diff options
Diffstat (limited to 'src/doc/reference/src/expressions/loop-expr.md')
| -rw-r--r-- | src/doc/reference/src/expressions/loop-expr.md | 34 |
1 files changed, 30 insertions, 4 deletions
diff --git a/src/doc/reference/src/expressions/loop-expr.md b/src/doc/reference/src/expressions/loop-expr.md index 308f3e346..204207ee0 100644 --- a/src/doc/reference/src/expressions/loop-expr.md +++ b/src/doc/reference/src/expressions/loop-expr.md @@ -1,4 +1,4 @@ -# Loops +# Loops and other breakable expressions > **<sup>Syntax</sup>**\ > _LoopExpression_ :\ @@ -7,6 +7,7 @@ > | [_PredicateLoopExpression_]\ > | [_PredicatePatternLoopExpression_]\ > | [_IteratorLoopExpression_]\ +> | [_LabelBlockExpression_]\ > ) [_LoopLabel_]: #loop-labels @@ -14,16 +15,19 @@ [_PredicateLoopExpression_]: #predicate-loops [_PredicatePatternLoopExpression_]: #predicate-pattern-loops [_IteratorLoopExpression_]: #iterator-loops +[_LabelBlockExpression_]: #labelled-block-expressions -Rust supports four loop expressions: +Rust supports five loop expressions: * A [`loop` expression](#infinite-loops) denotes an infinite loop. * A [`while` expression](#predicate-loops) loops until a predicate is false. * A [`while let` expression](#predicate-pattern-loops) tests a pattern. * A [`for` expression](#iterator-loops) extracts values from an iterator, looping until the iterator is empty. +* A [labelled block expression](#labelled-block-expressions) runs a loop exactly once, but allows exiting the loop early with `break`. -All four types of loop support [`break` expressions](#break-expressions), [`continue` expressions](#continue-expressions), and [labels](#loop-labels). -Only `loop` supports [evaluation to non-trivial values](#break-and-loop-values). +All five types of loop support [`break` expressions](#break-expressions), and [labels](#loop-labels). +All except labelled block expressions support [`continue` expressions](#continue-expressions). +Only `loop` and labelled block expressions support [evaluation to non-trivial values](#break-and-loop-values). ## Infinite loops @@ -193,6 +197,18 @@ A loop expression may optionally have a _label_. The label is written as a lifet If a label is present, then labeled `break` and `continue` expressions nested within this loop may exit out of this loop or return control to its head. See [break expressions](#break-expressions) and [continue expressions](#continue-expressions). +Labels follow the hygiene and shadowing rules of local variables. For example, this code will print "outer loop": + +```rust +'a: loop { + 'a: loop { + break 'a; + } + print!("outer loop"); + break 'a; +} +``` + ## `break` expressions > **<sup>Syntax</sup>**\ @@ -226,6 +242,16 @@ Example: A `break` expression is only permitted in the body of a loop, and has one of the forms `break`, `break 'label` or ([see below](#break-and-loop-values)) `break EXPR` or `break 'label EXPR`. +## Labelled block expressions + +> **<sup>Syntax</sup>**\ +> _LabelBlockExpression_ :\ +> [_BlockExpression_] + +Labelled block expressions are exactly like block expressions, except that they allow using `break` expressions within the block. +Unlike other loops, `break` expressions within a label expression *must* have a label (i.e. the label is not optional). +Unlike other loops, labelled block expressions *must* begin with a label. + ## `continue` expressions > **<sup>Syntax</sup>**\ |