Implement Try for ()

rust-lang · Sep 25, 2023 · 66623f6 · 66623f6
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commit 66623f6
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diff --git a/library/core/src/unit.rs b/library/core/src/unit.rs
@@ -1,4 +1,6 @@
+use crate::convert::Infallible;
 use crate::iter::FromIterator;
+use crate::ops::{ControlFlow, FromResidual, Try};
 
 /// Collapses all unit items from an iterator into one.
 ///
@@ -19,3 +21,88 @@ impl FromIterator<()> for () {
         iter.into_iter().for_each(|()| {})
     }
 }
+
+/// Allows the unit type to be used with the question mark operator.
+///
+/// This allows more easily writing internal iteration functions which can take
+/// both fallible and infallible closures, or visitors which can have both
+/// fallible and infallible implementations.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(try_trait_v2)]
+///
+/// fn foreach<I, R>(iter: I, mut f: impl FnMut(I::Item) -> R) -> R
+/// where
+///     I: IntoIterator,
+///     R: std::ops::Try<Output = ()>,
+/// {
+///     for x in iter {
+///         f(x)?;
+///     }
+///     R::from_output(())
+/// }
+///
+/// // prints everything
+/// foreach([1, 2, 3], |x| println!("{x}"));
+///
+/// // prints everything until an error occurs
+/// let _: Result<_, ()> = foreach([Ok(1), Ok(2), Err(()), Ok(3)], |x| {
+///     println!("{}", x?);
+///     Ok(())
+/// });
+/// ```
+///
+/// ```
+/// #![feature(try_trait_v2)]
+///
+/// fn walk_children<V: Visitor>(visitor: &mut V, item: ()) -> V::ResultTy {
+///     // can use the `?` operator if needed here.
+///     <<V as Visitor>::ResultTy as std::ops::Try>::from_output(())
+/// }
+///
+/// trait Visitor: Sized {
+///     type ResultTy: std::ops::Try<Output = ()>;
+///
+///     fn visit_x(&mut self, item: ()) -> Self::ResultTy {
+///         // can use the `?` operator if needed here.
+///         walk_children(self, item)
+///     }
+///     // some `visit_*` functions
+/// }
+///
+/// struct InfallibleVisitor;
+///
+/// impl Visitor for InfallibleVisitor {
+///     type ResultTy = ();
+///     // implement `visit_*` functions
+/// }
+///
+/// struct FallibleVisitor;
+///
+/// impl Visitor for FallibleVisitor {
+///     type ResultTy = std::ops::ControlFlow<()>;
+///     // implement `visit_*` functions
+/// }
+/// ```
+#[unstable(feature = "try_trait_v2", issue = "84277")]
+impl Try for () {
+    type Output = ();
+    type Residual = Infallible;
+
+    fn from_output(_: ()) -> Self {
+        ()
+    }
+
+    fn branch(self) -> ControlFlow<Infallible, ()> {
+        ControlFlow::Continue(())
+    }
+}
+
+#[unstable(feature = "try_trait_v2", issue = "84277")]
+impl FromResidual<Infallible> for () {
+    fn from_residual(_: Infallible) -> Self {
+        ()
+    }
+}
diff --git a/tests/ui/async-await/issue-84841.rs b/tests/ui/async-await/issue-84841.rs
@@ -8,7 +8,6 @@ async fn foo() {
     // Adding an .await here avoids the ICE
     test()?;
     //~^ ERROR the `?` operator can only be applied to values that implement `Try`
-    //~| ERROR the `?` operator can only be used in an async function that returns
 }
 
 // Removing the const generic parameter here avoids the ICE

diff --git a/tests/ui/async-await/issue-84841.stderr b/tests/ui/async-await/issue-84841.stderr
@@ -5,22 +5,11 @@ LL |     test()?;
    |     ^^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = ()>`
    |
    = help: the trait `Try` is not implemented for `impl Future<Output = ()>`
-
-error[E0277]: the `?` operator can only be used in an async function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/issue-84841.rs:9:11
-   |
-LL |   async fn foo() {
-   |  ________________-
-LL | |     // Adding an .await here avoids the ICE
-LL | |     test()?;
-   | |           ^ cannot use the `?` operator in an async function that returns `()`
-LL | |
-LL | |
-LL | | }
-   | |_- this function should return `Result` or `Option` to accept `?`
+help: consider `await`ing on the `Future`
    |
-   = help: the trait `FromResidual<_>` is not implemented for `()`
+LL |     test().await?;
+   |           ++++++
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/parser/ternary_operator.rs b/tests/ui/parser/ternary_operator.rs
@@ -1,69 +1,49 @@
 // A good chunk of these errors aren't shown to the user, but are still
 // required in the test for it to pass.
 
