<T, F>
}
}
impl<T, F> fmt::Debug for NowOrLater<T, F>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("NowOrLater")
.field("inner", &self.inner)
.finish()
}
}
pin_project! {
#[project = NowOrLaterProj]
enum Inner<T, F> {
#[non_exhaustive]
Now { value: Option<T> },
#[non_exhaustive]
Later { #[pin] future: F },
}
}
impl<T, F> fmt::Debug for Inner<T, F>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Self::Now { value } => f.debug_struct("Now").field("value", value).finish(),
Self::Later { .. } => f
.debug_struct("Later")
.field("future", &"<future>")
.finish(),
}
}
}
impl<T, F> NowOrLater<T, F> {
/// Creates a future that will resolve when `future` resolves
pub fn new(future: F) -> Self {
Self {
inner: Inner::Later { future },
}
}
/// Creates a future that immediately resolves to `value`
pub fn ready(value: T) -> NowOrLater<T, F> {
let value = Some(value);
Self {
inner: Inner::Now { value },
}
}
}
impl<T, F> Future for NowOrLater<T, F>
where
F: Future<Output = T>,
{
type Output = T;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.project().inner.project() {
NowOrLaterProj::Now { value } => {
Poll::Ready(value.take().expect("cannot be called twice"))
}
NowOrLaterProj::Later { future } => future.poll(cx),
}
}
}
impl<T> Future for NowOrLater<T, OnlyReady> {
type Output = T;
fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.project().inner.project() {
NowOrLaterProj::Now { value } => {
Poll::Ready(value.take().expect("cannot be called twice"))
}
NowOrLaterProj::Later { .. } => unreachable!(),
}
}
}
#[cfg(test)]
mod test {
use crate::future::now_or_later::{NowOrLater, OnlyReady};
use futures_util::FutureExt;
#[test]
fn ready_future_immediately_returns() {
let a = true;
let f = if a {
NowOrLater::ready(5)
} else {
NowOrLater::new(async { 5 })
};
use futures_util::FutureExt;
assert_eq!(f.now_or_never().expect("future was ready"), 5);
}
#[test]
fn only_ready_instantiation() {
assert_eq!(
NowOrLater::<i32, OnlyReady>::ready(5)
.now_or_never()
.expect("ready"),
5
);
}
#[tokio::test]
async fn box_dyn_future() {
let f = async { 5 };
let f = Box::pin(f);
let wrapped = NowOrLater::new(f);
assert_eq!(wrapped.await, 5);
}
#[tokio::test]
async fn async_fn_future() {
let wrapped = NowOrLater::new(async { 5 });
assert_eq!(wrapped.await, 5);
}
}