Stabilize #![feature(label_break_value)]

 # Stabilization proposal

The feature was implemented in #50045 by est31 and has been in nightly since 2018-05-16 (over 4 years now).
There are [no open issues][issue-label] other than the tracking issue. There is a strong consensus that `break` is the right keyword and we should not use `return`.

There have been several concerns raised about this feature on the tracking issue (other than the one about tests, which has been fixed, and an interaction with try blocks, which has been fixed).
1. nrc's original comment about cost-benefit analysis: #48594 (comment)
2. joshtriplett's comments about seeing use cases: #48594 (comment)
3. withoutboats's comments that Rust does not need more control flow constructs: #48594 (comment)

Many different examples of code that's simpler using this feature have been provided:
- A lexer by rpjohnst which must repeat code without label-break-value: #48594 (comment)
- A snippet by SergioBenitez which avoids using a new function and adding several new return points to a function: #48594 (comment). This particular case would also work if `try` blocks were stabilized (at the cost of making the code harder to optimize).
- Several examples by JohnBSmith: #48594 (comment)
- Several examples by Centril: #48594 (comment)
- An example by petrochenkov where this is used in the compiler itself to avoid duplicating error checking code: #48594 (comment)
- Amanieu recently provided another example related to complex conditions, where try blocks would not have helped: #48594 (comment)

Additionally, petrochenkov notes that this is strictly more powerful than labelled loops due to macros which accidentally exit a loop instead of being consumed by the macro matchers: #48594 (comment)

nrc later resolved their concern, mostly because of the aforementioned macro problems.
joshtriplett suggested that macros could be able to generate IR directly
(#48594 (comment)) but there are no open RFCs,
and the design space seems rather speculative.

joshtriplett later resolved his concerns, due to a symmetry between this feature and existing labelled break: #48594 (comment)

withoutboats has regrettably left the language team.

joshtriplett later posted that the lang team would consider starting an FCP given a stabilization report: #48594 (comment)

[issue-label]: https://github.com/rust-lang/rust/issues?q=is%3Aissue+is%3Aopen+label%3AF-label_break_value+

 ## Report

+ Feature gate:
    - https://github.com/rust-lang/rust/blob/d695a497bbf4b20d2580b75075faa80230d41667/src/test/ui/feature-gates/feature-gate-label_break_value.rs
+ Diagnostics:
    - https://github.com/rust-lang/rust/blob/6b2d3d5f3cd1e553d87b5496632132565b6779d3/compiler/rustc_parse/src/parser/diagnostics.rs#L2629
    - https://github.com/rust-lang/rust/blob/f65bf0b2bb1a99f73095c01a118f3c37d3ee614c/compiler/rustc_resolve/src/diagnostics.rs#L749
    - https://github.com/rust-lang/rust/blob/f65bf0b2bb1a99f73095c01a118f3c37d3ee614c/compiler/rustc_resolve/src/diagnostics.rs#L1001
    - https://github.com/rust-lang/rust/blob/111df9e6eda1d752233482c1309d00d20a4bbf98/compiler/rustc_passes/src/loops.rs#L254
    - https://github.com/rust-lang/rust/blob/d695a497bbf4b20d2580b75075faa80230d41667/compiler/rustc_parse/src/parser/expr.rs#L2079
    - https://github.com/rust-lang/rust/blob/d695a497bbf4b20d2580b75075faa80230d41667/compiler/rustc_parse/src/parser/expr.rs#L1569
+ Tests:
    - https://github.com/rust-lang/rust/blob/master/src/test/ui/label/label_break_value_continue.rs
    - https://github.com/rust-lang/rust/blob/master/src/test/ui/label/label_break_value_unlabeled_break.rs
    - https://github.com/rust-lang/rust/blob/master/src/test/ui/label/label_break_value_illegal_uses.rs
    - https://github.com/rust-lang/rust/blob/master/src/test/ui/lint/unused_labels.rs
    - https://github.com/rust-lang/rust/blob/master/src/test/ui/run-pass/for-loop-while/label_break_value.rs

 ## Interactions with other features

Labels follow the hygiene of local variables.

label-break-value is permitted within `try` blocks:
```rust
let _: Result<(), ()> = try {
    'foo: {
        Err(())?;
        break 'foo;
    }
};
```

label-break-value is disallowed within closures, generators, and async blocks:
```rust
'a: {
    || break 'a
    //~^ ERROR use of unreachable label `'a`
    //~| ERROR `break` inside of a closure
}
```

label-break-value is disallowed on [_BlockExpression_]; it can only occur as a [_LoopExpression_]:
```rust
fn labeled_match() {
    match false 'b: { //~ ERROR block label not supported here
        _ => {}
    }
}

macro_rules! m {
    ($b:block) => {
        'lab: $b; //~ ERROR cannot use a `block` macro fragment here
        unsafe $b; //~ ERROR cannot use a `block` macro fragment here
        |x: u8| -> () $b; //~ ERROR cannot use a `block` macro fragment here
    }
}

fn foo() {
    m!({});
}
```

