[stdlib] Adopt conditional conformance for Indices, Slice, ReversedCollection (#12913)

* Refactor Indices and Slice to use conditional conformance

* Replace ReversedRandomAccessCollection with a conditional extension

* Refactor some types into struct+extensions

* Revise Slice documentation

* Fix test cases for adoption of conditional conformances.

* [RangeReplaceableCollection] Eliminate unnecessary slicing subscript operator.

* Add -enable-experimental-conditional-conformances to test.

* Gruesome workaround for crasher in MutableSlice tests

Author: airspeedswift

stdlib/private/StdlibCollectionUnittest/CheckMutableCollectionType.swift.gyb

@@ -276,7 +276,8 @@ self.test("\(testNamePrefix).subscript(_: Range)/Set/semantics") {
       // Call setter implicitly through an inout mutation.
       var c = makeWrappedCollection(test.collection)
 
-      _mapInPlace(&c[test.bounds(in: c)]) {
+      var s = c[test.bounds(in: c)]
+      _mapInPlace(&s) {
         wrapValue(OpaqueValue(extractValue($0).value + 90000))
       }
 

stdlib/public/core/BidirectionalCollection.swift

@@ -44,14 +44,20 @@ public typealias BidirectionalIndexable = BidirectionalCollection
 ///   `c.index(before: c.index(after: i)) == i`.
 /// - If `i > c.startIndex && i <= c.endIndex`
 ///   `c.index(after: c.index(before: i)) == i`.
-public protocol BidirectionalCollection : Collection 
-{
+public protocol BidirectionalCollection: Collection
+where SubSequence: BidirectionalCollection, Indices: BidirectionalCollection {
   // FIXME(ABI): Associated type inference requires this.
   associatedtype Element
 
   // FIXME(ABI): Associated type inference requires this.
   associatedtype Index
 
+  // FIXME(ABI): Associated type inference requires this.
+  associatedtype SubSequence = Slice<Self>
+
+  // FIXME(ABI): Associated type inference requires this.
+  associatedtype Indices = DefaultIndices<Self>
+
   /// Returns the position immediately before the given index.
   ///
   /// - Parameter i: A valid index of the collection. `i` must be greater than
@@ -65,16 +71,6 @@ public protocol BidirectionalCollection : Collection
   ///   `startIndex`.
   func formIndex(before i: inout Index)
 
-  /// A sequence that can represent a contiguous subrange of the collection's
-  /// elements.
-  associatedtype SubSequence : BidirectionalCollection
-    = BidirectionalSlice<Self>
-
-  /// A type that represents the indices that are valid for subscripting the
-  /// collection, in ascending order.
-  associatedtype Indices : BidirectionalCollection
-    = DefaultBidirectionalIndices<Self>
-
   /// The indices that are valid for subscripting the collection, in ascending
   /// order.
   ///
@@ -201,17 +197,6 @@ extension BidirectionalCollection {
   }
 }
 
-/// Supply the default "slicing" `subscript` for `BidirectionalCollection`
-/// models that accept the default associated `SubSequence`,
-/// `BidirectionalSlice<Self>`.
-extension BidirectionalCollection where SubSequence == BidirectionalSlice<Self> {
-  @_inlineable // FIXME(sil-serialize-all)
-  public subscript(bounds: Range<Index>) -> BidirectionalSlice<Self> {
-    _failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)
-    return BidirectionalSlice(base: self, bounds: bounds)
-  }
-}
-
 extension BidirectionalCollection where SubSequence == Self {
   /// Removes and returns the last element of the collection.
   ///

stdlib/public/core/CMakeLists.txt

@@ -72,7 +72,7 @@ set(SWIFTLIB_ESSENTIAL
   HeapBuffer.swift
   ICU.swift
   ImplicitlyUnwrappedOptional.swift
-  Indices.swift.gyb
+  Indices.swift
   InputStream.swift
   IntegerParsing.swift
   Integers.swift.gyb
@@ -100,7 +100,7 @@ set(SWIFTLIB_ESSENTIAL
   Print.swift
   RandomAccessCollection.swift
   Range.swift.gyb
-  RangeReplaceableCollection.swift.gyb
+  RangeReplaceableCollection.swift
   ReflectionLegacy.swift
   Repeat.swift
   REPL.swift
@@ -115,7 +115,7 @@ set(SWIFTLIB_ESSENTIAL
   SetAlgebra.swift
   ShadowProtocols.swift
   Shims.swift
-  Slice.swift.gyb
+  Slice.swift
   Sort.swift.gyb
   StaticString.swift
   Stride.swift.gyb

stdlib/public/core/ClosedRange.swift

@@ -261,8 +261,8 @@ public struct CountableClosedRange<Bound> : RandomAccessCollection
 
