Make RangeInclusive just a two-field struct

Not being an enum improves ergonomics, especially since NonEmpty could be Empty.  It can still be iterable without an extra "done" bit by making the range have !(start <= end), which is even possible without changing the Step trait.

Implements RFC 1980

Author: scottmcm

src/libcollections/range.rs

@@ -106,16 +106,10 @@ impl<T> RangeArgument<T> for Range<T> {
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
 impl<T> RangeArgument<T> for RangeInclusive<T> {
     fn start(&self) -> Bound<&T> {
-        match *self {
-            RangeInclusive::Empty{ ref at }            => Included(at),
-            RangeInclusive::NonEmpty { ref start, .. } => Included(start),
-        }
+        Included(&self.start)
     }
     fn end(&self) -> Bound<&T> {
-        match *self {
-            RangeInclusive::Empty{ ref at }            => Excluded(at),
-            RangeInclusive::NonEmpty { ref end, .. }   => Included(end),
-        }
+        Included(&self.end)
     }
 }
 

src/libcore/iter/range.rs

@@ -403,61 +403,35 @@ impl<A: Step + Clone> Iterator for StepBy<A, ops::RangeInclusive<A>> {
 
     #[inline]
     fn next(&mut self) -> Option<A> {
-        use ops::RangeInclusive::*;
-
-        // this function has a sort of odd structure due to borrowck issues
-        // we may need to replace self.range, so borrows of start and end need to end early
-
-        let (finishing, n) = match self.range {
-            Empty { .. } => return None, // empty iterators yield no values
-
-            NonEmpty { ref mut start, ref mut end } => {
-                let rev = self.step_by.is_negative();
-
-                // march start towards (maybe past!) end and yield the old value
-                if (rev && start >= end) ||
-                   (!rev && start <= end)
-                {
-                    match start.step(&self.step_by) {
-                        Some(mut n) => {
-                            mem::swap(start, &mut n);
-                            (None, Some(n)) // yield old value, remain non-empty
-                        },
-                        None => {
-                            let mut n = end.clone();
-                            mem::swap(start, &mut n);
-                            (None, Some(n)) // yield old value, remain non-empty
-                        }
-                    }
-                } else {
-                    // found range in inconsistent state (start at or past end), so become empty
-                    (Some(end.replace_zero()), None)
-                }
-            }
-        };
+        let rev = self.step_by.is_negative();
 
-        // turn into an empty iterator if we've reached the end
-        if let Some(end) = finishing {
-            self.range = Empty { at: end };
+        if (rev && self.range.start >= self.range.end) ||
+           (!rev && self.range.start <= self.range.end)
+        {
+            match self.range.start.step(&self.step_by) {
+                Some(n) => {
+                    Some(mem::replace(&mut self.range.start, n))
+                },
+                None => {
+                    let last = self.range.start.replace_one();
+                    self.range.end.replace_zero();
+                    self.step_by.replace_one();
+                    Some(last)
+                },
+            }
+        }
+        else {
+            None
         }
-
-        n
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        use ops::RangeInclusive::*;
-
-        match self.range {
-            Empty { .. } => (0, Some(0)),
-
-            NonEmpty { ref start, ref end } =>
-                match Step::steps_between(start,
-                                          end,
-                                          &self.step_by) {
-                    Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
-                    None       => (0, None)
-                }
+        match Step::steps_between(&self.range.start,
+                                  &self.range.end,
+                                  &self.step_by) {
+            Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
+            None       => (0, None)
         }
     }
 }
@@ -583,56 +557,27 @@ impl<A: Step> Iterator for ops::RangeInclusive<A> where
 
