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
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
use crate::ops::ControlFlow;

/// The `?` operator and `try {}` blocks.
///
/// `try_*` methods typically involve a type implementing this trait.  For
/// example, the closures passed to [`Iterator::try_fold`] and
/// [`Iterator::try_for_each`] must return such a type.
///
/// `Try` types are typically those containing two or more categories of values,
/// some subset of which are so commonly handled via early returns that it's
/// worth providing a terse (but still visible) syntax to make that easy.
///
/// This is most often seen for error handling with [`Result`] and [`Option`].
/// The quintessential implementation of this trait is on [`ControlFlow`].
///
/// # Using `Try` in Generic Code
///
/// `Iterator::try_fold` was stabilized to call back in Rust 1.27, but
/// this trait is much newer.  To illustrate the various associated types and
/// methods, let's implement our own version.
///
/// As a reminder, an infallible version of a fold looks something like this:
/// ```
/// fn simple_fold<A, T>(
///     iter: impl Iterator<Item = T>,
///     mut accum: A,
///     mut f: impl FnMut(A, T) -> A,
/// ) -> A {
///     for x in iter {
///         accum = f(accum, x);
///     }
///     accum
/// }
/// ```
///
/// So instead of `f` returning just an `A`, we'll need it to return some other
/// type that produces an `A` in the "don't short circuit" path.  Conveniently,
/// that's also the type we need to return from the function.
///
/// Let's add a new generic parameter `R` for that type, and bound it to the
/// output type that we want:
/// ```
/// # #![feature(try_trait_v2)]
/// # use std::ops::Try;
/// fn simple_try_fold_1<A, T, R: Try<Output = A>>(
///     iter: impl Iterator<Item = T>,
///     mut accum: A,
///     mut f: impl FnMut(A, T) -> R,
/// ) -> R {
///     todo!()
/// }
/// ```
///
/// If we get through the entire iterator, we need to wrap up the accumulator
/// into the return type using [`Try::from_output`]:
/// ```
/// # #![feature(try_trait_v2)]
/// # use std::ops::{ControlFlow, Try};
/// fn simple_try_fold_2<A, T, R: Try<Output = A>>(
///     iter: impl Iterator<Item = T>,
///     mut accum: A,
///     mut f: impl FnMut(A, T) -> R,
/// ) -> R {
///     for x in iter {
///         let cf = f(accum, x).branch();
///         match cf {
///             ControlFlow::Continue(a) => accum = a,
///             ControlFlow::Break(_) => todo!(),
///         }
///     }
///     R::from_output(accum)
/// }
/// ```
///
/// We'll also need [`FromResidual::from_residual`] to turn the residual back
/// into the original type.  But because it's a supertrait of `Try`, we don't
/// need to mention it in the bounds.  All types which implement `Try` can be
/// recreated from their corresponding residual, so we'll just call it:
/// ```
/// # #![feature(try_trait_v2)]
/// # use std::ops::{ControlFlow, Try};
/// pub fn simple_try_fold_3<A, T, R: Try<Output = A>>(
///     iter: impl Iterator<Item = T>,
///     mut accum: A,
///     mut f: impl FnMut(A, T) -> R,
/// ) -> R {
///     for x in iter {
///         let cf = f(accum, x).branch();
///         match cf {
///             ControlFlow::Continue(a) => accum = a,
///             ControlFlow::Break(r) => return R::from_residual(r),
///         }
///     }
///     R::from_output(accum)
/// }
/// ```
///
/// But this "call `branch`, then `match` on it, and `return` if it was a
/// `Break`" is exactly what happens inside the `?` operator.  So rather than
/// do all this manually, we can just use `?` instead:
/// ```
/// # #![feature(try_trait_v2)]
/// # use std::ops::Try;
/// fn simple_try_fold<A, T, R: Try<Output = A>>(
///     iter: impl Iterator<Item = T>,
///     mut accum: A,
///     mut f: impl FnMut(A, T) -> R,
/// ) -> R {
///     for x in iter {
///         accum = f(accum, x)?;
///     }
///     R::from_output(accum)
/// }
/// ```
#[unstable(feature = "try_trait_v2", issue = "84277")]
#[rustc_on_unimplemented(
    on(
        all(from_desugaring = "TryBlock"),
        message = "a `try` block must return `Result` or `Option` \
                    (or another type that implements `{Try}`)",
        label = "could not wrap the final value of the block as `{Self}` doesn't implement `Try`",
    ),
    on(
        all(from_desugaring = "QuestionMark"),
        message = "the `?` operator can only be applied to values that implement `{Try}`",
        label = "the `?` operator cannot be applied to type `{Self}`"
    )
)]
#[doc(alias = "?")]
#[lang = "Try"]
pub trait Try: FromResidual {
    /// The type of the value produced by `?` when *not* short-circuiting.
    #[unstable(feature = "try_trait_v2", issue = "84277")]
    type Output;