   @_inlineable
   public subscript(bounds: Range<Index>)
-    -> RandomAccessSlice<CountableClosedRange<Bound>> {
-    return RandomAccessSlice(base: self, bounds: bounds)
+    -> Slice<CountableClosedRange<Bound>> {
+    return Slice(base: self, bounds: bounds)
   }
 
   /// Creates an instance with the given bounds.

stdlib/public/core/Collection.swift

@@ -82,9 +82,11 @@ public typealias Indexable = Collection
 ///     // Prints "15.0"
 ///     // Prints "20.0"
 @_fixed_layout
-public struct IndexingIterator<
-  Elements : Collection
-> : IteratorProtocol, Sequence {
+public struct IndexingIterator<Elements : Collection> {
+  @_versioned
+  internal let _elements: Elements
+  @_versioned
+  internal var _position: Elements.Index
 
   @_inlineable
   @inline(__always)
@@ -103,6 +105,12 @@ public struct IndexingIterator<
     self._elements = _elements
     self._position = _position
   }
+}
+
+extension IndexingIterator: IteratorProtocol, Sequence {
+  public typealias Element = Elements.Element
+  public typealias Iterator = IndexingIterator<Elements>
+  public typealias SubSequence = AnySequence<Element>
 
   /// Advances to the next element and returns it, or `nil` if no next element
   /// exists.
@@ -136,11 +144,6 @@ public struct IndexingIterator<
     _elements.formIndex(after: &_position)
     return element
   }
-  
-  @_versioned
-  internal let _elements: Elements
-  @_versioned
-  internal var _position: Elements.Index
 }
 
 /// A sequence whose elements can be traversed multiple times,
@@ -345,8 +348,7 @@ public struct IndexingIterator<
 /// or bidirectional collection must traverse the entire collection to count
 /// the number of contained elements, accessing its `count` property is an
 /// O(*n*) operation.
-public protocol Collection : Sequence
-{
+public protocol Collection: Sequence where SubSequence: Collection {
   // FIXME(ABI): Associated type inference requires this.
   associatedtype Element
 
@@ -403,7 +405,7 @@ public protocol Collection : Sequence
   /// This associated type appears as a requirement in the `Sequence`
   /// protocol, but it is restated here with stricter constraints. In a
   /// collection, the subsequence should also conform to `Collection`.
-  associatedtype SubSequence : Collection = Slice<Self>
+  associatedtype SubSequence = Slice<Self>
     where SubSequence.Index == Index,
           SubSequence.IndexDistance == IndexDistance
 

stdlib/public/core/CollectionOfOne.swift

@@ -109,10 +109,10 @@ public struct CollectionOfOne<Element>
 
   @_inlineable // FIXME(sil-serialize-all)
   public subscript(bounds: Range<Int>)
-    -> MutableRandomAccessSlice<CollectionOfOne<Element>> {
+    -> Slice<CollectionOfOne<Element>> {
     get {
       _failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)
-      return MutableRandomAccessSlice(base: self, bounds: bounds)
+      return Slice(base: self, bounds: bounds)
     }
     set {
       _failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)

stdlib/public/core/Flatten.swift.gyb

@@ -362,8 +362,8 @@ public struct ${Collection}<Base> : ${collectionForTraversal(traversal)}
 
   @_inlineable // FIXME(sil-serialize-all)
   public subscript(bounds: Range<Index>)
-    -> ${Slice}<${Collection}> {
-    return ${Slice}(base: self, bounds: bounds)
+    -> Slice<${Collection}> {
+    return Slice(base: self, bounds: bounds)
   }
 
   // To return any estimate of the number of elements, we have to start

stdlib/public/core/Indices.swift.gyb renamed to stdlib/public/core/Indices.swift

@@ -10,32 +10,15 @@
 //
 //===----------------------------------------------------------------------===//
 
-%{
-from gyb_stdlib_support import (
-    TRAVERSALS,
-    collectionForTraversal,
-    defaultIndicesForTraversal,
-    documentationNameForTraversal
-)
-}%
-
-% for Traversal in TRAVERSALS:
-%   Self = defaultIndicesForTraversal(Traversal)
-%   collection = documentationNameForTraversal(Traversal)
-
-// FIXME(ABI)#42 (Conditional Conformance): There should be just one default
-// indices type that has conditional conformances to
-// `BidirectionalCollection` and `RandomAccessCollection`.
-// <rdar://problem/17144340>
-
-/// A collection of indices for an arbitrary ${collection}.
+/// A collection of indices for an arbitrary collection
 @_fixed_layout
-public struct ${Self}<
-  Elements : ${collectionForTraversal(Traversal)}
-> : ${collectionForTraversal(Traversal)} {
-
-  public typealias Index = Elements.Index
-  public typealias Indices = ${Self}<Elements>
+public struct DefaultIndices<Elements: Collection> {
+  @_versioned
+  internal var _elements: Elements
+  @_versioned
+  internal var _startIndex: Elements.Index
+  @_versioned
+  internal var _endIndex: Elements.Index
 
