core/ops/control_flow.rs
1use crate::{convert, ops};
2
3/// Used to tell an operation whether it should exit early or go on as usual.
4///
5/// This is used when exposing things (like graph traversals or visitors) where
6/// you want the user to be able to choose whether to exit early.
7/// Having the enum makes it clearer -- no more wondering "wait, what did `false`
8/// mean again?" -- and allows including a value.
9///
10/// Similar to [`Option`] and [`Result`], this enum can be used with the `?` operator
11/// to return immediately if the [`Break`] variant is present or otherwise continue normally
12/// with the value inside the [`Continue`] variant.
13///
14/// # Examples
15///
16/// Early-exiting from [`Iterator::try_for_each`]:
17/// ```
18/// use std::ops::ControlFlow;
19///
20/// let r = (2..100).try_for_each(|x| {
21/// if 403 % x == 0 {
22/// return ControlFlow::Break(x)
23/// }
24///
25/// ControlFlow::Continue(())
26/// });
27/// assert_eq!(r, ControlFlow::Break(13));
28/// ```
29///
30/// A basic tree traversal:
31/// ```
32/// use std::ops::ControlFlow;
33///
34/// pub struct TreeNode<T> {
35/// value: T,
36/// left: Option<Box<TreeNode<T>>>,
37/// right: Option<Box<TreeNode<T>>>,
38/// }
39///
40/// impl<T> TreeNode<T> {
41/// pub fn traverse_inorder<B>(&self, f: &mut impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
42/// if let Some(left) = &self.left {
43/// left.traverse_inorder(f)?;
44/// }
45/// f(&self.value)?;
46/// if let Some(right) = &self.right {
47/// right.traverse_inorder(f)?;
48/// }
49/// ControlFlow::Continue(())
50/// }
51/// fn leaf(value: T) -> Option<Box<TreeNode<T>>> {
52/// Some(Box::new(Self { value, left: None, right: None }))
53/// }
54/// }
55///
56/// let node = TreeNode {
57/// value: 0,
58/// left: TreeNode::leaf(1),
59/// right: Some(Box::new(TreeNode {
60/// value: -1,
61/// left: TreeNode::leaf(5),
62/// right: TreeNode::leaf(2),
63/// }))
64/// };
65/// let mut sum = 0;
66///
67/// let res = node.traverse_inorder(&mut |val| {
68/// if *val < 0 {
69/// ControlFlow::Break(*val)
70/// } else {
71/// sum += *val;
72/// ControlFlow::Continue(())
73/// }
74/// });
75/// assert_eq!(res, ControlFlow::Break(-1));
76/// assert_eq!(sum, 6);
77/// ```
78///
79/// [`Break`]: ControlFlow::Break
80/// [`Continue`]: ControlFlow::Continue
81#[stable(feature = "control_flow_enum_type", since = "1.55.0")]
82#[rustc_diagnostic_item = "ControlFlow"]
83#[must_use]
84// ControlFlow should not implement PartialOrd or Ord, per RFC 3058:
85// https://rust-lang.github.io/rfcs/3058-try-trait-v2.html#traits-for-controlflow
86#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
87pub enum ControlFlow<B, C = ()> {
88 /// Move on to the next phase of the operation as normal.
89 #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
90 #[lang = "Continue"]
91 Continue(C),
92 /// Exit the operation without running subsequent phases.
93 #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
94 #[lang = "Break"]
95 Break(B),
96 // Yes, the order of the variants doesn't match the type parameters.
97 // They're in this order so that `ControlFlow<A, B>` <-> `Result<B, A>`
98 // is a no-op conversion in the `Try` implementation.
99}
100
101#[unstable(feature = "try_trait_v2", issue = "84277")]
102impl<B, C> ops::Try for ControlFlow<B, C> {
103 type Output = C;
104 type Residual = ControlFlow<B, convert::Infallible>;
105
106 #[inline]
107 fn from_output(output: Self::Output) -> Self {
108 ControlFlow::Continue(output)
109 }
110
111 #[inline]
112 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
113 match self {
114 ControlFlow::Continue(c) => ControlFlow::Continue(c),
115 ControlFlow::Break(b) => ControlFlow::Break(ControlFlow::Break(b)),
116 }
117 }
118}
119
120#[unstable(feature = "try_trait_v2", issue = "84277")]
121// Note: manually specifying the residual type instead of using the default to work around
122// https://github.com/rust-lang/rust/issues/99940
123impl<B, C> ops::FromResidual<ControlFlow<B, convert::Infallible>> for ControlFlow<B, C> {
124 #[inline]
125 fn from_residual(residual: ControlFlow<B, convert::Infallible>) -> Self {
126 match residual {
127 ControlFlow::Break(b) => ControlFlow::Break(b),
128 }
129 }
130}
131
132#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
133impl<B, C> ops::Residual<C> for ControlFlow<B, convert::Infallible> {
134 type TryType = ControlFlow<B, C>;
135}
136
137impl<B, C> ControlFlow<B, C> {
138 /// Returns `true` if this is a `Break` variant.