    /// The type of the value passed to [`FromResidual::from_residual`]
    /// as part of `?` when short-circuiting.
    ///
    /// This represents the possible values of the `Self` type which are *not*
    /// represented by the `Output` type.
    ///
    /// # Note to Implementors
    ///
    /// The choice of this type is critical to interconversion.
    /// Unlike the `Output` type, which will often be a raw generic type,
    /// this type is typically a newtype of some sort to "color" the type
    /// so that it's distinguishable from the residuals of other types.
    ///
    /// This is why `Result<T, E>::Residual` is not `E`, but `Result<Infallible, E>`.
    /// That way it's distinct from `ControlFlow<E>::Residual`, for example,
    /// and thus `?` on `ControlFlow` cannot be used in a method returning `Result`.
    ///
    /// If you're making a generic type `Foo<T>` that implements `Try<Output = T>`,
    /// then typically you can use `Foo<std::convert::Infallible>` as its `Residual`
    /// type: that type will have a "hole" in the correct place, and will maintain the
    /// "foo-ness" of the residual so other types need to opt-in to interconversion.
    #[unstable(feature = "try_trait_v2", issue = "84277")]
    type Residual;

    /// Constructs the type from its `Output` type.
    ///
    /// This should be implemented consistently with the `branch` method
    /// such that applying the `?` operator will get back the original value:
    /// `Try::from_output(x).branch() --> ControlFlow::Continue(x)`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_trait_v2)]
    /// use std::ops::Try;
    ///
    /// assert_eq!(<Result<_, String> as Try>::from_output(3), Ok(3));
    /// assert_eq!(<Option<_> as Try>::from_output(4), Some(4));
    /// assert_eq!(
    ///     <std::ops::ControlFlow<String, _> as Try>::from_output(5),
    ///     std::ops::ControlFlow::Continue(5),
    /// );
    ///
    /// # fn make_question_mark_work() -> Option<()> {
    /// assert_eq!(Option::from_output(4)?, 4);
    /// # None }
    /// # make_question_mark_work();
    ///
    /// // This is used, for example, on the accumulator in `try_fold`:
    /// let r = std::iter::empty().try_fold(4, |_, ()| -> Option<_> { unreachable!() });
    /// assert_eq!(r, Some(4));
    /// ```
    #[lang = "from_output"]
    #[unstable(feature = "try_trait_v2", issue = "84277")]
    fn from_output(output: Self::Output) -> Self;