-fn a() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+fn a() {
     let x = 5 > 2 ? true : false;
     //~^ ERROR Rust has no ternary operator
     //~| HELP use an `if-else` expression instead
     //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
     //~| HELP the trait `Try` is not implemented for `{integer}`
-    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
-    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
     //~| NOTE in this expansion of desugaring of operator `?`
     //~| NOTE the `?` operator cannot be applied to type `{integer}`
     //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE cannot use the `?` operator in a function that returns `()`
-    //~| NOTE in this expansion of desugaring of operator `?`
 }
 
-fn b() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+fn b() {
     let x = 5 > 2 ? { true } : { false };
     //~^ ERROR Rust has no ternary operator
     //~| HELP use an `if-else` expression instead
     //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
     //~| HELP the trait `Try` is not implemented for `{integer}`
-    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
-    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
     //~| NOTE in this expansion of desugaring of operator `?`
     //~| NOTE the `?` operator cannot be applied to type `{integer}`
     //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE cannot use the `?` operator in a function that returns `()`
-    //~| NOTE in this expansion of desugaring of operator `?`
 }
 
-fn c() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+fn c() {
     let x = 5 > 2 ? f32::MAX : f32::MIN;
     //~^ ERROR Rust has no ternary operator
     //~| HELP use an `if-else` expression instead
     //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
     //~| HELP the trait `Try` is not implemented for `{integer}`
-    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
-    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
     //~| NOTE in this expansion of desugaring of operator `?`
     //~| NOTE the `?` operator cannot be applied to type `{integer}`
     //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE cannot use the `?` operator in a function that returns `()`
-    //~| NOTE in this expansion of desugaring of operator `?`
 }
 
-fn main() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+fn main() {
     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
     //~^ ERROR Rust has no ternary operator
     //~| HELP use an `if-else` expression instead
     //~| ERROR expected one of `.`, `;`, `?`, `else`, or an operator, found `:`
     //~| NOTE expected one of `.`, `;`, `?`, `else`, or an operator
     //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
     //~| HELP the trait `Try` is not implemented for `{integer}`
-    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
-    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
     //~| NOTE type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
     //~| NOTE in this expansion of desugaring of operator `?`
     //~| NOTE the `?` operator cannot be applied to type `{integer}`
     //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE in this expansion of desugaring of operator `?`
-    //~| NOTE cannot use the `?` operator in a function that returns `()`
-    //~| NOTE in this expansion of desugaring of operator `?`
 }
diff --git a/tests/ui/parser/ternary_operator.stderr b/tests/ui/parser/ternary_operator.stderr
@@ -7,31 +7,31 @@ LL |     let x = 5 > 2 ? true : false;
    = help: use an `if-else` expression instead
 
 error: Rust has no ternary operator
-  --> $DIR/ternary_operator.rs:21:19
+  --> $DIR/ternary_operator.rs:16:19
    |
 LL |     let x = 5 > 2 ? { true } : { false };
    |                   ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: use an `if-else` expression instead
 
 error: Rust has no ternary operator
-  --> $DIR/ternary_operator.rs:37:19
+  --> $DIR/ternary_operator.rs:27:19
    |
 LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
    |                   ^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: use an `if-else` expression instead
 
 error: expected one of `.`, `;`, `?`, `else`, or an operator, found `:`
-  --> $DIR/ternary_operator.rs:53:37
+  --> $DIR/ternary_operator.rs:38:37
    |
 LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
    |                                     ^ expected one of `.`, `;`, `?`, `else`, or an operator
    |
    = note: type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
 
 error: Rust has no ternary operator
-  --> $DIR/ternary_operator.rs:53:19
+  --> $DIR/ternary_operator.rs:38:19
    |
 LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -46,70 +46,30 @@ LL |     let x = 5 > 2 ? true : false;
    |
    = help: the trait `Try` is not implemented for `{integer}`
 
-error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/ternary_operator.rs:5:19
-   |
-LL | fn a() {
-   | ------ this function should return `Result` or `Option` to accept `?`
-LL |     let x = 5 > 2 ? true : false;
-   |                   ^ cannot use the `?` operator in a function that returns `()`
-   |
-   = help: the trait `FromResidual<_>` is not implemented for `()`
-
 error[E0277]: the `?` operator can only be applied to values that implement `Try`
-  --> $DIR/ternary_operator.rs:21:17
+  --> $DIR/ternary_operator.rs:16:17
    |
 LL |     let x = 5 > 2 ? { true } : { false };
    |                 ^^^ the `?` operator cannot be applied to type `{integer}`
    |
    = help: the trait `Try` is not implemented for `{integer}`
 
-error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/ternary_operator.rs:21:19
-   |
-LL | fn b() {
-   | ------ this function should return `Result` or `Option` to accept `?`
-LL |     let x = 5 > 2 ? { true } : { false };
-   |                   ^ cannot use the `?` operator in a function that returns `()`
-   |
-   = help: the trait `FromResidual<_>` is not implemented for `()`
-
 error[E0277]: the `?` operator can only be applied to values that implement `Try`
-  --> $DIR/ternary_operator.rs:37:17
+  --> $DIR/ternary_operator.rs:27:17
    |
 LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
    |                 ^^^ the `?` operator cannot be applied to type `{integer}`
    |
    = help: the trait `Try` is not implemented for `{integer}`
 
