diff --git a/libraries/stdlib/src/generated/ArraysFromJLangIterables.kt b/libraries/stdlib/src/generated/ArraysFromJLangIterables.kt
index d8e2e51a4d1..a4a77729aa4 100644
--- a/libraries/stdlib/src/generated/ArraysFromJLangIterables.kt
+++ b/libraries/stdlib/src/generated/ArraysFromJLangIterables.kt
@@ -28,7 +28,7 @@ public inline fun <T> Array<T>.any(predicate: (T) -> Boolean) : Boolean {
 /**
  * Appends the string from all the elements separated using the *separator* and using the given *prefix* and *postfix* if supplied
  *
- * @includeFunctionBody ../../test/CollectionTest.kt makeString
+ * @includeFunctionBody ../../test/CollectionTest.kt appendString
  */
 public inline fun <T> Array<T>.appendString(buffer: Appendable, separator: String = ", ", prefix: String = "", postfix: String = "", limit: Int = -1): Unit {
     buffer.append(prefix)
@@ -180,22 +180,25 @@ public inline fun <T> Array<T>.reverse() : List<T> {
 }
 
 /** Copies all elements into the given collection */
-public inline fun <T, C: Collection<T>> Array<T>.to(result: C) : C {
+public inline fun <in T, C: Collection<in T>> Array<T>.to(result: C) : C {
     for (element in this) result.add(element)
     return result
 }
 
-/** Copies all elements into a [[LinkedList]] */
-public inline fun <T> Array<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
+/** Copies all elements into a [[LinkedList]]  */
+public inline fun <in T> Array<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
 
 /** Copies all elements into a [[List]] */
-public inline fun <T> Array<T>.toList() : List<T> = to(ArrayList<T>())
+public inline fun <in T> Array<T>.toList() : List<T> = to(ArrayList<T>())
+
+/** Copies all elements into a [[List] */
+public inline fun <in T> Array<T>.toCollection() : Collection<T> = to(ArrayList<T>())
 
 /** Copies all elements into a [[Set]] */
-public inline fun <T> Array<T>.toSet() : Set<T> = to(HashSet<T>())
+public inline fun <in T> Array<T>.toSet() : Set<T> = to(HashSet<T>())
 
 /** Copies all elements into a [[SortedSet]] */
-public inline fun <T> Array<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
+public inline fun <in T> Array<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
 
 /**
   TODO figure out necessary variance/generics ninja stuff... :)
diff --git a/libraries/stdlib/src/generated/ArraysFromJLangIterablesLazy.kt b/libraries/stdlib/src/generated/ArraysFromJLangIterablesLazy.kt
index 9e5b7f7d59c..f3bbcbc9cba 100644
--- a/libraries/stdlib/src/generated/ArraysFromJLangIterablesLazy.kt
+++ b/libraries/stdlib/src/generated/ArraysFromJLangIterablesLazy.kt
@@ -42,6 +42,23 @@ public inline fun <T> Array<T?>?.filterNotNull() : List<T> = filterNotNullTo<T,
  */
 public inline fun <T, R> Array<T>.flatMap(transform: (T)-> Collection<R>) : Collection<R> = flatMapTo(ArrayList<R>(), transform)
 