    /// Used in `?` to decide whether the operator should produce a value
    /// (because this returned [`ControlFlow::Continue`])
    /// or propagate a value back to the caller
    /// (because this returned [`ControlFlow::Break`]).
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_trait_v2)]
    /// use std::ops::{ControlFlow, Try};
    ///
    /// assert_eq!(Ok::<_, String>(3).branch(), ControlFlow::Continue(3));
    /// assert_eq!(Err::<String, _>(3).branch(), ControlFlow::Break(Err(3)));
    ///
    /// assert_eq!(Some(3).branch(), ControlFlow::Continue(3));
    /// assert_eq!(None::<String>.branch(), ControlFlow::Break(None));
    ///
    /// assert_eq!(ControlFlow::<String, _>::Continue(3).branch(), ControlFlow::Continue(3));
    /// assert_eq!(
    ///     ControlFlow::<_, String>::Break(3).branch(),
    ///     ControlFlow::Break(ControlFlow::Break(3)),
    /// );
    /// ```
    #[lang = "branch"]
    #[unstable(feature = "try_trait_v2", issue = "84277")]
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
}

/// Used to specify which residuals can be converted into which [`crate::ops::Try`] types.
///
/// Every `Try` type needs to be recreatable from its own associated
/// `Residual` type, but can also have additional `FromResidual` implementations
/// to support interconversion with other `Try` types.
#[rustc_on_unimplemented(
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::result::Result<T, E>",
            R = "std::option::Option<std::convert::Infallible>"
        ),
        message = "the `?` operator can only be used on `Result`s, not `Option`s, \
            in {ItemContext} that returns `Result`",
        label = "use `.ok_or(...)?` to provide an error compatible with `{Self}`",
        enclosing_scope = "this function returns a `Result`"
    ),
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::result::Result<T, E>",
        ),
        // There's a special error message in the trait selection code for
        // `From` in `?`, so this is not shown for result-in-result errors,
        // and thus it can be phrased more strongly than `ControlFlow`'s.
        message = "the `?` operator can only be used on `Result`s \
            in {ItemContext} that returns `Result`",
        label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
        enclosing_scope = "this function returns a `Result`"
    ),
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::option::Option<T>",
            R = "std::result::Result<T, E>",
        ),
        message = "the `?` operator can only be used on `Option`s, not `Result`s, \
            in {ItemContext} that returns `Option`",
        label = "use `.ok()?` if you want to discard the `{R}` error information",
        enclosing_scope = "this function returns an `Option`"
    ),
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::option::Option<T>",
        ),
        // `Option`-in-`Option` always works, as there's only one possible
        // residual, so this can also be phrased strongly.
        message = "the `?` operator can only be used on `Option`s \
            in {ItemContext} that returns `Option`",
        label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
        enclosing_scope = "this function returns an `Option`"
    ),
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::ops::ControlFlow<B, C>",
            R = "std::ops::ControlFlow<B, C>",
        ),
        message = "the `?` operator in {ItemContext} that returns `ControlFlow<B, _>` \
            can only be used on other `ControlFlow<B, _>`s (with the same Break type)",
        label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
        enclosing_scope = "this function returns a `ControlFlow`",
        note = "unlike `Result`, there's no `From`-conversion performed for `ControlFlow`"
    ),
    on(
        all(
            from_desugaring = "QuestionMark",
            _Self = "std::ops::ControlFlow<B, C>",
            // `R` is not a `ControlFlow`, as that case was matched previously
        ),
        message = "the `?` operator can only be used on `ControlFlow`s \
            in {ItemContext} that returns `ControlFlow`",
        label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
        enclosing_scope = "this function returns a `ControlFlow`",
    ),
    on(
        all(from_desugaring = "QuestionMark"),
        message = "the `?` operator can only be used in {ItemContext} \
                    that returns `Result` or `Option` \
                    (or another type that implements `{FromResidual}`)",
        label = "cannot use the `?` operator in {ItemContext} that returns `{Self}`",
        enclosing_scope = "this function should return `Result` or `Option` to accept `?`"
    ),
)]
#[rustc_diagnostic_item = "FromResidual"]
#[unstable(feature = "try_trait_v2", issue = "84277")]
pub trait FromResidual<R = <Self as Try>::Residual> {
    /// Constructs the type from a compatible `Residual` type.
    ///
    /// This should be implemented consistently with the `branch` method such
    /// that applying the `?` operator will get back an equivalent residual:
    /// `FromResidual::from_residual(r).branch() --> ControlFlow::Break(r)`.
    /// (It must not be an *identical* residual when interconversion is involved.)
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_trait_v2)]
    /// use std::ops::{ControlFlow, FromResidual};
    ///
    /// assert_eq!(Result::<String, i64>::from_residual(Err(3_u8)), Err(3));
    /// assert_eq!(Option::<String>::from_residual(None), None);
    /// assert_eq!(
    ///     ControlFlow::<_, String>::from_residual(ControlFlow::Break(5)),
    ///     ControlFlow::Break(5),
    /// );
    /// ```
    #[lang = "from_residual"]
    #[unstable(feature = "try_trait_v2", issue = "84277")]
    fn from_residual(residual: R) -> Self;
}

