[stdlib][DNM] Collapse sequence and collection wrappers (#20221)

* Concretize dropFirst/Last/sufix/prefix from Sequence

Remove split customization point

Eliminate SubSequence from Sequence protocol

Collapse LazyCollection

Collapse LazyMapCollection

Eliminate _SequenceWrapper

Collapse LazyFilterCollection

Collapse LazyDrop/PrefixWhileCollection

Fix tests, ABI stability update

Collapse FlattenSequence

* Add entries to source/ABI compatible expected results.

* Update tests to avoid pre-10.14 objc runtime bug

* Expunge _preprocessingPass

Author: airspeedswift

stdlib/private/StdlibCollectionUnittest/CheckSequenceInstance.swift

@@ -87,32 +87,6 @@ public func checkSequence<
   expectGE(
     expectedCount, sequence.underestimatedCount, message(),
       stackTrace: stackTrace.pushIf(showFrame, file: file, line: line))
-
-  // Test `_copyContents(initializing:)` if we can do so without destroying the
-  // sequence.
-  _ = sequence._preprocessingPass { () -> Void in
-    var count = 0
-    for _ in sequence { count += 1 }
-    let ptr = UnsafeMutablePointer<S.Element>.allocate(capacity: count)
-    let buf = UnsafeMutableBufferPointer(start: ptr, count: count)
-    var (remainders,writtenUpTo) = sequence._copyContents(initializing: buf)
-    expectTrue(remainders.next() == nil,
-      "_copyContents returned unwritten elements")
-    expectTrue(writtenUpTo == buf.endIndex,
-      "_copyContents failed to use entire buffer")
-    expectEqualSequence(expected, buf, message(),
-      stackTrace: stackTrace.pushIf(showFrame, file: file, line: line), sameValue: sameValue)
-    ptr.deinitialize(count: count)
-    ptr.deallocate()
-  }
-
-  // Test `_copyToContiguousArray()` if we can do so
-  // without destroying the sequence.
-  _ = sequence._preprocessingPass { () -> Void in
-    let copy = sequence._copyToContiguousArray()
-    expectEqualSequence(expected, copy, message(),
-      stackTrace: stackTrace.pushIf(showFrame, file: file, line: line), sameValue: sameValue)
-  }
 }
 
 public func checkSequence<
@@ -206,32 +180,6 @@ public func checkSequence<
   expectGE(
     expectedCount, sequence.underestimatedCount, message(),
     stackTrace: stackTrace.pushIf(showFrame, file: file, line: line))
-
-  // Test `_copyContents(initializing:)` if we can do so without destroying the
-  // sequence.
-  _ = sequence._preprocessingPass { () -> Void in
-    var count = 0
-    for _ in sequence { count += 1 }
-    let ptr = UnsafeMutablePointer<S.Element>.allocate(capacity: count)
-    let buf = UnsafeMutableBufferPointer(start: ptr, count: count)
-    var (remainders,writtenUpTo) = sequence._copyContents(initializing: buf)
-    expectTrue(remainders.next() == nil,
-      "_copyContents returned unwritten elements")
-    expectTrue(writtenUpTo == buf.endIndex,
-      "_copyContents failed to use entire buffer")
-    expectEqualSequence(expected, buf, message(),
-    stackTrace: stackTrace.pushIf(showFrame, file: file, line: line), sameValue: sameValue)
-    ptr.deinitialize(count: count)
-    ptr.deallocate()
-  }
-
-  // Test `_copyToContiguousArray()` if we can do so
-  // without destroying the sequence.
-  _ = sequence._preprocessingPass { () -> Void in
-    let copy = sequence._copyToContiguousArray()
-    expectEqualSequence(expected, copy, message(),
-    stackTrace: stackTrace.pushIf(showFrame, file: file, line: line), sameValue: sameValue)
-  }
 }
 
 public func checkSequence<

stdlib/private/StdlibCollectionUnittest/CheckSequenceType.swift

@@ -1643,14 +1643,6 @@ extension TestSuite {
 
     testNamePrefix += String(describing: S.Type.self)
 
-    let isMultiPass = makeSequence([])
-      ._preprocessingPass { true } ?? false
-    let isEquatableMultiPass = makeSequenceOfEquatable([])
-      ._preprocessingPass { true } ?? false
-    expectEqual(
-      isMultiPass, isEquatableMultiPass,
-      "Two sequence types are of different kinds?")
-
     // FIXME: swift-3-indexing-model: add tests for `underestimatedCount`
     // Check that it is non-negative, and an underestimate of the actual
     // element count.
@@ -1667,12 +1659,6 @@ self.test("\(testNamePrefix).contains()/WhereElementIsEquatable/semantics") {
       test.expected != nil,
       s.contains(wrapValueIntoEquatable(test.element)),
       stackTrace: SourceLocStack().with(test.loc))
-
-    if !isMultiPass {
-      expectEqualSequence(
-        test.expectedLeftoverSequence, s.map(extractValueFromEquatable),
-        stackTrace: SourceLocStack().with(test.loc))
-    }
   }
 }
 
