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

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

Tracking issue: #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 (#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.
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JohnTitor authored Jun 15, 2021
2 parents 1e14d39 + 590d452 commit 5936ecc
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2 changes: 1 addition & 1 deletion compiler/rustc_ast_lowering/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -331,7 +331,7 @@ pub fn lower_crate<'a, 'hir>(
lifetimes_to_define: Vec::new(),
is_collecting_in_band_lifetimes: false,
in_scope_lifetimes: Vec::new(),
allow_try_trait: Some([sym::control_flow_enum, sym::try_trait_v2][..].into()),
allow_try_trait: Some([sym::try_trait_v2][..].into()),
allow_gen_future: Some([sym::gen_future][..].into()),
}
.lower_crate(krate)
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1 change: 0 additions & 1 deletion compiler/rustc_resolve/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@
#![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
#![feature(box_patterns)]
#![feature(bool_to_option)]
#![feature(control_flow_enum)]
#![feature(crate_visibility_modifier)]
#![feature(format_args_capture)]
#![feature(iter_zip)]
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1 change: 0 additions & 1 deletion compiler/rustc_span/src/symbol.rs
Original file line number Diff line number Diff line change
Expand Up @@ -416,7 +416,6 @@ symbols! {
constructor,
contents,
context,
control_flow_enum,
convert,
copy,
copy_closures,
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1 change: 0 additions & 1 deletion compiler/rustc_traits/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,6 @@
#![feature(crate_visibility_modifier)]
#![feature(in_band_lifetimes)]
#![feature(nll)]
#![feature(control_flow_enum)]
#![recursion_limit = "256"]

#[macro_use]
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41 changes: 41 additions & 0 deletions library/core/src/iter/traits/iterator.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -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
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6 changes: 3 additions & 3 deletions library/core/src/ops/control_flow.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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| {
Expand All @@ -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> {
Expand All @@ -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.
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5 changes: 0 additions & 5 deletions library/core/src/ops/try_trait.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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>,
Expand All @@ -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>,
Expand Down Expand Up @@ -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));
Expand Down Expand Up @@ -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));
Expand Down Expand Up @@ -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));
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1 change: 0 additions & 1 deletion library/core/tests/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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)]
Expand Down

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