1 use crate::ops::ControlFlow;
3 /// The `?` operator and `try {}` blocks.
5 /// `try_*` methods typically involve a type implementing this trait. For
6 /// example, the closures passed to [`Iterator::try_fold`] and
7 /// [`Iterator::try_for_each`] must return such a type.
9 /// `Try` types are typically those containing two or more categories of values,
10 /// some subset of which are so commonly handled via early returns that it's
11 /// worth providing a terse (but still visible) syntax to make that easy.
13 /// This is most often seen for error handling with [`Result`] and [`Option`].
14 /// The quintessential implementation of this trait is on [`ControlFlow`].
16 /// # Using `Try` in Generic Code
18 /// `Iterator::try_fold` was stabilized to call back in Rust 1.27, but
19 /// this trait is much newer. To illustrate the various associated types and
20 /// methods, let's implement our own version.
22 /// As a reminder, an infallible version of a fold looks something like this:
24 /// fn simple_fold<A, T>(
25 /// iter: impl Iterator<Item = T>,
27 /// mut f: impl FnMut(A, T) -> A,
30 /// accum = f(accum, x);
36 /// So instead of `f` returning just an `A`, we'll need it to return some other
37 /// type that produces an `A` in the "don't short circuit" path. Conveniently,
38 /// that's also the type we need to return from the function.
40 /// Let's add a new generic parameter `R` for that type, and bound it to the
41 /// output type that we want:
43 /// # #![feature(try_trait_v2)]
44 /// # use std::ops::Try;
45 /// fn simple_try_fold_1<A, T, R: Try<Output = A>>(
46 /// iter: impl Iterator<Item = T>,
48 /// mut f: impl FnMut(A, T) -> R,
54 /// If we get through the entire iterator, we need to wrap up the accumulator
55 /// into the return type using [`Try::from_output`]:
57 /// # #![feature(try_trait_v2)]
58 /// # use std::ops::{ControlFlow, Try};
59 /// fn simple_try_fold_2<A, T, R: Try<Output = A>>(
60 /// iter: impl Iterator<Item = T>,
62 /// mut f: impl FnMut(A, T) -> R,
65 /// let cf = f(accum, x).branch();
67 /// ControlFlow::Continue(a) => accum = a,
68 /// ControlFlow::Break(_) => todo!(),
71 /// R::from_output(accum)
75 /// We'll also need [`FromResidual::from_residual`] to turn the residual back
76 /// into the original type. But because it's a supertrait of `Try`, we don't
77 /// need to mention it in the bounds. All types which implement `Try` can be
78 /// recreated from their corresponding residual, so we'll just call it:
80 /// # #![feature(try_trait_v2)]
81 /// # use std::ops::{ControlFlow, Try};
82 /// pub fn simple_try_fold_3<A, T, R: Try<Output = A>>(
83 /// iter: impl Iterator<Item = T>,
85 /// mut f: impl FnMut(A, T) -> R,
88 /// let cf = f(accum, x).branch();
90 /// ControlFlow::Continue(a) => accum = a,
91 /// ControlFlow::Break(r) => return R::from_residual(r),
94 /// R::from_output(accum)
98 /// But this "call `branch`, then `match` on it, and `return` if it was a
99 /// `Break`" is exactly what happens inside the `?` operator. So rather than
100 /// do all this manually, we can just use `?` instead:
102 /// # #![feature(try_trait_v2)]
103 /// # use std::ops::Try;
104 /// fn simple_try_fold<A, T, R: Try<Output = A>>(
105 /// iter: impl Iterator<Item = T>,
107 /// mut f: impl FnMut(A, T) -> R,
110 /// accum = f(accum, x)?;
112 /// R::from_output(accum)
115 #[unstable(feature = "try_trait_v2", issue = "84277")]
116 #[rustc_on_unimplemented(
118 all(from_desugaring = "TryBlock"),
119 message = "a `try` block must return `Result` or `Option` \
120 (or another type that implements `{Try}`)",
121 label = "could not wrap the final value of the block as `{Self}` doesn't implement `Try`",
124 all(from_desugaring = "QuestionMark"),
125 message = "the `?` operator can only be applied to values that implement `{Try}`",
126 label = "the `?` operator cannot be applied to type `{Self}`"
131 pub trait Try: FromResidual {
132 /// The type of the value produced by `?` when *not* short-circuiting.
133 #[unstable(feature = "try_trait_v2", issue = "84277")]
136 /// The type of the value passed to [`FromResidual::from_residual`]
137 /// as part of `?` when short-circuiting.
139 /// This represents the possible values of the `Self` type which are *not*
140 /// represented by the `Output` type.
142 /// # Note to Implementors
144 /// The choice of this type is critical to interconversion.
145 /// Unlike the `Output` type, which will often be a raw generic type,
146 /// this type is typically a newtype of some sort to "color" the type
147 /// so that it's distinguishable from the residuals of other types.
149 /// This is why `Result<T, E>::Residual` is not `E`, but `Result<Infallible, E>`.