#[unstable(
    feature = "yeet_desugar_details",
    issue = "none",
    reason = "just here to simplify the desugaring; will never be stabilized"
)]
#[inline]
#[track_caller] // because `Result::from_residual` has it
#[lang = "from_yeet"]
pub fn from_yeet<T, Y>(yeeted: Y) -> T
where
    T: FromResidual<Yeet<Y>>,
{
    FromResidual::from_residual(Yeet(yeeted))
}

/// Allows retrieving the canonical type implementing [`Try`] that has this type
/// as its residual and allows it to hold an `O` as its output.
///
/// If you think of the `Try` trait as splitting a type into its [`Try::Output`]
/// and [`Try::Residual`] components, this allows putting them back together.
///
/// For example,
/// `Result<T, E>: Try<Output = T, Residual = Result<Infallible, E>>`,
/// and in the other direction,
/// `<Result<Infallible, E> as Residual<T>>::TryType = Result<T, E>`.
#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
pub trait Residual<O> {
    /// The "return" type of this meta-function.
    #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
    type TryType: Try<Output = O, Residual = Self>;
}

#[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]
pub(crate) type ChangeOutputType<T, V> = <<T as Try>::Residual as Residual<V>>::TryType;

/// An adapter for implementing non-try methods via the `Try` implementation.
///
/// Conceptually the same as `Result<T, !>`, but requiring less work in trait
/// solving and inhabited-ness checking and such, by being an obvious newtype
/// and not having `From` bounds lying around.
///
/// Not currently planned to be exposed publicly, so just `pub(crate)`.
#[repr(transparent)]
pub(crate) struct NeverShortCircuit<T>(pub T);

impl<T> NeverShortCircuit<T> {
    /// Wrap a binary `FnMut` to return its result wrapped in a `NeverShortCircuit`.
    #[inline]
    pub fn wrap_mut_2<A, B>(mut f: impl FnMut(A, B) -> T) -> impl FnMut(A, B) -> Self {
        move |a, b| NeverShortCircuit(f(a, b))
    }
}

pub(crate) enum NeverShortCircuitResidual {}

impl<T> Try for NeverShortCircuit<T> {
    type Output = T;
    type Residual = NeverShortCircuitResidual;

    #[inline]
    fn branch(self) -> ControlFlow<NeverShortCircuitResidual, T> {
        ControlFlow::Continue(self.0)
    }

    #[inline]
    fn from_output(x: T) -> Self {
        NeverShortCircuit(x)
    }
}

impl<T> FromResidual for NeverShortCircuit<T> {
    #[inline]
    fn from_residual(never: NeverShortCircuitResidual) -> Self {
        match never {}
    }
}

impl<T> Residual<T> for NeverShortCircuitResidual {
    type TryType = NeverShortCircuit<T>;
}

/// Implement `FromResidual<Yeet<T>>` on your type to enable
/// `do yeet expr` syntax in functions returning your type.
#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
#[derive(Debug)]
pub struct Yeet<T>(pub T);