Rollup merge of rust-lang#85608 - scottmcm:stabilize-control-flow-enum-basics, r=m-ou-se

Stabilize `ops::ControlFlow` (just the type)

Tracking issue: rust-lang#75744 (which also tracks items *not* closed by this PR).

With the new `?` desugar implemented, [it's no longer possible to mix `Result` and `ControlFlow`](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=13feec97f5c96a9d791d97f7de2d49a6).  (At the time of making this PR, godbolt was still on the 2021-05-01 nightly, where you can see that [the mixing example compiled](https://rust.godbolt.org/z/13Ke54j16).)  That resolves the only blocker I know of, so I'd like to propose that `ControlFlow` be considered for stabilization.

Its basic existence was part of rust-lang/rfcs#3058, where it got a bunch of positive comments (examples [1](rust-lang/rfcs#3058 (comment)) [2](rust-lang/rfcs#3058 (review)) [3](rust-lang/rfcs#3058 (comment)) [4](rust-lang/rfcs#3058 (comment))).  Its use in the compiler has been well received (rust-lang#78182 (comment)), and there are ecosystem updates interested in using it (rust-itertools/itertools#469 (comment), jonhoo/rust-imap#194).

As this will need an FCP, picking a libs member manually:
r? `@m-ou-se`

## Stabilized APIs

```rust
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ControlFlow<B, C = ()> {
    /// Exit the operation without running subsequent phases.
    Break(B),
    /// Move on to the next phase of the operation as normal.
    Continue(C),
}
```

As well as using `?` on a `ControlFlow<B, _>` in a function returning `ControlFlow<B, _>`.  (Note, in particular, that there's no `From::from`-conversion on the `Break` value, the way there is for `Err`s.)

## Existing APIs *not* stabilized here

All the associated methods and constants: `break_value`, `is_continue`, `map_break`, [`CONTINUE`](https://doc.rust-lang.org/nightly/std/ops/enum.ControlFlow.html#associatedconstant.CONTINUE), etc.

Some of the existing methods in nightly seem reasonable, some seem like they should be removed, and some need more discussion to decide.  But none of them are *essential*, so [as in the RFC](https://rust-lang.github.io/rfcs/3058-try-trait-v2.html#methods-on-controlflow), they're all omitted from this PR.

They can be considered separately later, as further usage demonstrates which are important.

Author: JohnTitor

core/src/iter/traits/iterator.rs

@@ -1959,6 +1959,31 @@ pub trait Iterator {
     /// assert_eq!(it.len(), 2);
     /// assert_eq!(it.next(), Some(&40));
     /// ```
+    ///
+    /// While you cannot `break` from a closure, the [`crate::ops::ControlFlow`]
+    /// type allows a similar idea:
+    ///
+    /// ```
+    /// use std::ops::ControlFlow;
+    ///
+    /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
+    ///     if let Some(next) = prev.checked_add(x) {
+    ///         ControlFlow::Continue(next)
+    ///     } else {
+    ///         ControlFlow::Break(prev)
+    ///     }
+    /// });
+    /// assert_eq!(triangular, ControlFlow::Break(120));
+    ///
+    /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
+    ///     if let Some(next) = prev.checked_add(x) {
+    ///         ControlFlow::Continue(next)
+    ///     } else {
+    ///         ControlFlow::Break(prev)
+    ///     }
+    /// });
+    /// assert_eq!(triangular, ControlFlow::Continue(435));
+    /// ```
     #[inline]
     #[stable(feature = "iterator_try_fold", since = "1.27.0")]
     fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
@@ -2001,6 +2026,22 @@ pub trait Iterator {
     /// // It short-circuited, so the remaining items are still in the iterator:
     /// assert_eq!(it.next(), Some("stale_bread.json"));
     /// ```
+    ///
+    /// The [`crate::ops::ControlFlow`] type can be used with this method for the
+    /// situations in which you'd use `break` and `continue` in a normal loop:
+    ///
+    /// ```
+    /// use std::ops::ControlFlow;
+    ///
+    /// let r = (2..100).try_for_each(|x| {
+    ///     if 323 % x == 0 {
+    ///         return ControlFlow::Break(x)
+    ///     }
+    ///
+    ///     ControlFlow::Continue(())
+    /// });
+    /// assert_eq!(r, ControlFlow::Break(17));
+    /// ```
     #[inline]
     #[stable(feature = "iterator_try_fold", since = "1.27.0")]
     fn try_for_each<F, R>(&mut self, f: F) -> R