+/**
+ * Returns a list containing all the non-*null* elements, throwing an [[IllegalArgumentException]] if there are any null elements
+ *
+ * @includeFunctionBody ../../test/CollectionTest.kt requireNoNulls
+ */
+public inline fun <in T> Array<T?>?.requireNoNulls() : List<T> {
+    val list = ArrayList<T>()
+    for (element in this) {
+        if (element == null) {
+            throw IllegalArgumentException("null element found in $this")
+        } else {
+            list.add(element)
+        }
+    }
+    return list
+}
+
 /**
  * Returns a list containing the first *n* elements
  *
diff --git a/libraries/stdlib/src/generated/JUnilIterablesFromJUtilCollections.kt b/libraries/stdlib/src/generated/JUtilIterablesFromJUtilCollections.kt
similarity index 100%
rename from libraries/stdlib/src/generated/JUnilIterablesFromJUtilCollections.kt
rename to libraries/stdlib/src/generated/JUtilIterablesFromJUtilCollections.kt
diff --git a/libraries/stdlib/src/generated/JUnilIteratorsFromJLangIterables.kt b/libraries/stdlib/src/generated/JUtilIteratorsFromJLangIterables.kt
similarity index 90%
rename from libraries/stdlib/src/generated/JUnilIteratorsFromJLangIterables.kt
rename to libraries/stdlib/src/generated/JUtilIteratorsFromJLangIterables.kt
index 7acb05ebde7..a147f2efe82 100644
--- a/libraries/stdlib/src/generated/JUnilIteratorsFromJLangIterables.kt
+++ b/libraries/stdlib/src/generated/JUtilIteratorsFromJLangIterables.kt
@@ -26,7 +26,7 @@ public inline fun <T> java.util.Iterator<T>.any(predicate: (T) -> Boolean) : Boo
 /**
  * Appends the string from all the elements separated using the *separator* and using the given *prefix* and *postfix* if supplied
  *
- * @includeFunctionBody ../../test/CollectionTest.kt makeString
+ * @includeFunctionBody ../../test/CollectionTest.kt appendString
  */
 public inline fun <T> java.util.Iterator<T>.appendString(buffer: Appendable, separator: String = ", ", prefix: String = "", postfix: String = "", limit: Int = -1): Unit {
     buffer.append(prefix)
@@ -178,22 +178,25 @@ public inline fun <T> java.util.Iterator<T>.reverse() : List<T> {
 }
 
 /** Copies all elements into the given collection */
-public inline fun <T, C: Collection<T>> java.util.Iterator<T>.to(result: C) : C {
+public inline fun <in T, C: Collection<in T>> java.util.Iterator<T>.to(result: C) : C {
     for (element in this) result.add(element)
     return result
 }
 
-/** Copies all elements into a [[LinkedList]] */
-public inline fun <T> java.util.Iterator<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
+/** Copies all elements into a [[LinkedList]]  */
+public inline fun <in T> java.util.Iterator<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
 
 /** Copies all elements into a [[List]] */
-public inline fun <T> java.util.Iterator<T>.toList() : List<T> = to(ArrayList<T>())
+public inline fun <in T> java.util.Iterator<T>.toList() : List<T> = to(ArrayList<T>())
+
+/** Copies all elements into a [[List] */
+public inline fun <in T> java.util.Iterator<T>.toCollection() : Collection<T> = to(ArrayList<T>())
 
 /** Copies all elements into a [[Set]] */
-public inline fun <T> java.util.Iterator<T>.toSet() : Set<T> = to(HashSet<T>())
+public inline fun <in T> java.util.Iterator<T>.toSet() : Set<T> = to(HashSet<T>())
 
 /** Copies all elements into a [[SortedSet]] */
-public inline fun <T> java.util.Iterator<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
+public inline fun <in T> java.util.Iterator<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
 
 /**
   TODO figure out necessary variance/generics ninja stuff... :)
diff --git a/libraries/stdlib/src/generated/StandardFromJLangIterables.kt b/libraries/stdlib/src/generated/StandardFromJLangIterables.kt
index 6ca1df36585..29a82738db7 100644
--- a/libraries/stdlib/src/generated/StandardFromJLangIterables.kt
+++ b/libraries/stdlib/src/generated/StandardFromJLangIterables.kt
@@ -28,7 +28,7 @@ public inline fun <T> Iterable<T>.any(predicate: (T) -> Boolean) : Boolean {
 /**
  * Appends the string from all the elements separated using the *separator* and using the given *prefix* and *postfix* if supplied
  *
- * @includeFunctionBody ../../test/CollectionTest.kt makeString
+ * @includeFunctionBody ../../test/CollectionTest.kt appendString
  */
 public inline fun <T> Iterable<T>.appendString(buffer: Appendable, separator: String = ", ", prefix: String = "", postfix: String = "", limit: Int = -1): Unit {
     buffer.append(prefix)
@@ -180,22 +180,25 @@ public inline fun <T> Iterable<T>.reverse() : List<T> {
 }
 
