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

library/src/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 + (if (isEmpty) "()" else "(<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.

library/src/scala/collection/concurrent/TrieMap.scala

@@ -1002,6 +1002,7 @@ final class TrieMap[K, V] private (r: AnyRef, rtupd: AtomicReferenceFieldUpdater
     else cachedSize()
   override def isEmpty: Boolean = size == 0
   override protected[this] def className = "TrieMap"
+  override def toString: String = mkString(className + "(", ", ", ")")
 
 }