[stdlib] Improve default implementation of Collection.distance(from:to:)

(cherry picked from commit 72948ca)

Author: Max Moiseev

stdlib/private/StdlibCollectionUnittest/CheckCollectionType.swift.gyb

@@ -1584,11 +1584,6 @@ extension TestSuite {
       .forEach(in: distanceFromToTests) {
       test in
       let c = toCollection(0..<20)
-      let backwards = (test.startOffset > test.endOffset)
-      if backwards && !collectionIsBidirectional {
-        expectCrashLater()
-      }
-
       let d = c.distance(
         from: c.nthIndex(test.startOffset), to: c.nthIndex(test.endOffset))
       expectEqual(

stdlib/private/StdlibCollectionUnittest/MinimalCollections.swift.gyb

@@ -657,8 +657,6 @@ public struct ${Self}<T> : ${SelfProtocols} {
   public func distance(from start: ${Index}, to end: ${Index})
     -> Int {
 %     if Traversal == 'Forward':
-    _precondition(start <= end,
-      "Only BidirectionalCollections can have end come before start")
 %     end
     // FIXME: swift-3-indexing-model: perform a range check properly.
     if start != endIndex {

stdlib/public/core/Collection.swift

@@ -701,16 +701,11 @@ public protocol Collection: Sequence where SubSequence: Collection {
 
   /// Returns the distance between two indices.
   ///
-  /// Unless the collection conforms to the `BidirectionalCollection` protocol,
-  /// `start` must be less than or equal to `end`.
-  ///
   /// - Parameters:
   ///   - start: A valid index of the collection.
   ///   - end: Another valid index of the collection. If `end` is equal to
   ///     `start`, the result is zero.
-  /// - Returns: The distance between `start` and `end`. The result can be
-  ///   negative only if the collection conforms to the
-  ///   `BidirectionalCollection` protocol.
+  /// - Returns: The distance between `start` and `end`.
   ///
   /// - Complexity: O(1) if the collection conforms to
   ///   `RandomAccessCollection`; otherwise, O(*n*), where *n* is the
@@ -962,30 +957,34 @@ extension Collection {
 
   /// Returns the distance between two indices.
   ///
-  /// Unless the collection conforms to the `BidirectionalCollection` protocol,
-  /// `start` must be less than or equal to `end`.
-  ///
   /// - Parameters:
   ///   - start: A valid index of the collection.
   ///   - end: Another valid index of the collection. If `end` is equal to
   ///     `start`, the result is zero.
-  /// - Returns: The distance between `start` and `end`. The result can be
-  ///   negative only if the collection conforms to the
-  ///   `BidirectionalCollection` protocol.
+  /// - Returns: The distance between `start` and `end`.
   ///
   /// - Complexity: O(1) if the collection conforms to
   ///   `RandomAccessCollection`; otherwise, O(*n*), where *n* is the
   ///   resulting distance.
   @_inlineable
   public func distance(from start: Index, to end: Index) -> Int {
-    _precondition(start <= end,
-      "Only BidirectionalCollections can have end come before start")
-
-    var start = start
+    var _start: Index
+    let _end: Index
+    let step: Int
+    if start > end {
+      _start = end
+      _end = start
+      step = -1
+    }
+    else {
+      _start = start
+      _end = end
+      step = 1
+    }
     var count = 0
-    while start != end {
-      count = count + 1
-      formIndex(after: &start)
+    while _start != _end {
+      count += step
+      formIndex(after: &_start)
     }
     return count
   }