[stdlib] Simplify internal DropFirst/PrefixSequence types (#19970)

* Simplify internal DropFirstSequence type

* Simplify internal PrefixSequence type

Author: airspeedswift

stdlib/public/core/Sequence.swift

@@ -646,52 +646,44 @@ extension Sequence where Self.Iterator == Self {
 /// `Base` iterator before possibly returning the first available element.
 ///
 /// The underlying iterator's sequence may be infinite.
-@usableFromInline
 @_fixed_layout
-internal struct _DropFirstSequence<Base : IteratorProtocol>
-    : Sequence, IteratorProtocol {
-
+@usableFromInline
+internal struct DropFirstSequence<Base: Sequence> {
   @usableFromInline
-  internal var _iterator: Base
+  internal let _base: Base
   @usableFromInline
   internal let _limit: Int
-  @usableFromInline
-  internal var _dropped: Int
-
-  @inlinable
-  internal init(_iterator: Base, limit: Int, dropped: Int = 0) {
-    self._iterator = _iterator
-    self._limit = limit
-    self._dropped = dropped
-  }
-
-  @inlinable
-  __consuming internal func makeIterator() -> _DropFirstSequence<Base> {
-    return self
+  
+  @inlinable 
+  public init(_ base: Base, dropping limit: Int) {
+    _precondition(limit >= 0, 
+      "Can't drop a negative number of elements from a sequence")
+    _base = base
+    _limit = limit
   }
+}
 
+extension DropFirstSequence: Sequence {
+  public typealias Element = Base.Element
+  public typealias Iterator = Base.Iterator
+  public typealias SubSequence = AnySequence<Element>
+  
   @inlinable
-  internal mutating func next() -> Base.Element? {
-    while _dropped < _limit {
-      if _iterator.next() == nil {
-        _dropped = _limit
-        return nil
-      }
-      _dropped += 1
-    }
-    return _iterator.next()
+  public __consuming func makeIterator() -> Iterator {
+    var it = _base.makeIterator()
+    var dropped = 0
+    while dropped < _limit, it.next() != nil { dropped &+= 1 }
+    return it
   }
 
   @inlinable
-  internal __consuming func dropFirst(_ k: Int) -> AnySequence<Base.Element> {
+  public __consuming func dropFirst(_ k: Int) -> AnySequence<Element> {
     // If this is already a _DropFirstSequence, we need to fold in
     // the current drop count and drop limit so no data is lost.
     //
     // i.e. [1,2,3,4].dropFirst(1).dropFirst(1) should be equivalent to
     // [1,2,3,4].dropFirst(2).
-    return AnySequence(
-      _DropFirstSequence(
-        _iterator: _iterator, limit: _limit + k, dropped: _dropped))
+    return AnySequence(DropFirstSequence(_base, dropping: _limit + k))
   }
 }
 
@@ -701,47 +693,60 @@ internal struct _DropFirstSequence<Base : IteratorProtocol>
 /// The underlying iterator's sequence may be infinite.
 @_fixed_layout
 @usableFromInline
-internal struct _PrefixSequence<Base : IteratorProtocol>
-    : Sequence, IteratorProtocol {
-  @usableFromInline
-  internal let _maxLength: Int
+internal struct PrefixSequence<Base: Sequence> {
   @usableFromInline
-  internal var _iterator: Base
+  internal var _base: Base
   @usableFromInline
-  internal var _taken: Int
+  internal let _maxLength: Int
 
   @inlinable
-  internal init(_iterator: Base, maxLength: Int, taken: Int = 0) {
-    self._iterator = _iterator
-    self._maxLength = maxLength
-    self._taken = taken
+  public init(_ base: Base, maxLength: Int) {
+    _precondition(maxLength >= 0, "Can't take a prefix of negative length")
+    _base = base
+    _maxLength = maxLength
   }
+}
 
-  @inlinable
-  __consuming internal func makeIterator() -> _PrefixSequence<Base> {
-    return self
-  }
+extension PrefixSequence {
+  @_fixed_layout
+  public struct Iterator {
+    @usableFromInline
+    internal var _base: Base.Iterator
+    @usableFromInline
+    internal var _remaining: Int
+    
+    @inlinable
+    internal init(_ base: Base.Iterator, maxLength: Int) {
+      _base = base
+      _remaining = maxLength
+    }
+  }  
+}
 
