src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @Deprecated
    public final Completable toCompletable() {
        return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(this));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @Deprecated
    public final Completable toCompletable() {
        return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(this));
    }
Returns a {@link Completable} that discards result of the {@link Single}
and calls {@code onComplete} when this source {@link Single} calls
{@code onSuccess}. Error terminal event is propagated.
<p>
<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>
</dl>

@return a {@link Completable} that calls {@code onComplete} on it's subscriber when the source {@link Single}
calls {@code onSuccess}.
@since 2.0
@deprecated see {@link #ignoreElement()} instead, will be removed in 3.0