[_BlockExpression_]: https://doc.rust-lang.org/nightly/reference/expressions/block-expr.html
[_LoopExpression_]: https://doc.rust-lang.org/nightly/reference/expressions/loop-expr.html

Author: jyn514

compiler/rustc_ast/src/lib.rs

@@ -13,7 +13,7 @@
 #![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 #![feature(if_let_guard)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(slice_internals)]

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -647,14 +647,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::ExprKind::TryBlock(_) => {
                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
             }
-            ast::ExprKind::Block(_, Some(label)) => {
-                gate_feature_post!(
-                    &self,
-                    label_break_value,
-                    label.ident.span,
-                    "labels on blocks are unstable"
-                );
-            }
             _ => {}
         }
         visit::walk_expr(self, e)
@@ -823,7 +815,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(box_patterns, "box pattern syntax is experimental");
     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
     gate_all!(try_blocks, "`try` blocks are unstable");
-    gate_all!(label_break_value, "labels on blocks are unstable");
     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
     gate_all!(type_ascription, "type ascription is experimental");
 

compiler/rustc_error_codes/src/error_codes/E0695.md

@@ -3,7 +3,6 @@ A `break` statement without a label appeared inside a labeled block.
 Erroneous code example:
 
 ```compile_fail,E0695
-# #![feature(label_break_value)]
 loop {
     'a: {
         break;
@@ -14,7 +13,6 @@ loop {
 Make sure to always label the `break`:
 
 ```
-# #![feature(label_break_value)]
 'l: loop {
     'a: {
         break 'l;
@@ -25,7 +23,6 @@ Make sure to always label the `break`:
 Or if you want to `break` the labeled block:
 
 ```
-# #![feature(label_break_value)]
 loop {
     'a: {
         break 'a;

compiler/rustc_feature/src/accepted.rs

@@ -186,6 +186,8 @@ declare_features! (
     /// Allows some increased flexibility in the name resolution rules,
     /// especially around globs and shadowing (RFC 1560).
     (accepted, item_like_imports, "1.15.0", Some(35120), None),
+    /// Allows `'a: { break 'a; }`.
+    (accepted, label_break_value, "1.65.0", Some(48594), None),
     /// Allows `if/while p && let q = r && ...` chains.
     (accepted, let_chains, "1.64.0", Some(53667), None),
     /// Allows `break {expr}` with a value inside `loop`s.

compiler/rustc_feature/src/active.rs

@@ -420,8 +420,6 @@ declare_features! (
     (active, intra_doc_pointers, "1.51.0", Some(80896), None),
     /// Allows `#[instruction_set(_)]` attribute
     (active, isa_attribute, "1.48.0", Some(74727), None),
-    /// Allows `'a: { break 'a; }`.
-    (active, label_break_value, "1.28.0", Some(48594), None),
     // Allows setting the threshold for the `large_assignments` lint.
     (active, large_assignments, "1.52.0", Some(83518), None),
     /// Allows `let...else` statements.

compiler/rustc_infer/src/lib.rs

@@ -17,7 +17,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_parse/src/parser/expr.rs

@@ -2075,10 +2075,6 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if let Some(label) = opt_label {
-            self.sess.gated_spans.gate(sym::label_break_value, label.ident.span);
-        }
-
         if self.token.is_whole_block() {
             self.struct_span_err(self.token.span, "cannot use a `block` macro fragment here")
                 .span_label(lo.to(self.token.span), "the `block` fragment is within this context")

compiler/rustc_trait_selection/src/lib.rs

@@ -16,7 +16,7 @@
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
 #![feature(hash_drain_filter)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_else)]
 #![feature(if_let_guard)]
 #![feature(never_type)]

compiler/rustc_typeck/src/lib.rs

@@ -64,7 +64,7 @@ This API is completely unstable and subject to change.
 #![feature(if_let_guard)]
 #![feature(is_sorted)]
 #![feature(iter_intersperse)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

library/std/src/lib.rs

@@ -252,7 +252,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exhaustive_patterns)]
 #![feature(intra_doc_pointers)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
 #![feature(let_else)]
 #![feature(linkage)]