Stabilize `#![feature(label_break_value)]`

 # Stabilization proposal

The feature was implemented in https://github.com/rust-lang/rust/pull/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: https://github.com/rust-lang/rust/issues/48594#issuecomment-422235234
2. joshtriplett's comments about seeing use cases: https://github.com/rust-lang/rust/issues/48594#issuecomment-422281176
3. withoutboats's comments that Rust does not need more control flow constructs: https://github.com/rust-lang/rust/issues/48594#issuecomment-450050630

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: https://github.com/rust-lang/rust/issues/48594#issuecomment-422502014
- A snippet by SergioBenitez which avoids using a new function and adding several new return points to a function: https://github.com/rust-lang/rust/issues/48594#issuecomment-427628251. 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: https://github.com/rust-lang/rust/issues/48594#issuecomment-434651395
- Several examples by Centril: https://github.com/rust-lang/rust/issues/48594#issuecomment-440154733
- An example by petrochenkov where this is used in the compiler itself to avoid duplicating error checking code: https://github.com/rust-lang/rust/issues/48594#issuecomment-443557569
- Amanieu recently provided another example related to complex conditions, where try blocks would not have helped: https://github.com/rust-lang/rust/issues/48594#issuecomment-1184213006

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: https://github.com/rust-lang/rust/issues/48594#issuecomment-450246249

nrc later resolved their concern, mostly because of the aforementioned macro problems.
joshtriplett suggested that macros could be able to generate IR directly
(https://github.com/rust-lang/rust/issues/48594#issuecomment-451685983) 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: https://github.com/rust-lang/rust/issues/48594#issuecomment-632960804

withoutboats has regrettably left the language team.

joshtriplett later posted that the lang team would consider starting an FCP given a stabilization report: https://github.com/rust-lang/rust/issues/48594#issuecomment-1111269353

[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

7 files changed, 17 insertions, 23 deletions

diff --git a/src/test/ui/parser/bad-interpolated-block.rs b/src/test/ui/parser/bad-interpolated-block.rs
index 38d53a14bc0..c6d7ae383b2 100644
--- a/src/test/ui/parser/bad-interpolated-block.rs
+++ b/src/test/ui/parser/bad-interpolated-block.rs
@@ -1,5 +1,3 @@
-#![feature(label_break_value)]
-
 fn main() {}
 
 macro_rules! m {
diff --git a/src/test/ui/parser/bad-interpolated-block.stderr b/src/test/ui/parser/bad-interpolated-block.stderr
index 77933b1bcec..2a0999afdfa 100644
--- a/src/test/ui/parser/bad-interpolated-block.stderr
+++ b/src/test/ui/parser/bad-interpolated-block.stderr
@@ -1,5 +1,5 @@
 error: cannot use a `block` macro fragment here
-  --> $DIR/bad-interpolated-block.rs:7:15
+  --> $DIR/bad-interpolated-block.rs:5:15
    |
 LL |         'lab: $b;
    |         ------^^
@@ -12,7 +12,7 @@ LL |     m!({});
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: cannot use a `block` macro fragment here
-  --> $DIR/bad-interpolated-block.rs:8:16
+  --> $DIR/bad-interpolated-block.rs:6:16
    |
 LL |         unsafe $b;
    |         -------^^
@@ -25,7 +25,7 @@ LL |     m!({});
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: cannot use a `block` macro fragment here
-  --> $DIR/bad-interpolated-block.rs:9:23
+  --> $DIR/bad-interpolated-block.rs:7:23
    |
 LL |         |x: u8| -> () $b;
    |                       ^^ the `block` fragment is within this context
diff --git a/src/test/ui/parser/labeled-no-colon-expr.rs b/src/test/ui/parser/labeled-no-colon-expr.rs
index db9ef52c1ae..d9ebd7473bc 100644
--- a/src/test/ui/parser/labeled-no-colon-expr.rs
+++ b/src/test/ui/parser/labeled-no-colon-expr.rs
@@ -1,5 +1,3 @@
-#![feature(label_break_value)]
-
 fn main() {
     'l0 while false {} //~ ERROR labeled expression must be followed by `:`
     'l1 for _ in 0..1 {} //~ ERROR labeled expression must be followed by `:`
diff --git a/src/test/ui/parser/labeled-no-colon-expr.stderr b/src/test/ui/parser/labeled-no-colon-expr.stderr
index a258bd3ccde..62288fe152d 100644
--- a/src/test/ui/parser/labeled-no-colon-expr.stderr
+++ b/src/test/ui/parser/labeled-no-colon-expr.stderr
@@ -1,5 +1,5 @@
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:4:5
+  --> $DIR/labeled-no-colon-expr.rs:2:5
    |
 LL |     'l0 while false {}
    |     ----^^^^^^^^^^^^^^
@@ -10,7 +10,7 @@ LL |     'l0 while false {}
    = note: labels are used before loops and blocks, allowing e.g., `break 'label` to them
 
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:5:5
+  --> $DIR/labeled-no-colon-expr.rs:3:5
    |
 LL |     'l1 for _ in 0..1 {}
    |     ----^^^^^^^^^^^^^^^^
@@ -21,7 +21,7 @@ LL |     'l1 for _ in 0..1 {}
    = note: labels are used before loops and blocks, allowing e.g., `break 'label` to them
 