-error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/ternary_operator.rs:37:19
-   |
-LL | fn c() {
-   | ------ this function should return `Result` or `Option` to accept `?`
-LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
-   |                   ^ cannot use the `?` operator in a function that returns `()`
-   |
-   = help: the trait `FromResidual<_>` is not implemented for `()`
-
 error[E0277]: the `?` operator can only be applied to values that implement `Try`
-  --> $DIR/ternary_operator.rs:53:17
+  --> $DIR/ternary_operator.rs:38:17
    |
 LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
    |                 ^^^ the `?` operator cannot be applied to type `{integer}`
    |
    = help: the trait `Try` is not implemented for `{integer}`
 
-error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/ternary_operator.rs:53:19
-   |
-LL | fn main() {
-   | --------- this function should return `Result` or `Option` to accept `?`
-LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
-   |                   ^ cannot use the `?` operator in a function that returns `()`
-   |
-   = help: the trait `FromResidual<_>` is not implemented for `()`
-
-error: aborting due to 13 previous errors
+error: aborting due to 9 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/try-block/try-block-bad-type.rs b/tests/ui/try-block/try-block-bad-type.rs
@@ -14,8 +14,5 @@ pub fn main() {
 
     let res: Result<i32, i32> = try { }; //~ ERROR type mismatch
 
-    let res: () = try { };
-    //~^ ERROR a `try` block must return `Result` or `Option`
-
     let res: i32 = try { 5 }; //~ ERROR a `try` block must return `Result` or `Option`
 }
diff --git a/tests/ui/try-block/try-block-bad-type.stderr b/tests/ui/try-block/try-block-bad-type.stderr
@@ -21,22 +21,14 @@ LL |     let res: Result<i32, i32> = try { };
    |                                       ^ expected `i32`, found `()`
 
 error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
-  --> $DIR/try-block-bad-type.rs:17:25
-   |
-LL |     let res: () = try { };
-   |                         ^ could not wrap the final value of the block as `()` doesn't implement `Try`
-   |
-   = help: the trait `Try` is not implemented for `()`
-
-error[E0277]: a `try` block must return `Result` or `Option` (or another type that implements `Try`)
-  --> $DIR/try-block-bad-type.rs:20:26
+  --> $DIR/try-block-bad-type.rs:17:26
    |
 LL |     let res: i32 = try { 5 };
    |                          ^ could not wrap the final value of the block as `i32` doesn't implement `Try`
    |
    = help: the trait `Try` is not implemented for `i32`
 
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0271, E0277.
 For more information about an error, try `rustc --explain E0271`.
diff --git a/tests/ui/try-trait/try-on-option-diagnostics.stderr b/tests/ui/try-trait/try-on-option-diagnostics.stderr
@@ -30,6 +30,7 @@ LL |             x?;
    |              ^ cannot use the `?` operator in a method that returns `()`
    |
    = help: the trait `FromResidual<Option<Infallible>>` is not implemented for `()`
+   = help: the trait `FromResidual<Infallible>` is implemented for `()`
 
 error[E0277]: the `?` operator can only be used in a trait method that returns `Result` or `Option` (or another type that implements `FromResidual`)
   --> $DIR/try-on-option-diagnostics.rs:39:14
@@ -41,6 +42,7 @@ LL |             x?;
    |              ^ cannot use the `?` operator in a trait method that returns `()`
    |
    = help: the trait `FromResidual<Option<Infallible>>` is not implemented for `()`
+   = help: the trait `FromResidual<Infallible>` is implemented for `()`
 
 error: aborting due to 4 previous errors
 

diff --git a/tests/ui/try-trait/try-operator-on-main.rs b/tests/ui/try-trait/try-operator-on-main.rs
@@ -7,16 +7,15 @@ fn main() {
     std::fs::File::open("foo")?; //~ ERROR the `?` operator can only
 
     // a non-`Try` type on a non-`Try` fn
-    ()?; //~ ERROR the `?` operator can only be applied to
-    //~^ ERROR the `?` operator can only be used in a function that
+    0i32?; //~ ERROR the `?` operator can only be applied to
 
     // an unrelated use of `Try`
-    try_trait_generic::<()>(); //~ ERROR the trait bound
+    try_trait_generic::<i32>(); //~ ERROR the trait bound
 }
 
 fn try_trait_generic<T: Try>() -> T {
     // and a non-`Try` object on a `Try` fn.
-    ()?; //~ ERROR the `?` operator can only be applied to values that implement `Try`
+    0i32?; //~ ERROR the `?` operator can only be applied to values that implement `Try`
 
     loop {}
 }