Use a more efficient SubSequence type for lazy map and filter.

Rather than using the default slice type when slicing the collection produced
by a lazy map or filter, slice the base collection and form a new
lazy map/filter collection from it. This allows any optimizations provided by
the collection SubSequence type to kick in, as well as ensuring that slicing
a lazy collection provides the same type as producing a lazy collection of a
slice.

This is technically source-breaking, because someone could have spelled out
the types of slicing a lazy filter or map… but it seems unlikely to matter
in practice and the benefits could be significant.

Fixes ABI FIXME’s swiftlang#28 and swiftlang#46.

Author: DougGregor

stdlib/public/core/Filter.swift.gyb

@@ -12,8 +12,7 @@
 
 %{
 from gyb_stdlib_support import (
-    collectionForTraversal,
-    sliceTypeName
+    collectionForTraversal
 )
 }%
 
@@ -115,7 +114,6 @@ public typealias LazyFilterIndex<Base : Collection> = Base.Index
 
 % for Traversal in ['Forward', 'Bidirectional']:
 %   Self = "LazyFilter" + collectionForTraversal(Traversal)
-%   Slice = sliceTypeName(traversal=Traversal, mutable=False, rangeReplaceable=False)
 
 /// A lazy `Collection` wrapper that includes the elements of an
 /// underlying collection that satisfy a predicate.
@@ -224,9 +222,11 @@ public struct ${Self}<
     return _base[position]
   }
 
+  public typealias SubSequence = ${Self}<Base.SubSequence>
+
   @_inlineable // FIXME(sil-serialize-all)
-  public subscript(bounds: Range<Index>) -> ${Slice}<${Self}<Base>> {
-    return ${Slice}(base: self, bounds: bounds)
+  public subscript(bounds: Range<Index>) -> SubSequence {
+    return SubSequence(_base: _base[bounds], _predicate)
   }
 
   // Any estimate of the number of elements that pass `_predicate` requires
@@ -247,16 +247,6 @@ public struct ${Self}<
     return _copySequenceToContiguousArray(self)
   }
 
-  // FIXME(ABI)#28 (Associated Types with where clauses): we actually want to add:
-  //
-  //   typealias SubSequence = ${Self}<Base.SubSequence>
-  //
-  // so that all slicing optimizations of the base collection can kick in.
-  //
-  // We can't do that right now though, because that would force a lot of
-  // constraints on `Base.SubSequence`, limiting the possible contexts where
-  // the `.lazy.filter` API can be used.
-
   /// Returns an iterator over the elements of this sequence.
   ///
   /// - Complexity: O(1).

stdlib/public/core/Map.swift.gyb

@@ -13,8 +13,7 @@
 %{
 from gyb_stdlib_support import (
     TRAVERSALS,
-    collectionForTraversal,
-    sliceTypeName
+    collectionForTraversal
 )
 }%
 
@@ -104,7 +103,6 @@ public struct LazyMapSequence<Base : Sequence, Element>
 
 % for Traversal in TRAVERSALS:
 %   Self = "LazyMap" + collectionForTraversal(Traversal)
-%   Slice = sliceTypeName(traversal=Traversal, mutable=False, rangeReplaceable=False)
 
 /// A `Collection` whose elements consist of those in a `Base`
 /// `Collection` passed through a transform function returning `Element`.
@@ -150,21 +148,13 @@ public struct ${Self}<
     return _transform(_base[position])
   }
 
+  public typealias SubSequence = ${Self}<Base.SubSequence, Element>
+
   @_inlineable
-  public subscript(bounds: Range<Base.Index>) -> ${Slice}<${Self}> {
-    return ${Slice}(base: self, bounds: bounds)
+  public subscript(bounds: Range<Base.Index>) -> SubSequence {
+    return SubSequence(_base: _base[bounds], transform: _transform)
   }
 
