Fixes Iterable#toString forcing elements

Fixes scala/bug#11785

This changes the implementation of Iterable#toString to not force the evaluation of its elements unless it extends StrictOptimizedIterableOps.

Author: eed3si9n

src/library/scala/collection/Iterable.scala

@@ -68,12 +68,12 @@ trait Iterable[+A] extends IterableOnce[A]
   protected[this] def stringPrefix: String = "Iterable"
 
   /** Converts this $coll to a string.
-    *
-    *  @return   a string representation of this collection. By default this
-    *            string consists of the `className` of this $coll, followed
-    *            by all elements separated by commas and enclosed in parentheses.
     */
-  override def toString = mkString(className + "(", ", ", ")")
+  override def toString: String  =
+    this match {
+      case _: StrictOptimizedIterableOps[_, _, _] => mkString(className + "(", ", ", ")")
+      case _                                      => className + "(<iterable>)"
+    }
 
   /** Analogous to `zip` except that the elements in each collection are not consumed until a strict operation is
     * invoked on the returned `LazyZip2` decorator.

test/junit/scala/collection/IterableTest.scala

@@ -243,7 +243,7 @@ class IterableTest {
       override def className = "Fu"
     }
     val foo = new Foo
-    Assert.assertEquals("Fu()", foo.toString)
+    Assert.assertEquals("Fu(<iterable>)", foo.toString)
   }
 
   @Test
@@ -254,7 +254,7 @@ class IterableTest {
 
     }
     val foo = new Foo
-    Assert.assertEquals("Bar()", foo.toString)
+    Assert.assertEquals("Bar(<iterable>)", foo.toString)
   }
 
   @Test
@@ -266,7 +266,17 @@ class IterableTest {
 
     }
     val foo = new Foo
-    Assert.assertEquals("Fu()", foo.toString)
+    Assert.assertEquals("Fu(<iterable>)", foo.toString)
+  }
+
+  @Test
+  def infiniteIterable: Unit = {
+    class Foo extends Iterable[Int] {
+      def iterator = Iterator.continually(42)
+      override def className = "Fu"
+    }
+    val foo = new Foo
+    Assert.assertEquals("Fu(<iterable>)", foo.toString)
   }
 
   @Test