library/core/src/ops/try_trait.rs - toolchain/rustc - Git at Google

 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_method = "from_output", 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_method = "branch", 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_method = "from_residual",
             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_method = "from_residual",
             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_method = "from_residual",
             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_method = "from_residual",
             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_method = "from_residual",
             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_method = "from_residual",
             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_method = "from_residual",
             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 `?`"
     ),
 )]
 #[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 may 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;
 }
	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_method = "from_output", 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_method = "branch", 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_method = "from_residual",
	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_method = "from_residual",
	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_method = "from_residual",
	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_method = "from_residual",
	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_method = "from_residual",
	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_method = "from_residual",
	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_method = "from_residual",
	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 `?`"
	),
	)]
	#[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 may 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;
	}