 /** Copies all elements into the given collection */
-public inline fun <T, C: Collection<T>> Iterable<T>.to(result: C) : C {
+public inline fun <in T, C: Collection<in T>> Iterable<T>.to(result: C) : C {
     for (element in this) result.add(element)
     return result
 }
 
-/** Copies all elements into a [[LinkedList]] */
-public inline fun <T> Iterable<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
+/** Copies all elements into a [[LinkedList]]  */
+public inline fun <in T> Iterable<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
 
 /** Copies all elements into a [[List]] */
-public inline fun <T> Iterable<T>.toList() : List<T> = to(ArrayList<T>())
+public inline fun <in T> Iterable<T>.toList() : List<T> = to(ArrayList<T>())
+
+/** Copies all elements into a [[List] */
+public inline fun <in T> Iterable<T>.toCollection() : Collection<T> = to(ArrayList<T>())
 
 /** Copies all elements into a [[Set]] */
-public inline fun <T> Iterable<T>.toSet() : Set<T> = to(HashSet<T>())
+public inline fun <in T> Iterable<T>.toSet() : Set<T> = to(HashSet<T>())
 
 /** Copies all elements into a [[SortedSet]] */
-public inline fun <T> Iterable<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
+public inline fun <in T> Iterable<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
 
 /**
   TODO figure out necessary variance/generics ninja stuff... :)
diff --git a/libraries/stdlib/src/generated/StandardFromJLangIterablesLazy.kt b/libraries/stdlib/src/generated/StandardFromJLangIterablesLazy.kt
index b4c66b93d7a..3fbfefb60fd 100644
--- a/libraries/stdlib/src/generated/StandardFromJLangIterablesLazy.kt
+++ b/libraries/stdlib/src/generated/StandardFromJLangIterablesLazy.kt
@@ -42,6 +42,23 @@ public inline fun <T> Iterable<T?>?.filterNotNull() : List<T> = filterNotNullTo<
  */
 public inline fun <T, R> Iterable<T>.flatMap(transform: (T)-> Collection<R>) : Collection<R> = flatMapTo(ArrayList<R>(), transform)
 
+/**
+ * Returns a list containing all the non-*null* elements, throwing an [[IllegalArgumentException]] if there are any null elements
+ *
+ * @includeFunctionBody ../../test/CollectionTest.kt requireNoNulls
+ */
+public inline fun <in T> Iterable<T?>?.requireNoNulls() : List<T> {
+    val list = ArrayList<T>()
+    for (element in this) {
+        if (element == null) {
+            throw IllegalArgumentException("null element found in $this")
+        } else {
+            list.add(element)
+        }
+    }
+    return list
+}
+
 /**
  * Returns a list containing the first *n* elements
  *
diff --git a/libraries/stdlib/src/kotlin/Iterators.kt b/libraries/stdlib/src/kotlin/Iterators.kt
index 84e8435185d..53a5dd04765 100644
--- a/libraries/stdlib/src/kotlin/Iterators.kt
+++ b/libraries/stdlib/src/kotlin/Iterators.kt
@@ -110,6 +110,19 @@ private class FlatMapIterator<T, R>(val iterator : java.util.Iterator<T>, val tr
     }
 }
 