150 /// That way it's distinct from `ControlFlow<E>::Residual`, for example,
151 /// and thus `?` on `ControlFlow` cannot be used in a method returning `Result`.
153 /// If you're making a generic type `Foo<T>` that implements `Try<Output = T>`,
154 /// then typically you can use `Foo<std::convert::Infallible>` as its `Residual`
155 /// type: that type will have a "hole" in the correct place, and will maintain the
156 /// "foo-ness" of the residual so other types need to opt-in to interconversion.
157 #[unstable(feature = "try_trait_v2", issue = "84277")]
160 /// Constructs the type from its `Output` type.
162 /// This should be implemented consistently with the `branch` method
163 /// such that applying the `?` operator will get back the original value:
164 /// `Try::from_output(x).branch() --> ControlFlow::Continue(x)`.
169 /// #![feature(try_trait_v2)]
170 /// use std::ops::Try;
172 /// assert_eq!(<Result<_, String> as Try>::from_output(3), Ok(3));
173 /// assert_eq!(<Option<_> as Try>::from_output(4), Some(4));
175 /// <std::ops::ControlFlow<String, _> as Try>::from_output(5),
176 /// std::ops::ControlFlow::Continue(5),
179 /// # fn make_question_mark_work() -> Option<()> {
180 /// assert_eq!(Option::from_output(4)?, 4);
182 /// # make_question_mark_work();
184 /// // This is used, for example, on the accumulator in `try_fold`:
185 /// let r = std::iter::empty().try_fold(4, |_, ()| -> Option<_> { unreachable!() });
186 /// assert_eq!(r, Some(4));
188 #[lang = "from_output"]
189 #[unstable(feature = "try_trait_v2", issue = "84277")]
190 fn from_output(output: Self::Output) -> Self;
192 /// Used in `?` to decide whether the operator should produce a value
193 /// (because this returned [`ControlFlow::Continue`])
194 /// or propagate a value back to the caller
195 /// (because this returned [`ControlFlow::Break`]).
200 /// #![feature(try_trait_v2)]
201 /// use std::ops::{ControlFlow, Try};
203 /// assert_eq!(Ok::<_, String>(3).branch(), ControlFlow::Continue(3));
204 /// assert_eq!(Err::<String, _>(3).branch(), ControlFlow::Break(Err(3)));
206 /// assert_eq!(Some(3).branch(), ControlFlow::Continue(3));
207 /// assert_eq!(None::<String>.branch(), ControlFlow::Break(None));
209 /// assert_eq!(ControlFlow::<String, _>::Continue(3).branch(), ControlFlow::Continue(3));
211 /// ControlFlow::<_, String>::Break(3).branch(),
212 /// ControlFlow::Break(ControlFlow::Break(3)),
216 #[unstable(feature = "try_trait_v2", issue = "84277")]
217 fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
220 /// Used to specify which residuals can be converted into which [`crate::ops::Try`] types.
222 /// Every `Try` type needs to be recreatable from its own associated
223 /// `Residual` type, but can also have additional `FromResidual` implementations
224 /// to support interconversion with other `Try` types.
225 #[rustc_on_unimplemented(
228 from_desugaring = "QuestionMark",
229 _Self = "std::result::Result<T, E>",
230 R = "std::option::Option<std::convert::Infallible>"
232 message = "the `?` operator can only be used on `Result`s, not `Option`s, \
233 in {ItemContext} that returns `Result`",
234 label = "use `.ok_or(...)?` to provide an error compatible with `{Self}`",
235 enclosing_scope = "this function returns a `Result`"
239 from_desugaring = "QuestionMark",
240 _Self = "std::result::Result<T, E>",
242 // There's a special error message in the trait selection code for
243 // `From` in `?`, so this is not shown for result-in-result errors,
244 // and thus it can be phrased more strongly than `ControlFlow`'s.
245 message = "the `?` operator can only be used on `Result`s \
246 in {ItemContext} that returns `Result`",
247 label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
248 enclosing_scope = "this function returns a `Result`"
252 from_desugaring = "QuestionMark",
253 _Self = "std::option::Option<T>",
254 R = "std::result::Result<T, E>",
256 message = "the `?` operator can only be used on `Option`s, not `Result`s, \
257 in {ItemContext} that returns `Option`",
258 label = "use `.ok()?` if you want to discard the `{R}` error information",
259 enclosing_scope = "this function returns an `Option`"
263 from_desugaring = "QuestionMark",
264 _Self = "std::option::Option<T>",
266 // `Option`-in-`Option` always works, as there's only one possible
267 // residual, so this can also be phrased strongly.