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:6:5
+  --> $DIR/labeled-no-colon-expr.rs:4:5
    |
 LL |     'l2 loop {}
    |     ----^^^^^^^
@@ -32,7 +32,7 @@ LL |     'l2 loop {}
    = note: labels are used before loops and blocks, allowing e.g., `break 'label` to them
 
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:7:5
+  --> $DIR/labeled-no-colon-expr.rs:5:5
    |
 LL |     'l3 {}
    |     ----^^
@@ -43,7 +43,7 @@ LL |     'l3 {}
    = note: labels are used before loops and blocks, allowing e.g., `break 'label` to them
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/labeled-no-colon-expr.rs:8:9
+  --> $DIR/labeled-no-colon-expr.rs:6:9
    |
 LL |     'l4 0;
    |         ^ expected `while`, `for`, `loop` or `{` after a label
@@ -55,7 +55,7 @@ LL +     0;
    |
 
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:8:9
+  --> $DIR/labeled-no-colon-expr.rs:6:9
    |
 LL |     'l4 0;
    |     ----^
@@ -66,7 +66,7 @@ LL |     'l4 0;
    = note: labels are used before loops and blocks, allowing e.g., `break 'label` to them
 
 error: cannot use a `block` macro fragment here
-  --> $DIR/labeled-no-colon-expr.rs:13:17
+  --> $DIR/labeled-no-colon-expr.rs:11:17
    |
 LL |             'l5 $b;
    |             ----^^
@@ -79,7 +79,7 @@ LL |     m!({});
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: labeled expression must be followed by `:`
-  --> $DIR/labeled-no-colon-expr.rs:16:8
+  --> $DIR/labeled-no-colon-expr.rs:14:8
    |
 LL |             'l5 $b;
    |             ---- help: add `:` after the label
diff --git a/src/test/ui/parser/recover-labeled-non-block-expr.fixed b/src/test/ui/parser/recover-labeled-non-block-expr.fixed
index fe546a71971..c2e76444d11 100644
--- a/src/test/ui/parser/recover-labeled-non-block-expr.fixed
+++ b/src/test/ui/parser/recover-labeled-non-block-expr.fixed
@@ -1,5 +1,4 @@
 // run-rustfix
-#![feature(label_break_value)]
 fn main() {
     let _ = 1 + 1; //~ ERROR expected `while`, `for`, `loop` or `{` after a label
 
diff --git a/src/test/ui/parser/recover-labeled-non-block-expr.rs b/src/test/ui/parser/recover-labeled-non-block-expr.rs
index 35862e2eef9..fc11c646a8c 100644
--- a/src/test/ui/parser/recover-labeled-non-block-expr.rs
+++ b/src/test/ui/parser/recover-labeled-non-block-expr.rs
@@ -1,5 +1,4 @@
 // run-rustfix
-#![feature(label_break_value)]
 fn main() {
     let _ = 'label: 1 + 1; //~ ERROR expected `while`, `for`, `loop` or `{` after a label
 
diff --git a/src/test/ui/parser/recover-labeled-non-block-expr.stderr b/src/test/ui/parser/recover-labeled-non-block-expr.stderr
index 04fc1203e90..d66ce695090 100644
--- a/src/test/ui/parser/recover-labeled-non-block-expr.stderr
+++ b/src/test/ui/parser/recover-labeled-non-block-expr.stderr
@@ -1,5 +1,5 @@
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:4:21
+  --> $DIR/recover-labeled-non-block-expr.rs:3:21
    |
 LL |     let _ = 'label: 1 + 1;
    |                     ^ expected `while`, `for`, `loop` or `{` after a label
@@ -11,7 +11,7 @@ LL +     let _ = 1 + 1;
    |
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:6:13
+  --> $DIR/recover-labeled-non-block-expr.rs:5:13
    |
 LL |     'label: match () { () => {}, };
    |             ^^^^^ expected `while`, `for`, `loop` or `{` after a label
@@ -23,7 +23,7 @@ LL +     match () { () => {}, };
    |
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:7:13
+  --> $DIR/recover-labeled-non-block-expr.rs:6:13
    |
 LL |     'label: match () { () => break 'label, };
    |             ^^^^^ expected `while`, `for`, `loop` or `{` after a label
@@ -34,7 +34,7 @@ LL |     'label: { match () { () => break 'label, } };
    |             +                                  +
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:9:13
+  --> $DIR/recover-labeled-non-block-expr.rs:8:13
    |
 LL |     'label: match () { () => 'lp: loop { break 'lp 0 }, };
    |             ^^^^^ expected `while`, `for`, `loop` or `{` after a label
@@ -45,7 +45,7 @@ LL |     'label: { match () { () => 'lp: loop { break 'lp 0 }, } };
    |             +                                               +
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:12:22
+  --> $DIR/recover-labeled-non-block-expr.rs:11:22
    |
 LL |     let _i = 'label: match x {
    |                      ^^^^^ expected `while`, `for`, `loop` or `{` after a label
@@ -60,7 +60,7 @@ LL ~     } };
    |
 
 error: expected `while`, `for`, `loop` or `{` after a label
-  --> $DIR/recover-labeled-non-block-expr.rs:26:24
+  --> $DIR/recover-labeled-non-block-expr.rs:25:24
    |
 LL |     let _val = 'label: (1, if other == 3 { break 'label (2, 3) } else { other });
    |                        ^ expected `while`, `for`, `loop` or `{` after a label