[Stdlib] Add a test for the performance of CollectionType.first

test/1_stdlib/Collection.swift

@@ -1,6 +1,13 @@
-// RUN: %target-run-simple-swift | FileCheck %s
+// RUN: %target-run-simple-swift --stdlib-unittest-in-process | tee %t.txt
+// RUN: FileCheck %s < %t.txt
+// note: remove the --stdlib-unittest-in-process once all the FileCheck tests
+// have been converted to StdlibUnittest
 // REQUIRES: executable_test
 
+import StdlibUnittest
+
+var CollectionTests = TestSuite("CollectionTests")
+
 struct X : CollectionType {
   typealias Element = String.CharacterView.Generator.Element
   typealias Index = String.Index
@@ -202,5 +209,38 @@ func testIsEmptyFirstLast() {
 }
 testIsEmptyFirstLast()
 
+/// A `CollectionType` that vends just the default implementations for
+/// `CollectionType` methods.
+struct CollectionOnly<T: CollectionType> : CollectionType {
+  var base: T
+
+  var startIndex: T.Index {
+    return base.startIndex
+  }
+
+  var endIndex: T.Index {
+    return base.endIndex
+  }
+
+  func generate() -> T.Generator {
+    return base.generate()
+  }
+
+  subscript(position: T.Index) -> T.Generator.Element {
+    return base[position]
+  }
+}
+
 // CHECK: all done.
 print("all done.")
+
+CollectionTests.test("first/performance") {
+  // accessing `first` should not perform duplicate work on lazy collections
+  var log: [Int] = []
+  let col_ = (0..<10).lazy.filter({ log.append($0); return (2..<8).contains($0) })
+  let col = CollectionOnly(base: col_)
+  expectEqual(2, col.first)
+  expectEqual([0, 1, 2], log)
+}
+
+runAllTests()