139 ///
140 /// # Examples
141 ///
142 /// ```
143 /// use std::ops::ControlFlow;
144 ///
145 /// assert!(ControlFlow::<&str, i32>::Break("Stop right there!").is_break());
146 /// assert!(!ControlFlow::<&str, i32>::Continue(3).is_break());
147 /// ```
148 #[inline]
149 #[stable(feature = "control_flow_enum_is", since = "1.59.0")]
150 pub fn is_break(&self) -> bool {
151 matches!(*self, ControlFlow::Break(_))
152 }
153
154 /// Returns `true` if this is a `Continue` variant.
155 ///
156 /// # Examples
157 ///
158 /// ```
159 /// use std::ops::ControlFlow;
160 ///
161 /// assert!(!ControlFlow::<&str, i32>::Break("Stop right there!").is_continue());
162 /// assert!(ControlFlow::<&str, i32>::Continue(3).is_continue());
163 /// ```
164 #[inline]
165 #[stable(feature = "control_flow_enum_is", since = "1.59.0")]
166 pub fn is_continue(&self) -> bool {
167 matches!(*self, ControlFlow::Continue(_))
168 }
169
170 /// Converts the `ControlFlow` into an `Option` which is `Some` if the
171 /// `ControlFlow` was `Break` and `None` otherwise.
172 ///
173 /// # Examples
174 ///
175 /// ```
176 /// use std::ops::ControlFlow;
177 ///
178 /// assert_eq!(ControlFlow::<&str, i32>::Break("Stop right there!").break_value(), Some("Stop right there!"));
179 /// assert_eq!(ControlFlow::<&str, i32>::Continue(3).break_value(), None);
180 /// ```
181 #[inline]
182 #[stable(feature = "control_flow_enum", since = "1.83.0")]
183 pub fn break_value(self) -> Option<B> {
184 match self {
185 ControlFlow::Continue(..) => None,
186 ControlFlow::Break(x) => Some(x),
187 }
188 }
189
190 /// Maps `ControlFlow<B, C>` to `ControlFlow<T, C>` by applying a function
191 /// to the break value in case it exists.
192 #[inline]
193 #[stable(feature = "control_flow_enum", since = "1.83.0")]
194 pub fn map_break<T>(self, f: impl FnOnce(B) -> T) -> ControlFlow<T, C> {
195 match self {
196 ControlFlow::Continue(x) => ControlFlow::Continue(x),
197 ControlFlow::Break(x) => ControlFlow::Break(f(x)),
198 }
199 }
200
201 /// Converts the `ControlFlow` into an `Option` which is `Some` if the
202 /// `ControlFlow` was `Continue` and `None` otherwise.
203 ///
204 /// # Examples
205 ///
206 /// ```
207 /// use std::ops::ControlFlow;
208 ///
209 /// assert_eq!(ControlFlow::<&str, i32>::Break("Stop right there!").continue_value(), None);
210 /// assert_eq!(ControlFlow::<&str, i32>::Continue(3).continue_value(), Some(3));
211 /// ```
212 #[inline]
213 #[stable(feature = "control_flow_enum", since = "1.83.0")]
214 pub fn continue_value(self) -> Option<C> {
215 match self {
216 ControlFlow::Continue(x) => Some(x),
217 ControlFlow::Break(..) => None,
218 }
219 }
220
221 /// Maps `ControlFlow<B, C>` to `ControlFlow<B, T>` by applying a function
222 /// to the continue value in case it exists.
223 #[inline]
224 #[stable(feature = "control_flow_enum", since = "1.83.0")]
225 pub fn map_continue<T>(self, f: impl FnOnce(C) -> T) -> ControlFlow<B, T> {
226 match self {
227 ControlFlow::Continue(x) => ControlFlow::Continue(f(x)),
228 ControlFlow::Break(x) => ControlFlow::Break(x),
229 }
230 }
231}
232
233impl<T> ControlFlow<T, T> {
234 /// Extracts the value `T` that is wrapped by `ControlFlow<T, T>`.
235 ///
236 /// # Examples
237 ///
238 /// ```
239 /// #![feature(control_flow_into_value)]
240 /// use std::ops::ControlFlow;
241 ///
242 /// assert_eq!(ControlFlow::<i32, i32>::Break(1024).into_value(), 1024);
243 /// assert_eq!(ControlFlow::<i32, i32>::Continue(512).into_value(), 512);
244 /// ```
245 #[unstable(feature = "control_flow_into_value", issue = "137461")]
246 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
247 pub const fn into_value(self) -> T {
248 match self {
249 ControlFlow::Continue(x) | ControlFlow::Break(x) => x,
250 }
251 }
252}
253
254/// These are used only as part of implementing the iterator adapters.
255/// They have mediocre names and non-obvious semantics, so aren't
256/// currently on a path to potential stabilization.
257impl<R: ops::Try> ControlFlow<R, R::Output> {
258 /// Creates a `ControlFlow` from any type implementing `Try`.
259 #[inline]
260 pub(crate) fn from_try(r: R) -> Self {
261 match R::branch(r) {
262 ControlFlow::Continue(v) => ControlFlow::Continue(v),
263 ControlFlow::Break(v) => ControlFlow::Break(R::from_residual(v)),
264 }
265 }
266
267 /// Converts a `ControlFlow` into any type implementing `Try`.
268 #[inline]
269 pub(crate) fn into_try(self) -> R {
270 match self {
271 ControlFlow::Continue(v) => R::from_output(v),
272 ControlFlow::Break(v) => v,
273 }
274 }
275}