268 message = "the `?` operator can only be used on `Option`s \
269 in {ItemContext} that returns `Option`",
270 label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
271 enclosing_scope = "this function returns an `Option`"
275 from_desugaring = "QuestionMark",
276 _Self = "std::ops::ControlFlow<B, C>",
277 R = "std::ops::ControlFlow<B, C>",
279 message = "the `?` operator in {ItemContext} that returns `ControlFlow<B, _>` \
280 can only be used on other `ControlFlow<B, _>`s (with the same Break type)",
281 label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
282 enclosing_scope = "this function returns a `ControlFlow`",
283 note = "unlike `Result`, there's no `From`-conversion performed for `ControlFlow`"
287 from_desugaring = "QuestionMark",
288 _Self = "std::ops::ControlFlow<B, C>",
289 // `R` is not a `ControlFlow`, as that case was matched previously
291 message = "the `?` operator can only be used on `ControlFlow`s \
292 in {ItemContext} that returns `ControlFlow`",
293 label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
294 enclosing_scope = "this function returns a `ControlFlow`",
297 all(from_desugaring = "QuestionMark"),
298 message = "the `?` operator can only be used in {ItemContext} \
299 that returns `Result` or `Option` \
300 (or another type that implements `{FromResidual}`)",
301 label = "cannot use the `?` operator in {ItemContext} that returns `{Self}`",
302 enclosing_scope = "this function should return `Result` or `Option` to accept `?`"
305 #[rustc_diagnostic_item = "FromResidual"]
306 #[unstable(feature = "try_trait_v2", issue = "84277")]
307 pub trait FromResidual<R = <Self as Try>::Residual> {
308 /// Constructs the type from a compatible `Residual` type.
310 /// This should be implemented consistently with the `branch` method such
311 /// that applying the `?` operator will get back an equivalent residual:
312 /// `FromResidual::from_residual(r).branch() --> ControlFlow::Break(r)`.
313 /// (It must not be an *identical* residual when interconversion is involved.)
318 /// #![feature(try_trait_v2)]
319 /// use std::ops::{ControlFlow, FromResidual};
321 /// assert_eq!(Result::<String, i64>::from_residual(Err(3_u8)), Err(3));
322 /// assert_eq!(Option::<String>::from_residual(None), None);
324 /// ControlFlow::<_, String>::from_residual(ControlFlow::Break(5)),
325 /// ControlFlow::Break(5),
328 #[lang = "from_residual"]
329 #[unstable(feature = "try_trait_v2", issue = "84277")]
330 fn from_residual(residual: R) -> Self;
334 feature = "yeet_desugar_details",
336 reason = "just here to simplify the desugaring; will never be stabilized"
339 #[track_caller] // because `Result::from_residual` has it
340 #[lang = "from_yeet"]
341 pub fn from_yeet<T, Y>(yeeted: Y) -> T
343 T: FromResidual<Yeet<Y>>,
345 FromResidual::from_residual(Yeet(yeeted))
348 /// Allows retrieving the canonical type implementing [`Try`] that has this type
349 /// as its residual and allows it to hold an `O` as its output.
351 /// If you think of the `Try` trait as splitting a type into its [`Try::Output`]
352 /// and [`Try::Residual`] components, this allows putting them back together.
355 /// `Result<T, E>: Try<Output = T, Residual = Result<Infallible, E>>`,
356 /// and in the other direction,
357 /// `<Result<Infallible, E> as Residual<T>>::TryType = Result<T, E>`.
358 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
359 pub trait Residual<O> {
360 /// The "return" type of this meta-function.
361 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
362 type TryType: Try<Output = O, Residual = Self>;
365 #[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]
366 pub(crate) type ChangeOutputType<T, V> = <<T as Try>::Residual as Residual<V>>::TryType;
368 /// An adapter for implementing non-try methods via the `Try` implementation.
370 /// Conceptually the same as `Result<T, !>`, but requiring less work in trait
371 /// solving and inhabited-ness checking and such, by being an obvious newtype
372 /// and not having `From` bounds lying around.
374 /// Not currently planned to be exposed publicly, so just `pub(crate)`.
376 pub(crate) struct NeverShortCircuit<T>(pub T);
378 impl<T> NeverShortCircuit<T> {
379 /// Wrap a binary `FnMut` to return its result wrapped in a `NeverShortCircuit`.
381 pub fn wrap_mut_2<A, B>(mut f: impl FnMut(A, B) -> T) -> impl FnMut(A, B) -> Self {
382 move |a, b| NeverShortCircuit(f(a, b))
386 pub(crate) enum NeverShortCircuitResidual {}
388 impl<T> Try for NeverShortCircuit<T> {
390 type Residual = NeverShortCircuitResidual;
393 fn branch(self) -> ControlFlow<NeverShortCircuitResidual, T> {
394 ControlFlow::Continue(self.0)
398 fn from_output(x: T) -> Self {
403 impl<T> FromResidual for NeverShortCircuit<T> {
405 fn from_residual(never: NeverShortCircuitResidual) -> Self {
410 impl<T> Residual<T> for NeverShortCircuitResidual {
411 type TryType = NeverShortCircuit<T>;
414 /// Implement `FromResidual<Yeet<T>>` on your type to enable
415 /// `do yeet expr` syntax in functions returning your type.
416 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
418 pub struct Yeet<T>(pub T);