@@ -1992,52 +1978,6 @@ self.test("\(testNamePrefix).first(where:)/semantics") {
   }
 }
 
-//===----------------------------------------------------------------------===//
-// _preprocessingPass()
-//===----------------------------------------------------------------------===//
-
-self.test("\(testNamePrefix)._preprocessingPass/semantics") {
-  for test in forEachTests {
-    let s = makeWrappedSequence(test.sequence.map(OpaqueValue.init))
-    var wasInvoked = false
-    let result = s._preprocessingPass {
-      () -> OpaqueValue<Int> in
-      wasInvoked = true
-
-      expectEqualSequence(
-        test.sequence,
-        s.map { extractValue($0).value })
-
-      return OpaqueValue(42)
-    }
-    if wasInvoked {
-      expectEqual(42, result?.value)
-    } else {
-      expectNil(result)
-    }
-  }
-
-  for test in forEachTests {
-    let s = makeWrappedSequence(test.sequence.map(OpaqueValue.init))
-    var wasInvoked = false
-    var caughtError: Error?
-    var result: OpaqueValue<Int>?
-    do {
-      result = try s._preprocessingPass {
-        () -> OpaqueValue<Int> in
-        wasInvoked = true
-        throw TestError.error2
-      }
-    } catch {
-      caughtError = error
-    }
-    expectNil(result)
-    if wasInvoked {
-      expectEqual(TestError.error2, caughtError as? TestError)
-    }
-  }
-}
-
 //===----------------------------------------------------------------------===//
   } // addSequenceTests
 }

stdlib/private/StdlibCollectionUnittest/LoggingWrappers.swift

@@ -83,9 +83,7 @@ public class SequenceLog {
   public static var prefixWhile = TypeIndexed(0)
   public static var prefixMaxLength = TypeIndexed(0)
   public static var suffixMaxLength = TypeIndexed(0)
-  public static var split = TypeIndexed(0)
   public static var _customContainsEquatableElement = TypeIndexed(0)
-  public static var _preprocessingPass = TypeIndexed(0)
   public static var _copyToContiguousArray = TypeIndexed(0)
   public static var _copyContents = TypeIndexed(0)  
   // Collection
@@ -202,7 +200,6 @@ extension LoggingSequence: LoggingType {
 extension LoggingSequence: Sequence {
   public typealias Element = Base.Element
   public typealias Iterator = LoggingIterator<Base.Iterator>
-  public typealias SubSequence = Base.SubSequence
 
   public func makeIterator() -> Iterator {
     SequenceLog.makeIterator[selfType] += 1
@@ -214,67 +211,11 @@ extension LoggingSequence: Sequence {
     return base.underestimatedCount
   }
 
-  public func dropFirst(_ n: Int) -> SubSequence {
-    SequenceLog.dropFirst[selfType] += 1
-    return base.dropFirst(n)
-  }
-
-  public func dropLast(_ n: Int) -> SubSequence {
-    SequenceLog.dropLast[selfType] += 1
-    return base.dropLast(n)
-  }
-
-  public func drop(
-    while predicate: (Element) throws -> Bool
-  ) rethrows -> SubSequence {
-    SequenceLog.dropWhile[selfType] += 1
-    return try base.drop(while: predicate)
-  }
-
-  public func prefix(_ maxLength: Int) -> SubSequence {
-    SequenceLog.prefixMaxLength[selfType] += 1
-    return base.prefix(maxLength)
-  }
-
-  public func prefix(
-    while predicate: (Element) throws -> Bool
-  ) rethrows -> SubSequence {
-    SequenceLog.prefixWhile[selfType] += 1
-    return try base.prefix(while: predicate)
-  }
-
-  public func suffix(_ maxLength: Int) -> SubSequence {
-    SequenceLog.suffixMaxLength[selfType] += 1
-    return base.suffix(maxLength)
-  }
-
-  public func split(
-    maxSplits: Int = Int.max,
-    omittingEmptySubsequences: Bool = true,
-    whereSeparator isSeparator: (Element) throws -> Bool
-  ) rethrows -> [SubSequence] {
-    SequenceLog.split[selfType] += 1
-    return try base.split(
-      maxSplits: maxSplits,
-      omittingEmptySubsequences: omittingEmptySubsequences,
-      whereSeparator: isSeparator)
-  }
-
   public func _customContainsEquatableElement(_ element: Element) -> Bool? {
     SequenceLog._customContainsEquatableElement[selfType] += 1
     return base._customContainsEquatableElement(element)
   }
 