-  // FIXME(ABI)#46 (Associated Types with where clauses): we actually want to add:
-  //
-  //   typealias SubSequence = ${Self}<Base.SubSequence, Element>
-  //
-  // so that all slicing optimizations of the base collection can kick in.
-  //
-  // We can't do that right now though, because that would force a lot of
-  // constraints on `Base.SubSequence`, limiting the possible contexts where
-  // the `.lazy.map` API can be used.
-
   public typealias IndexDistance = Base.IndexDistance
 
   public typealias Indices = Base.Indices

validation-test/stdlib/Lazy.swift.gyb

@@ -874,8 +874,7 @@ tests.test("LazyMapCollection/AssociatedTypes") {
   expectCollectionAssociatedTypes(
     collectionType: Subject.self,
     iteratorType: LazyMapIterator<Base.Iterator, OpaqueValue<Int32>>.self,
-    // FIXME(ABI)#77 (Associated Types with where clauses): SubSequence should be `LazyMapCollection<Base.Slice>`.
-    subSequenceType: Slice<Subject>.self,
+    subSequenceType: LazyMapCollection<Base.SubSequence, OpaqueValue<Int32>>.self,
     indexType: Base.Index.self,
     indexDistanceType: Base.IndexDistance.self,
     indicesType: Base.Indices.self)
@@ -887,8 +886,7 @@ tests.test("LazyMapBidirectionalCollection/AssociatedTypes") {
   expectBidirectionalCollectionAssociatedTypes(
     collectionType: Subject.self,
     iteratorType: LazyMapIterator<Base.Iterator, OpaqueValue<Int32>>.self,
-    // FIXME(ABI)#78 (Associated Types with where clauses): SubSequence should be `LazyMapBidirectionalCollection<Base.Slice>`.
-    subSequenceType: BidirectionalSlice<Subject>.self,
+    subSequenceType: LazyMapBidirectionalCollection<Base.SubSequence, OpaqueValue<Int32>>.self,
     indexType: Base.Index.self,
     indexDistanceType: Base.IndexDistance.self,
     indicesType: Base.Indices.self)
@@ -900,8 +898,7 @@ tests.test("LazyMapRandomAccessCollection/AssociatedTypes") {
   expectRandomAccessCollectionAssociatedTypes(
     collectionType: Subject.self,
     iteratorType: LazyMapIterator<Base.Iterator, OpaqueValue<Int32>>.self,
-    // FIXME(ABI)#79 (Associated Types with where clauses): SubSequence should be `LazyMapRandomAccessCollection<Base.Slice>`.
-    subSequenceType: RandomAccessSlice<Subject>.self,
+    subSequenceType: LazyMapRandomAccessCollection<Base.SubSequence, OpaqueValue<Int32>>.self,
     indexType: Base.Index.self,
     indexDistanceType: Base.IndexDistance.self,
     indicesType: Base.Indices.self)
@@ -1167,8 +1164,7 @@ tests.test("LazyFilterCollection/AssociatedTypes") {
   expectCollectionAssociatedTypes(
     collectionType: Subject.self,
     iteratorType: LazyFilterIterator<Base.Iterator>.self,
-    // FIXME(ABI)#80 (Associated Types with where clauses): SubSequence should be `LazyFilterCollection<Base.Slice>`.
-    subSequenceType: Slice<Subject>.self,
+    subSequenceType: LazyFilterCollection<Base.SubSequence>.self,
     indexType: LazyFilterIndex<Base>.self,
     indexDistanceType: Base.IndexDistance.self,
     indicesType: DefaultIndices<Subject>.self)
@@ -1180,8 +1176,7 @@ tests.test("LazyFilterBidirectionalCollection/AssociatedTypes") {
   expectBidirectionalCollectionAssociatedTypes(
     collectionType: Subject.self,
     iteratorType: LazyFilterIterator<Base.Iterator>.self,
-    // FIXME(ABI)#81 (Associated Types with where clauses): SubSequence should be `LazyFilterBidirectionalCollection<Base.Slice>`.
-    subSequenceType: BidirectionalSlice<Subject>.self,
+    subSequenceType: LazyFilterBidirectionalCollection<Base.SubSequence>.self,
     indexType: LazyFilterIndex<Base>.self,
     indexDistanceType: Base.IndexDistance.self,
     indicesType: DefaultBidirectionalIndices<Subject>.self)