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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>>(
/// 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`]:
/// # use std::ops::{ControlFlow, Try};
/// fn simple_try_fold_2<A, T, R: Try<Output = A>>(
/// 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:
/// pub fn simple_try_fold_3<A, T, R: Try<Output = A>>(
/// ControlFlow::Break(r) => return R::from_residual(r),
/// 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:
/// fn simple_try_fold<A, T, R: Try<Output = A>>(
/// accum = f(accum, x)?;
#[unstable(feature = "try_trait_v2", issue = "84277", old_name = "try_trait")]
#[rustc_on_unimplemented(
on(
all(from_desugaring = "TryBlock"),
message = "a `try` block must return `Result` or `Option` \
(or another type that implements `{This}`)",
label = "could not wrap the final value of the block as `{Self}` doesn't implement `Try`",
),
all(from_desugaring = "QuestionMark"),
message = "the `?` operator can only be applied to values that implement `{This}`",
label = "the `?` operator cannot be applied to type `{Self}`"
)
)]
#[doc(alias = "?")]
#[lang = "Try"]
#[const_trait]
#[rustc_const_unstable(feature = "const_try", issue = "74935")]
pub trait Try: [const] FromResidual {
/// The type of the value produced by `?` when *not* short-circuiting.
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.
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"]
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`]).
/// 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));
/// ControlFlow::<_, String>::Break(3).branch(),
/// ControlFlow::Break(ControlFlow::Break(3)),
#[lang = "branch"]
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.
all(
from_desugaring = "QuestionMark",
Self = "core::result::Result<T, E>",
R = "core::option::Option<core::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}`",
parent_label = "this function returns a `Result`"
// 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 \
label = "this `?` produces `{R}`, which is incompatible with `{Self}`",
Self = "core::option::Option<T>",
R = "core::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",
parent_label = "this function returns an `Option`"
// `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 \
Self = "core::ops::control_flow::ControlFlow<B, C>",
R = "core::ops::control_flow::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)",
parent_label = "this function returns a `ControlFlow`",
note = "unlike `Result`, there's no `From`-conversion performed for `ControlFlow`"
// `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`",
message = "the `?` operator can only be used in {ItemContext} \
that returns `Result` or `Option` \
label = "cannot use the `?` operator in {ItemContext} that returns `{Self}`",
parent_label = "this function should return `Result` or `Option` to accept `?`"
#[rustc_diagnostic_item = "FromResidual"]
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)`.
/// (The residual is not mandated to be *identical* when interconversion is involved.)
/// 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);
/// ControlFlow::<_, String>::from_residual(ControlFlow::Break(5)),
/// ControlFlow::Break(5),
#[lang = "from_residual"]
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"]
#[allow(unreachable_pub)] // not-exposed but still used via lang-item
#[cfg(not(feature = "ferrocene_certified"))]
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.
type TryType: Try<Output = O, Residual = Self>;
#[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]
#[allow(type_alias_bounds)]
pub(crate) type ChangeOutputType<T: Try<Residual: Residual<V>>, V> =
<T::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> {
/// Wraps a unary function to produce one that wraps the output into a `NeverShortCircuit`.
/// This is useful for implementing infallible functions in terms of the `try_` ones,
/// without accidentally capturing extra generic parameters in a closure.
pub(crate) fn wrap_mut_1<A>(
mut f: impl FnMut(A) -> T,
) -> impl FnMut(A) -> NeverShortCircuit<T> {
move |a| NeverShortCircuit(f(a))
pub(crate) 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;
fn branch(self) -> ControlFlow<NeverShortCircuitResidual, T> {
ControlFlow::Continue(self.0)
fn from_output(x: T) -> Self {
NeverShortCircuit(x)
impl<T> FromResidual for NeverShortCircuit<T> {
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);