Merge pull request #1330 from dtolnay-contrib/rangepattern

Eliminate 'half open' terminology from range pattern grammar

Author: ehuss

src/patterns.md

@@ -380,16 +380,19 @@ match tuple {
 
 > **<sup>Syntax</sup>**\
 > _RangePattern_ :\
-> &nbsp;&nbsp; &nbsp;&nbsp; _InclusiveRangePattern_\
-> &nbsp;&nbsp; | _HalfOpenRangePattern_\
+> &nbsp;&nbsp; &nbsp;&nbsp; _RangeInclusivePattern_\
+> &nbsp;&nbsp; | _RangeFromPattern_\
+> &nbsp;&nbsp; | _RangeToInclusivePattern_\
 > &nbsp;&nbsp; | _ObsoleteRangePattern_
 >
-> _InclusiveRangePattern_ :\
+> _RangeInclusivePattern_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; _RangePatternBound_ `..=` _RangePatternBound_
 >
-> _HalfOpenRangePattern_ :\
+> _RangeFromPattern_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; _RangePatternBound_ `..`
-> &nbsp;&nbsp; | `..=` _RangePatternBound_
+>
+> _RangeToInclusivePattern_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; `..=` _RangePatternBound_
 >
 > _ObsoleteRangePattern_ :\
 > &nbsp;&nbsp; _RangePatternBound_ `...` _RangePatternBound_
@@ -402,36 +405,29 @@ match tuple {
 > &nbsp;&nbsp; | [_PathExpression_]
 
 *Range patterns* match scalar values within the range defined by their bounds.
-A bound on the left of its sigils is a *lower bound*.
+They comprise a *sigil* (one of `..`, `..=`, or `...`) and a bound on one or both sides.
+A bound on the left of the sigil is a *lower bound*.
 A bound on the right is an *upper bound*.
-A range pattern may be closed or half-open.
-
-A range pattern is *closed* if it has both a lower and an upper bound.
-The only closed ranged pattern is the inclusive range pattern.
 
-*Inclusive range patterns* match all the values between and including both of its bounds.
-It is written as its lower bounds, followed by `..=`, followed by its upper bounds.
-The type of it is the type unification of its upper and lower bounds.
+A range pattern with both a lower and upper bound will match all values between and including both of its bounds.
+It is written as its lower bound, followed by `..=`, followed by its upper bound.
+The type of the range pattern is the type unification of its upper and lower bounds.
 
 For example, a pattern `'m'..='p'` will match only the values `'m'`, `'n'`, `'o'`, and `'p'`.
 
 The lower bound cannot be greater than the upper bound.
 That is, in `a..=b`, a &le; b must be the case.
 For example, it is an error to have a range pattern `10..=0`.
 
-Range patterns are *half-open* if they have only an upper or lower bound.
-They have the same type as their upper or lower bound.
-
-A half open range with only a lower bound is written as its lower bound followed by `..`.
-These range patterns will match on any value greater than or equal to the lower bound.
+A range pattern with only a lower bound will match any value greater than or equal to the lower bound.
+It is written as its lower bound followed by `..`, and has the same type as its lower bound.
 For example, `1..` will match 1, 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 with only an upper bound is written as `..=` followed by its upper bound.
-These range patterns will match on any value less than or equal to the upper bound.
+A range pattern with only an upper bound matches any value less than or equal to the upper bound.
+It is written as `..=` followed by its upper bound, and has the same type as its upper bound.
 For example, `..=10` will match 10, 1, 0, and for signed integer types, all negative values.
 
-Half-open range patterns cannot be used as the top-level pattern for subpatterns in [slice patterns](#slice-patterns).
+Range patterns with only one bound cannot be used as the top-level pattern for subpatterns in [slice patterns](#slice-patterns).
 
 The bounds is written as one of:
 
@@ -529,7 +525,7 @@ The range of values for a `char` type are precisely those ranges containing all
 Floating point range patterns are deprecated and may be removed in a future Rust release.
 See [issue #41620](https://github.com/rust-lang/rust/issues/41620) for more information.
 
-> **Edition Differences**: Before the 2021 edition, closed range patterns may also be written using `...` as an alternative to `..=`, with the same meaning.
+> **Edition Differences**: Before the 2021 edition, range patterns with both a lower and upper bound may also be written using `...` in place of `..=`, with the same meaning.
 
 > **Note**: Although range patterns use the same syntax as [range expressions], there are no exclusive range patterns.
 > That is, neither `x .. y` nor `.. x` are valid range patterns.
@@ -747,7 +743,8 @@ match v[..] {
 Slice patterns are irrefutable when matching an array as long as each element 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.
+Within a slice, a range pattern without both lower and upper bound must be enclosed in parentheses, as in `(a..)`, to clarify it is intended to match against a single slice element.
+A range pattern with both lower and upper bound, like `a..=b`, is not required to be enclosed in parentheses.
 
 ## Path patterns