Merge pull request #900 from workingjubilee/rangefrom-pat

Document RangeFrom patterns

Author: ehuss

src/patterns.md

@@ -13,7 +13,6 @@
 >    | [_IdentifierPattern_]\
 >    | [_WildcardPattern_]\
 >    | [_RestPattern_]\
->    | [_ObsoleteRangePattern_]\
 >    | [_ReferencePattern_]\
 >    | [_StructPattern_]\
 >    | [_TupleStructPattern_]\
@@ -407,7 +406,15 @@ match tuple {
 
 > **<sup>Syntax</sup>**\
 > _RangePattern_ :\
-> &nbsp;&nbsp; _RangePatternBound_ `..=` _RangePatternBound_
+> &nbsp;&nbsp; &nbsp;&nbsp; _InclusiveRangePattern_\
+> &nbsp;&nbsp; | _HalfOpenRangePattern_\
+> &nbsp;&nbsp; | _ObsoleteRangePattern_
+>
+> _InclusiveRangePattern_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; _RangePatternBound_ `..=` _RangePatternBound_
+>
+> _HalfOpenRangePattern_ :\
+> &nbsp;&nbsp; | _RangePatternBound_ `..`
 >
 > _ObsoleteRangePattern_ :\
 > &nbsp;&nbsp; _RangePatternBound_ `...` _RangePatternBound_
@@ -420,11 +427,20 @@ match tuple {
 > &nbsp;&nbsp; | [_PathInExpression_]\
 > &nbsp;&nbsp; | [_QualifiedPathInExpression_]
 
-Range patterns match values that are within the closed range defined by its lower and
-upper bounds. For example, a pattern `'m'..='p'` will match only the values `'m'`, `'n'`,
-`'o'`, and `'p'`. The bounds can be literals or paths that point to constant values.
+Range patterns match values within the range defined by their bounds. A range pattern may be
+closed or half-open. A range pattern is closed if it has both a lower and an upper bound, and
+it matches all the values between and including both of its bounds. A range pattern that is
+half-open is written with a lower bound but not an upper bound, and matches any value equal to
+or greater than the specified lower bound.
+
+For example, a pattern `'m'..='p'` will match only the values `'m'`, `'n'`, `'o'`, and `'p'`. For an integer the
+pattern `1..` will match 9, or 9001, or 9007199254740991 (if it is of an appropriate size), but
+not 0, and not negative numbers for signed integers. The bounds can be literals or paths that point
+to constant values.
+
+A half-open range pattern in the style `a..` cannot be used to match within the context of a slice.
 
-A pattern a `..=` b must always have a &le; b. It is an error to have a range pattern
+A pattern `a..=b` must always have a &le; b. It is an error to have a range pattern
 `10..=0`, for example.
 
 The `...` syntax is kept for backwards compatibility.
@@ -456,6 +472,12 @@ println!("{}", match ph {
     _ => unreachable!(),
 });
 
+# let uint: u32 = 5;
+match uint {
+    0 => "zero!",
+    1.. => "positive number!",
+};
+
 // using paths to constants:
 # const TROPOSPHERE_MIN : u8 = 6;
 # const TROPOSPHERE_MAX : u8 = 20;
@@ -736,6 +758,10 @@ is irrefutable. When matching a slice, it is irrefutable only in the form with
 a single `..` [rest pattern](#rest-patterns) or [identifier
 pattern](#identifier-patterns) with the `..` rest pattern as a subpattern.
 
+Within a slice, a half-open range pattern like `a..` must be enclosed in parentheses,
+as in `(a..)`, to clarify it is intended to match a single value.
+A future version of Rust may give the non-parenthesized version an alternate meaning.
+
 ## Path patterns
 
 > **<sup>Syntax</sup>**\

src/tokens.md

@@ -585,7 +585,7 @@ usages and meanings are defined in the linked pages.
 | `@`    | At          | [Subpattern binding]
 | `_`    | Underscore  | [Wildcard patterns], [Inferred types], Unnamed items in [constants], [extern crates], and [use declarations]
 | `.`    | Dot         | [Field access][field], [Tuple index]
-| `..`   | DotDot      | [Range][range], [Struct expressions], [Patterns]
+| `..`   | DotDot      | [Range][range], [Struct expressions], [Patterns], [Range Patterns][rangepat]
 | `...`  | DotDotDot   | [Variadic functions][extern], [Range patterns]
 | `..=`  | DotDotEq    | [Inclusive Range][range], [Range patterns]
 | `,`    | Comma       | Various separators
@@ -646,6 +646,7 @@ them are referred to as "token trees" in [macros].  The three types of brackets
 [patterns]: patterns.md
 [question]: expressions/operator-expr.md#the-question-mark-operator
 [range]: expressions/range-expr.md
+[rangepat]: patterns.md#range-patterns
 [raw pointers]: types/pointer.md#raw-pointers-const-and-mut
 [references]: types/pointer.md
 [sized]: trait-bounds.md#sized