     #[inline]
     fn next(&mut self) -> Option<A> {
-        use ops::RangeInclusive::*;
-
-        // this function has a sort of odd structure due to borrowck issues
-        // we may need to replace self, so borrows of self.start and self.end need to end early
-
-        let (finishing, n) = match *self {
-            Empty { .. } => (None, None), // empty iterators yield no values
-
-            NonEmpty { ref mut start, ref mut end } => {
-                if start == end {
-                    (Some(end.replace_one()), Some(start.replace_one()))
-                } else if start < end {
-                    let mut n = start.add_one();
-                    mem::swap(&mut n, start);
-
-                    // if the iterator is done iterating, it will change from
-                    // NonEmpty to Empty to avoid unnecessary drops or clones,
-                    // we'll reuse either start or end (they are equal now, so
-                    // it doesn't matter which) to pull out end, we need to swap
-                    // something back in
-
-                    (if n == *end { Some(end.replace_one()) } else { None },
-                    // ^ are we done yet?
-                    Some(n)) // < the value to output
-                } else {
-                    (Some(start.replace_one()), None)
-                }
-            }
-        };
-
-        // turn into an empty iterator if this is the last value
-        if let Some(end) = finishing {
-            *self = Empty { at: end };
+        use cmp::Ordering::*;
+
+        match self.start.partial_cmp(&self.end) {
+            Some(Less) => {
+                let n = self.start.add_one();
+                Some(mem::replace(&mut self.start, n))
+            },
+            Some(Equal) => {
+                let last = self.start.replace_one();
+                self.end.replace_zero();
+                Some(last)
+            },
+            _ => None,
         }
-
-        n
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        use ops::RangeInclusive::*;
-
-        match *self {
-            Empty { .. } => (0, Some(0)),
-
-            NonEmpty { ref start, ref end } =>
-                match Step::steps_between_by_one(start, end) {
-                    Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
-                    None => (0, None),
-                }
+        match Step::steps_between_by_one(&self.start, &self.end) {
+            Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
+            None => (0, None),
         }
     }
 }
@@ -644,33 +589,20 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> where
 {
     #[inline]
     fn next_back(&mut self) -> Option<A> {
-        use ops::RangeInclusive::*;
-
-        // see Iterator::next for comments
-
-        let (finishing, n) = match *self {
-            Empty { .. } => return None,
-
-            NonEmpty { ref mut start, ref mut end } => {
-                if start == end {
-                    (Some(start.replace_one()), Some(end.replace_one()))
-                } else if start < end {
-                    let mut n = end.sub_one();
-                    mem::swap(&mut n, end);
-
-                    (if n == *start { Some(start.replace_one()) } else { None },
-                     Some(n))
-                } else {
-                    (Some(end.replace_one()), None)
-                }
-            }
-        };
-
-        if let Some(start) = finishing {
-            *self = Empty { at: start };
+        use cmp::Ordering::*;
+
+        match self.start.partial_cmp(&self.end) {
+            Some(Less) => {
+                let n = self.end.sub_one();
+                Some(mem::replace(&mut self.end, n))
+            },
+            Some(Equal) => {
+                let last = self.end.replace_zero();
+                self.start.replace_one();
+                Some(last)
+            },
+            _ => None,
         }
-
-        n
     }
 }
 

src/libcore/ops.rs

@@ -2271,7 +2271,7 @@ impl<Idx: PartialOrd<Idx>> RangeTo<Idx> {
 /// ```
 /// #![feature(inclusive_range,inclusive_range_syntax)]
 /// fn main() {
-///     assert_eq!((3...5), std::ops::RangeInclusive::NonEmpty{ start: 3, end: 5 });
+///     assert_eq!((3...5), std::ops::RangeInclusive{ start: 3, end: 5 });
 ///     assert_eq!(3+4+5, (3...5).sum());
 ///
 ///     let arr = [0, 1, 2, 3];
@@ -2281,45 +2281,23 @@ impl<Idx: PartialOrd<Idx>> RangeTo<Idx> {
 /// ```
 #[derive(Clone, PartialEq, Eq, Hash)]  // not Copy -- see #27186
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
-pub enum RangeInclusive<Idx> {
-    /// Empty range (iteration has finished)
+pub struct RangeInclusive<Idx> {
+    /// The lower bound of the range (inclusive).
     #[unstable(feature = "inclusive_range",
                reason = "recently added, follows RFC",
                issue = "28237")]
-    Empty {
-        /// The point at which iteration finished
-        #[unstable(feature = "inclusive_range",
-                   reason = "recently added, follows RFC",
-                   issue = "28237")]
-        at: Idx
-    },
-    /// Non-empty range (iteration will yield value(s))
+    pub start: Idx,
+    /// The upper bound of the range (inclusive).
     #[unstable(feature = "inclusive_range",
                reason = "recently added, follows RFC",
                issue = "28237")]
-    NonEmpty {
-        /// The lower bound of the range (inclusive).
-        #[unstable(feature = "inclusive_range",
-                   reason = "recently added, follows RFC",
-                   issue = "28237")]
-        start: Idx,
-        /// The upper bound of the range (inclusive).
-        #[unstable(feature = "inclusive_range",
-                   reason = "recently added, follows RFC",
-                   issue = "28237")]
-        end: Idx,
-    },
+    pub end: Idx,
 }
 
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
 impl<Idx: fmt::Debug> fmt::Debug for RangeInclusive<Idx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        use self::RangeInclusive::*;
-
-        match *self {
-            Empty { ref at } => write!(fmt, "[empty range @ {:?}]", at),
-            NonEmpty { ref start, ref end } => write!(fmt, "{:?}...{:?}", start, end),
-        }
+        write!(fmt, "{:?}...{:?}", self.start, self.end)
     }
 }
 
@@ -2341,9 +2319,7 @@ impl<Idx: PartialOrd<Idx>> RangeInclusive<Idx> {
     /// }
     /// ```
     pub fn contains(&self, item: Idx) -> bool {
-        if let &RangeInclusive::NonEmpty{ref start, ref end} = self {
-            (*start <= item) && (item <= *end)
-        } else { false }
+        self.start <= item && item <= self.end
     }
 }