   @_inlineable
   @_versioned
@@ -48,6 +31,16 @@ public struct ${Self}<
     self._startIndex = startIndex
     self._endIndex = endIndex
   }
+}
+
+extension DefaultIndices: Collection {
+
+  public typealias Index = Elements.Index
+  public typealias Element = Elements.Index
+  public typealias Indices = DefaultIndices<Elements>
+  public typealias SubSequence = DefaultIndices<Elements>
+	public typealias IndexDistance = Elements.IndexDistance
+	public typealias Iterator = IndexingIterator<DefaultIndices<Elements>>
 
   @_inlineable
   public var startIndex: Index {
@@ -65,12 +58,10 @@ public struct ${Self}<
     return i
   }
 
-  public typealias SubSequence = ${Self}<Elements>
-
   @_inlineable
-  public subscript(bounds: Range<Index>) -> ${Self}<Elements> {
+  public subscript(bounds: Range<Index>) -> DefaultIndices<Elements> {
     // FIXME: swift-3-indexing-model: range check.
-    return ${Self}(
+    return DefaultIndices(
       _elements: _elements,
       startIndex: bounds.lowerBound,
       endIndex: bounds.upperBound)
@@ -88,7 +79,14 @@ public struct ${Self}<
     _elements.formIndex(after: &i)
   }
 
-%     if Traversal in ['Bidirectional', 'RandomAccess']:
+  @_inlineable
+  public var indices: Indices {
+    return self
+  }
+}
+
+extension DefaultIndices: BidirectionalCollection
+where Elements: BidirectionalCollection {
   @_inlineable
   public func index(before i: Index) -> Index {
     // FIXME: swift-3-indexing-model: range check.
@@ -100,23 +98,12 @@ public struct ${Self}<
     // FIXME: swift-3-indexing-model: range check.
     _elements.formIndex(before: &i)
   }
-%     end
-
-  @_inlineable
-  public var indices: Indices {
-    return self
-  }
-
-  @_versioned
-  internal var _elements: Elements
-  @_versioned
-  internal var _startIndex: Elements.Index
-  @_versioned
-  internal var _endIndex: Elements.Index
 }
 
-extension ${collectionForTraversal(Traversal)}
-where Indices == ${Self}<Self> {
+extension DefaultIndices: RandomAccessCollection
+where Elements: RandomAccessCollection { }
+
+extension Collection where Indices == DefaultIndices<Self> {
   /// The indices that are valid for subscripting the collection, in ascending
   /// order.
   ///
@@ -135,16 +122,13 @@ where Indices == ${Self}<Self> {
   ///     }
   ///     // c == MyFancyCollection([2, 4, 6, 8, 10])
   @_inlineable // FIXME(sil-serialize-all)
-  public var indices: ${Self}<Self> {
-    return ${Self}(
+  public var indices: DefaultIndices<Self> {
+    return DefaultIndices(
       _elements: self,
       startIndex: self.startIndex,
       endIndex: self.endIndex)
   }
 }
 
-% end
-
-// ${'Local Variables'}:
-// eval: (read-only-mode 1)
-// End:
+public typealias DefaultBidirectionalIndices<T: BidirectionalCollection> = DefaultIndices<T>
+public typealias DefaultRandomAccessIndices<T: RandomAccessCollection> = DefaultIndices<T>

stdlib/public/core/Integers.swift.gyb

@@ -3208,7 +3208,7 @@ ${assignmentOperatorComment(x.operator, True)}
   @_fixed_layout // FIXME(sil-serialize-all)
   public struct Words : BidirectionalCollection {
     public typealias Indices = CountableRange<Int>
-    public typealias SubSequence = BidirectionalSlice<${Self}.Words>
+    public typealias SubSequence = Slice<${Self}.Words>
 
     @_versioned // FIXME(sil-serialize-all)
     internal var _value: ${Self}

stdlib/public/core/LazyCollection.swift.gyb

@@ -162,8 +162,8 @@ extension ${Self} : ${TraversalCollection} {
   ///
   /// - Complexity: O(1)
   @_inlineable
-  public subscript(bounds: Range<Index>) -> ${Slice}<${Self}<Base>> {
-    return ${Slice}(base: self, bounds: bounds)
+  public subscript(bounds: Range<Index>) -> Slice<${Self}<Base>> {
+    return Slice(base: self, bounds: bounds)
   }
 
   /// A Boolean value indicating whether the collection is empty.