+/**
+ * Returns an iterator containing all the non-*null* elements, lazily throwing an [[IllegalArgumentException]]
+ if there are any null elements
+ *
+ * @includeFunctionBody ../../test/iterators/IteratorsTest.kt requireNoNulls
+ */
+public inline fun <in T> java.util.Iterator<T?>.requireNoNulls(): java.util.Iterator<T> {
+    return map<T?, T>{
+        if (it == null) throw IllegalArgumentException("null element in iterator $this") else it
+    }
+}
+
+
 /**
  * Returns an iterator restricted to the first *n* elements
  *
diff --git a/libraries/stdlib/src/kotlin/JLangIterables.kt b/libraries/stdlib/src/kotlin/JLangIterables.kt
index bbd0e979971..5f1bf8009a2 100644
--- a/libraries/stdlib/src/kotlin/JLangIterables.kt
+++ b/libraries/stdlib/src/kotlin/JLangIterables.kt
@@ -182,20 +182,20 @@ public inline fun <in T, C: Collection<in T>> java.lang.Iterable<T>.to(result: C
     return result
 }
 
-/** Copies all elements into a [[LinkedList]] if its not already a [[LinkedList]]  */
-public inline fun <in T> java.lang.Iterable<T>.toLinkedList() : LinkedList<T> = if (this is LinkedList<T>) this else to(LinkedList<T>())
+/** Copies all elements into a [[LinkedList]]  */
+public inline fun <in T> java.lang.Iterable<T>.toLinkedList() : LinkedList<T> = to(LinkedList<T>())
 
-/** Copies all elements into a [[List]] if its not already a [[List]] */
-public inline fun <in T> java.lang.Iterable<T>.toList() : List<T> = if (this is List<T>) this else to(ArrayList<T>())
+/** Copies all elements into a [[List]] */
+public inline fun <in T> java.lang.Iterable<T>.toList() : List<T> = to(ArrayList<T>())
 
-/** Copies all elements into a [[List]] if it is not already a [[Collection]] */
-public inline fun <in T> java.lang.Iterable<T>.toCollection() : Collection<T> = if (this is Collection<T>) this else to(ArrayList<T>())
+/** Copies all elements into a [[List] */
+public inline fun <in T> java.lang.Iterable<T>.toCollection() : Collection<T> = to(ArrayList<T>())
 
-/** Copies all elements into a [[Set]] if its not already a [[Set]] */
-public inline fun <in T> java.lang.Iterable<T>.toSet() : Set<T> = if (this is Set<T>) this else to(HashSet<T>())
+/** Copies all elements into a [[Set]] */
+public inline fun <in T> java.lang.Iterable<T>.toSet() : Set<T> = to(HashSet<T>())
 
-/** Copies all elements into a [[SortedSet]] if its not already a [[SortedSet]] */
-public inline fun <in T> java.lang.Iterable<T>.toSortedSet() : SortedSet<T> = if (this is SortedSet<T>) this else to(TreeSet<T>())
+/** Copies all elements into a [[SortedSet]] */
+public inline fun <in T> java.lang.Iterable<T>.toSortedSet() : SortedSet<T> = to(TreeSet<T>())
 
 /**
   TODO figure out necessary variance/generics ninja stuff... :)
diff --git a/libraries/stdlib/src/kotlin/JLangIterablesLazy.kt b/libraries/stdlib/src/kotlin/JLangIterablesLazy.kt
index 5829abe2dd5..8804b1cb7d7 100644
--- a/libraries/stdlib/src/kotlin/JLangIterablesLazy.kt
+++ b/libraries/stdlib/src/kotlin/JLangIterablesLazy.kt
@@ -39,6 +39,23 @@ public inline fun <T> java.lang.Iterable<T?>?.filterNotNull() : List<T> = filter
  */
 public inline fun <T, R> java.lang.Iterable<T>.flatMap(transform: (T)-> Collection<R>) : Collection<R> = flatMapTo(ArrayList<R>(), transform)
 
