2.x: Add Single.ignoreElement, deprecate toCompletable (#5957)

* 2.x: Add Single.ignoreElement, deprecate toCompletable

* Fix javadoc method name

* Have Single.ignoreElement standard from the start

Author: akarnokd

src/main/java/io/reactivex/Maybe.java

@@ -3084,7 +3084,7 @@ public final Maybe<T> hide() {
     /**
      * Ignores the item emitted by the source Maybe and only calls {@code onComplete} or {@code onError}.
      * <p>
-     * <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/ignoreElements.png" alt="">
+     * <img width="640" height="389" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.ignoreElement.png" alt="">
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code ignoreElement} does not operate by default on a particular {@link Scheduler}.</dd>

src/main/java/io/reactivex/Single.java

@@ -3491,9 +3491,7 @@ public final <R> R to(Function<? super Single<T>, R> convert) {
      * and calls {@code onComplete} when this source {@link Single} calls
      * {@code onSuccess}. Error terminal event is propagated.
      * <p>
-     * <img width="640" height="295" src=
-     * "https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.toCompletable.png"
-     * alt="">
+     * <img width="640" height="436" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.toCompletable.png" alt="">
      * <dl>
      * <dt><b>Scheduler:</b></dt>
      * <dd>{@code toCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
@@ -3503,13 +3501,36 @@ public final <R> R to(Function<? super Single<T>, R> convert) {
      *         calls {@code onSuccess}.
      * @see <a href="http://reactivex.io/documentation/completable.html">ReactiveX documentation: Completable</a>
      * @since 2.0
+     * @deprecated see {@link #ignoreElement()} instead, will be removed in 3.0
      */
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
+    @Deprecated
     public final Completable toCompletable() {
         return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(this));
     }
 
+    /**
+     * Returns a {@link Completable} that ignores the success value of this {@link Single}
+     * and calls {@code onComplete} instead on the returned {@code Completable}.
+     * <p>
+     * <img width="640" height="436" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.ignoreElement.png" alt="">
+     * <dl>
+     * <dt><b>Scheduler:</b></dt>
+     * <dd>{@code ignoreElement} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @return a {@link Completable} that calls {@code onComplete} on it's observer when the source {@link Single}
+     *         calls {@code onSuccess}.
+     * @see <a href="http://reactivex.io/documentation/completable.html">ReactiveX documentation: Completable</a>
+     * @since 2.1.13
+     */
+    @CheckReturnValue
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public final Completable ignoreElement() {
+        return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(this));
+    }
+
     /**
      * Converts this Single into a {@link Flowable}.
      * <p>

src/test/java/io/reactivex/internal/operators/single/SingleMiscTest.java

@@ -254,6 +254,7 @@ public void timeoutOther() throws Exception {
     }
 
     @Test
+    @SuppressWarnings("deprecation")
     public void toCompletable() {
         Single.just(1)
         .toCompletable()
@@ -266,6 +267,19 @@ public void toCompletable() {
         .assertFailure(TestException.class);
     }
 
+    @Test
+    public void ignoreElement() {
+        Single.just(1)
+        .ignoreElement()
+        .test()
+        .assertResult();
+
+        Single.error(new TestException())
+        .ignoreElement()
+        .test()
+        .assertFailure(TestException.class);
+    }
+
     @Test
     public void toObservable() {
         Single.just(1)