-  /// If `self` is multi-pass (i.e., a `Collection`), invoke
-  /// `preprocess` on `self` and return its result.  Otherwise, return
-  /// `nil`.
-  public func _preprocessingPass<R>(
-    _ preprocess: () throws -> R
-  ) rethrows -> R? {
-    SequenceLog._preprocessingPass[selfType] += 1
-    return try base._preprocessingPass(preprocess)
-  }
-
   /// Create a native array buffer containing the elements of `self`,
   /// in the same order.
   public func _copyToContiguousArray() -> ContiguousArray<Element> {
@@ -297,6 +238,7 @@ public typealias LoggingCollection<Base: Collection> = LoggingSequence<Base>
 extension LoggingCollection: Collection {  
   public typealias Index = Base.Index
   public typealias Indices = Base.Indices
+  public typealias SubSequence = Base.SubSequence
 
   public var startIndex: Index {
     CollectionLog.startIndex[selfType] += 1

stdlib/private/StdlibUnittest/StdlibUnittest.swift

@@ -499,8 +499,7 @@ public func expectMutableSliceType<X : MutableCollection>(
 /// to be.
 public func expectSequenceAssociatedTypes<X : Sequence>(
   sequenceType: X.Type,
-  iteratorType: X.Iterator.Type,
-  subSequenceType: X.SubSequence.Type
+  iteratorType: X.Iterator.Type
 ) {}
 
 /// Check that all associated types of a `Collection` are what we expect them

stdlib/public/core/CMakeLists.txt

@@ -122,7 +122,6 @@ set(SWIFTLIB_ESSENTIAL
   SipHash.swift
   Sequence.swift
   SequenceAlgorithms.swift
-  SequenceWrapper.swift
   Set.swift
   SetAlgebra.swift
   SetAnyHashableExtensions.swift

stdlib/public/core/Collection.swift

@@ -332,7 +332,7 @@ extension IndexingIterator: IteratorProtocol, Sequence {
 /// 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 where SubSequence: Collection {
+public protocol Collection: Sequence {
   // FIXME: ideally this would be in MigrationSupport.swift, but it needs
   // to be on the protocol instead of as an extension
   @available(*, deprecated/*, obsoleted: 5.0*/, message: "all index distances are now of type Int")
@@ -390,7 +390,10 @@ public protocol Collection: Sequence where SubSequence: Collection {
   /// 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 = Slice<Self> where SubSequence.Index == Index
+  associatedtype SubSequence: Collection = Slice<Self>
+  where SubSequence.Index == Index,
+        Element == SubSequence.Element,
+        SubSequence.SubSequence == SubSequence
 
   /// Accesses the element at the specified position.
   ///
@@ -1223,7 +1226,7 @@ extension Collection {
   ///   `RandomAccessCollection`; otherwise, O(*k*), where *k* is the number of
   ///   elements to drop from the beginning of the collection.
   @inlinable
-  public __consuming func dropFirst(_ k: Int) -> SubSequence {
+  public __consuming func dropFirst(_ k: Int = 1) -> SubSequence {
     _precondition(k >= 0, "Can't drop a negative number of elements from a collection")
     let start = index(startIndex, offsetBy: k, limitedBy: endIndex) ?? endIndex
     return self[start..<endIndex]
@@ -1250,15 +1253,15 @@ extension Collection {
   ///   `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length of
   ///   the collection.
   @inlinable
-  public __consuming func dropLast(_ k: Int) -> SubSequence {
+  public __consuming func dropLast(_ k: Int = 1) -> SubSequence {
     _precondition(
       k >= 0, "Can't drop a negative number of elements from a collection")
     let amount = Swift.max(0, count - k)
     let end = index(startIndex,
       offsetBy: amount, limitedBy: endIndex) ?? endIndex
     return self[startIndex..<end]
   }
-  
+    
   /// Returns a subsequence by skipping elements while `predicate` returns
   /// `true` and returning the remaining elements.
   ///
@@ -1665,13 +1668,3 @@ extension Collection where SubSequence == Self {
     self = self[index(startIndex, offsetBy: k)..<endIndex]
   }
 }
-
-extension Collection {
-  @inlinable
-  @inline(__always)
-  public func _preprocessingPass<R>(
-    _ preprocess: () throws -> R
-  ) rethrows -> R? {
-    return try preprocess()
-  }
-}

stdlib/public/core/Dictionary.swift

@@ -618,6 +618,8 @@ extension Dictionary {
 }
 
 extension Dictionary: Collection {
+  public typealias SubSequence = Slice<Dictionary>
+  
   /// The position of the first element in a nonempty dictionary.
   ///
   /// If the collection is empty, `startIndex` is equal to `endIndex`.
@@ -1321,6 +1323,7 @@ extension Dictionary {
     : Collection, Equatable,
       CustomStringConvertible, CustomDebugStringConvertible {
     public typealias Element = Key
+    public typealias SubSequence = Slice<Dictionary.Keys>
 
     @usableFromInline
     internal var _variant: Dictionary<Key, Value>._Variant