+/**
+ * Returns a list containing all the non-*null* elements, throwing an [[IllegalArgumentException]] if there are any null elements
+ *
+ * @includeFunctionBody ../../test/CollectionTest.kt requireNoNulls
+ */
+public inline fun <in T> java.lang.Iterable<T?>?.requireNoNulls() : List<T> {
+    val list = ArrayList<T>()
+    for (element in this) {
+        if (element == null) {
+            throw IllegalArgumentException("null element found in $this")
+        } else {
+            list.add(element)
+        }
+    }
+    return list
+}
+
 /**
  * Returns a list containing the first *n* elements
  *
diff --git a/libraries/stdlib/src/kotlin/test/Test.kt b/libraries/stdlib/src/kotlin/test/Test.kt
index 5183ac193c4..9abd522b90e 100644
--- a/libraries/stdlib/src/kotlin/test/Test.kt
+++ b/libraries/stdlib/src/kotlin/test/Test.kt
@@ -101,13 +101,15 @@ public fun fails(block: ()-> Unit): Throwable? {
 }
 
 /** Asserts that a block fails with a specific exception being thrown */
-public fun <T: Throwable> failsWith(block: ()-> Unit) {
+public fun <T: Throwable> failsWith(block: ()-> Any): T {
     try {
         block()
         asserter.fail("Expected an exception to be thrown")
+        throw IllegalStateException("Should have failed")
     } catch (e: T) {
         //println("Caught expected exception: $e")
         // OK
+        return e
     }
 }
 
diff --git a/libraries/stdlib/test/CollectionTest.kt b/libraries/stdlib/test/CollectionTest.kt
index b2f3aea751a..5d82b637a7b 100644
--- a/libraries/stdlib/test/CollectionTest.kt
+++ b/libraries/stdlib/test/CollectionTest.kt
@@ -271,6 +271,18 @@ class CollectionTest {
 */
     }
 
+    test fun requireNoNulls() {
+        val data = arrayList<String?>("foo", "bar")
+        val notNull = data.requireNoNulls()
+        assertEquals(arrayList("foo", "bar"), notNull)
+
+        val hasNulls = arrayList("foo", null, "bar")
+        failsWith<IllegalArgumentException> {
+            // should throw an exception as we have a null
+            hasNulls.requireNoNulls()
+        }
+    }
+
     test fun reverse() {
         val data = arrayList("foo", "bar")
         val rev = data.reverse()
diff --git a/libraries/stdlib/test/GenerateStandardLib.kt b/libraries/stdlib/test/GenerateStandardLib.kt
index bb238bc645f..5e9470c716d 100644
--- a/libraries/stdlib/test/GenerateStandardLib.kt
+++ b/libraries/stdlib/test/GenerateStandardLib.kt
@@ -63,7 +63,7 @@ fun main(args: Array<String>) {
         it.replaceAll("java.lang.Iterable<T", "Iterable<T")
     }
 
-    generateFile(File(outDir, "JUnilIteratorsFromJLangIterables.kt"), "package kotlin", File(srcDir, "JLangIterables.kt")) {
+    generateFile(File(outDir, "JUtilIteratorsFromJLangIterables.kt"), "package kotlin", File(srcDir, "JLangIterables.kt")) {
         it.replaceAll("java.lang.Iterable<T", "java.util.Iterator<T")
     }
 
@@ -74,7 +74,7 @@ fun main(args: Array<String>) {
         it.replaceAll("java.util.Collection<T", "Array<T")
     }
 