core/src/ops/control_flow.rs

@@ -11,7 +11,6 @@ use crate::{convert, ops};
 ///
 /// Early-exiting from [`Iterator::try_for_each`]:
 /// ```
-/// #![feature(control_flow_enum)]
 /// use std::ops::ControlFlow;
 ///
 /// let r = (2..100).try_for_each(|x| {
@@ -26,7 +25,6 @@ use crate::{convert, ops};
 ///
 /// A basic tree traversal:
 /// ```no_run
-/// #![feature(control_flow_enum)]
 /// use std::ops::ControlFlow;
 ///
 /// pub struct TreeNode<T> {
@@ -48,13 +46,15 @@ use crate::{convert, ops};
 ///     }
 /// }
 /// ```
-#[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
+#[stable(feature = "control_flow_enum_type", since = "1.55.0")]
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum ControlFlow<B, C = ()> {
     /// Move on to the next phase of the operation as normal.
+    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
     #[cfg_attr(not(bootstrap), lang = "Continue")]
     Continue(C),
     /// Exit the operation without running subsequent phases.
+    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
     #[cfg_attr(not(bootstrap), lang = "Break")]
     Break(B),
     // Yes, the order of the variants doesn't match the type parameters.

core/src/ops/try_trait.rs

@@ -55,7 +55,6 @@ use crate::ops::ControlFlow;
 /// into the return type using [`Try::from_output`]:
 /// ```
 /// # #![feature(try_trait_v2)]
-/// # #![feature(control_flow_enum)]
 /// # use std::ops::{ControlFlow, Try};
 /// fn simple_try_fold_2<A, T, R: Try<Output = A>>(
 ///     iter: impl Iterator<Item = T>,
@@ -79,7 +78,6 @@ use crate::ops::ControlFlow;
 /// recreated from their corresponding residual, so we'll just call it:
 /// ```
 /// # #![feature(try_trait_v2)]
-/// # #![feature(control_flow_enum)]
 /// # use std::ops::{ControlFlow, Try};
 /// pub fn simple_try_fold_3<A, T, R: Try<Output = A>>(
 ///     iter: impl Iterator<Item = T>,
@@ -170,7 +168,6 @@ pub trait Try: FromResidual {
     ///
     /// ```
     /// #![feature(try_trait_v2)]
-    /// #![feature(control_flow_enum)]
     /// use std::ops::Try;
     ///
     /// assert_eq!(<Result<_, String> as Try>::from_output(3), Ok(3));
@@ -202,7 +199,6 @@ pub trait Try: FromResidual {
     ///
     /// ```
     /// #![feature(try_trait_v2)]
-    /// #![feature(control_flow_enum)]
     /// use std::ops::{ControlFlow, Try};
     ///
     /// assert_eq!(Ok::<_, String>(3).branch(), ControlFlow::Continue(3));
@@ -329,7 +325,6 @@ pub trait FromResidual<R = <Self as Try>::Residual> {
     ///
     /// ```
     /// #![feature(try_trait_v2)]
-    /// #![feature(control_flow_enum)]
     /// use std::ops::{ControlFlow, FromResidual};
     ///
     /// assert_eq!(Result::<String, i64>::from_residual(Err(3_u8)), Err(3));

core/tests/lib.rs

@@ -15,7 +15,6 @@
 #![feature(const_ptr_read)]
 #![feature(const_ptr_write)]
 #![feature(const_ptr_offset)]
-#![feature(control_flow_enum)]
 #![feature(core_intrinsics)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]