+extension PrefixSequence.Iterator: IteratorProtocol {
+  public typealias Element = Base.Element
+  
   @inlinable
-  internal mutating func next() -> Base.Element? {
-    if _taken >= _maxLength { return nil }
-    _taken += 1
-
-    if let next = _iterator.next() {
-      return next
+  internal mutating func next() -> Element? {
+    if _remaining != 0 {
+      _remaining &-= 1
+      return _base.next()
+    } else {
+      return nil
     }
+  }  
+}
 
-    _taken = _maxLength
-    return nil
+extension PrefixSequence: Sequence {
+  @inlinable
+  public __consuming func makeIterator() -> Iterator {
+    return Iterator(_base.makeIterator(), maxLength: _maxLength)
   }
 
   @inlinable
-  internal __consuming func prefix(_ maxLength: Int) -> AnySequence<Base.Element> {
-    return AnySequence(
-      _PrefixSequence(
-        _iterator: _iterator,
-        maxLength: Swift.min(maxLength, self._maxLength),
-        taken: _taken))
+  public __consuming func prefix(_ maxLength: Int) -> AnySequence<Element> {
+    let length = Swift.min(maxLength, self._maxLength)
+    return AnySequence(PrefixSequence(_base, maxLength: length))
   }
 }
 
@@ -1223,9 +1228,7 @@ extension Sequence where SubSequence == AnySequence<Element> {
   ///   the sequence.
   @inlinable
   public __consuming func dropFirst(_ k: Int) -> AnySequence<Element> {
-    _precondition(k >= 0, "Can't drop a negative number of elements from a sequence")
-    if k == 0 { return AnySequence(self) }
-    return AnySequence(_DropFirstSequence(_iterator: makeIterator(), limit: k))
+    return AnySequence(DropFirstSequence(self, dropping: k))
   }
 
   /// Returns a subsequence containing all but the given number of final
@@ -1325,12 +1328,7 @@ extension Sequence where SubSequence == AnySequence<Element> {
   /// - Complexity: O(1)
   @inlinable
   public __consuming func prefix(_ maxLength: Int) -> AnySequence<Element> {
-    _precondition(maxLength >= 0, "Can't take a prefix of negative length from a sequence")
-    if maxLength == 0 {
-      return AnySequence(EmptyCollection<Element>())
-    }
-    return AnySequence(
-      _PrefixSequence(_iterator: makeIterator(), maxLength: maxLength))
+    return AnySequence(PrefixSequence(self, maxLength: maxLength))
   }
 
   /// Returns a subsequence containing the initial, consecutive elements that

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -1,16 +1,39 @@
 
 /* Generic Signature Changes */
+Func _DropFirstSequence.dropFirst(_:) has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
+Func _DropFirstSequence.makeIterator() has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
+Func _PrefixSequence.makeIterator() has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
+Func _PrefixSequence.prefix(_:) has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
+Struct _DropFirstSequence has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
+Struct _PrefixSequence has generic signature change from <τ_0_0 where τ_0_0 : IteratorProtocol> to <τ_0_0 where τ_0_0 : Sequence>
 
 /* RawRepresentable Changes */
 
 /* Removed Decls */
+Constructor _DropFirstSequence.init(_iterator:limit:dropped:) has been removed
+Constructor _PrefixSequence.init(_iterator:maxLength:taken:) has been removed
+Func _DropFirstSequence.next() has been removed
+Func _PrefixSequence.next() has been removed
+Var _DropFirstSequence._dropped has been removed
+Var _DropFirstSequence._iterator has been removed
+Var _PrefixSequence._iterator has been removed
+Var _PrefixSequence._taken has been removed
 
 /* Moved Decls */
+Struct _DropFirstSequence has been renamed to Struct DropFirstSequence
+Struct _PrefixSequence has been renamed to Struct PrefixSequence
 
 /* Renamed Decls */
 
 /* Type Changes */
+Func _DropFirstSequence.makeIterator() has return type change from _DropFirstSequence<τ_0_0> to τ_0_0.Iterator
+Func _PrefixSequence.makeIterator() has return type change from _PrefixSequence<τ_0_0> to PrefixSequence<τ_0_0>.Iterator
+Var DropFirstSequence._base is added to a non-resilient type
+Var PrefixSequence._base is added to a non-resilient type
+Var _PrefixSequence._maxLength in a non-resilient type changes position from 0 to 1
 
 /* Decl Attribute changes */
 
 /* Protocol Requirement Changes */
+Struct _DropFirstSequence has removed conformance to IteratorProtocol
+Struct _PrefixSequence has removed conformance to IteratorProtocol