-    generateFile(File(outDir, "JUnilIterablesFromJUtilCollections.kt"), "package kotlin", File(srcDir, "JUtilCollections.kt")) {
+    generateFile(File(outDir, "JUtilIterablesFromJUtilCollections.kt"), "package kotlin", File(srcDir, "JUtilCollections.kt")) {
         it.replaceAll("java.util.Collection<T", "java.lang.Iterable<T").replaceAll("(this.size)", "")
     }
 
diff --git a/libraries/stdlib/test/iterators/IteratorsTest.kt b/libraries/stdlib/test/iterators/IteratorsTest.kt
index 43cc51920bf..e788a658e9a 100644
--- a/libraries/stdlib/test/iterators/IteratorsTest.kt
+++ b/libraries/stdlib/test/iterators/IteratorsTest.kt
@@ -1,7 +1,8 @@
 package iterators
 
 import kotlin.test.assertEquals
-import org.junit.Test
+import org.junit.Test as test
+import kotlin.test.failsWith
 
 class IteratorsTest {
 
@@ -11,24 +12,24 @@ class IteratorsTest {
         return iterate<Int> { when (index++) { 0 -> a; 1 -> b; else -> { val result = a + b; a = b; b = result; result } } }
     }
 
-    Test fun filterAndTakeWhileExtractTheElementsWithinRange() {
+    test fun filterAndTakeWhileExtractTheElementsWithinRange() {
         assertEquals(arrayList(144, 233, 377, 610, 987), fibonacci().filter { it > 100 }.takeWhile { it < 1000 }.toList())
     }
 
-    Test fun foldReducesTheFirstNElements() {
+    test fun foldReducesTheFirstNElements() {
         val sum = { (a: Int, b: Int) -> a + b }
         assertEquals(arrayList(13, 21, 34, 55, 89).fold(0, sum), fibonacci().filter { it > 10 }.take(5).fold(0, sum))
     }
 
-    Test fun takeExtractsTheFirstNElements() {
+    test fun takeExtractsTheFirstNElements() {
         assertEquals(arrayList(0, 1, 1, 2, 3, 5, 8, 13, 21, 34), fibonacci().take(10).toList())
     }
 
-    Test fun mapAndTakeWhileExtractTheTransformedElements() {
+    test fun mapAndTakeWhileExtractTheTransformedElements() {
         assertEquals(arrayList(0, 3, 3, 6, 9, 15), fibonacci().map { it * 3 }.takeWhile { (i: Int) -> i < 20 }.toList())
     }
 
-    Test fun flatMapAndTakeExtractTheTransformedElements() {
+    test fun flatMapAndTakeExtractTheTransformedElements() {
         fun intToBinaryDigits() = { (i: Int) ->
             val binary = Integer.toBinaryString(i).sure()
             var index = 0
@@ -47,11 +48,24 @@ class IteratorsTest {
         assertEquals(expected, fibonacci().flatMap<Int, Char>(intToBinaryDigits()).take(10).toList())
     }
 
-    Test fun joinConcatenatesTheFirstNElementsAboveAThreshold() {
+    test fun joinConcatenatesTheFirstNElementsAboveAThreshold() {
         assertEquals("13, 21, 34, 55, 89, ...", fibonacci().filter { it > 10 }.makeString(separator = ", ", limit = 5))
     }
 
-    Test fun toStringJoinsNoMoreThanTheFirstTenElements() {
+    test fun requireNoNulls() {
+        val iter = arrayList<String?>("foo", "bar").iterator()
+        val notNull = iter.requireNoNulls()
+        assertEquals(arrayList("foo", "bar"), notNull.toList())
+
+        val iterWithNulls = arrayList("foo", null, "bar").iterator()
+        val notNull2 = iterWithNulls.requireNoNulls()
+        failsWith<IllegalArgumentException> {
+            // should throw an exception as we have a null
+            notNull2.toList()
+        }
+    }
+
+    test fun toStringJoinsNoMoreThanTheFirstTenElements() {
         assertEquals("0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...", fibonacci().makeString(limit = 10))
         assertEquals("13, 21, 34, 55, 89, 144, 233, 377, 610, 987, ...", fibonacci().filter {  it > 10 }.makeString(limit = 10))
         assertEquals("144, 233, 377, 610, 987", fibonacci().filter { it > 100 }.takeWhile { it < 1000 }.makeString())