refactor: simplify inner and outer subscription

- Reduces memory pressure by no longer capturing outer values where they are not needed. For example, every inner subscription in mergeMap was retaining the outer value, if the outer value was a large array, that could add up
- Makes subscribeToResult less ridiculous, I am not sure what I was thinking there
- Leaves a few operators with more complex inner subscriptions, to be refactored later.
- Removes some redundant code that removed inner subscriptions manually, this was necessary in the early days of RxJS 5, but has not been necessary in a long time

src/internal/innies-and-outies.ts

@@ -0,0 +1,114 @@
+/** @prettier */
+import { Subscription } from './Subscription';
+import { Subscriber } from './Subscriber';
+import { Observable } from './Observable';
+import { subscribeTo } from './util/subscribeTo';
+
+export interface ISimpleOuterSubscriber<T> {
+  /**
+   * A handler for inner next notifications from the inner subscription
+   * @param innerValue the value nexted by the inner producer
+   */
+  notifyNext(innerValue: T): void;
+  /**
+   * A handler for inner error notifications from the inner subscription
+   * @param err the error from the inner producer
+   */
+  notifyError(err: any): void;
+  /**
+   * A handler for inner complete notifications from the inner subscription.
+   */
+  notifyComplete(): void;
+}
+
+export class SimpleInnerSubscriber<T> extends Subscriber<T> {
+  constructor(private parent: ISimpleOuterSubscriber<any>) {
+    super();
+  }
+
+  protected _next(value: T): void {
+    this.parent.notifyNext(value);
+  }
+
+  protected _error(error: any): void {
+    this.parent.notifyError(error);
+    this.unsubscribe();
+  }
+
+  protected _complete(): void {
+    this.parent.notifyComplete();
+    this.unsubscribe();
+  }
+}
+
+export class ComplexInnerSubscriber<T, R> extends Subscriber<R> {
+  constructor(private parent: ComplexOuterSubscriber<T, R>, public outerValue: T, public outerIndex: number) {
+    super();
+  }
+
+  protected _next(value: R): void {
+    this.parent.notifyNext(this.outerValue, value, this.outerIndex, this);
+  }
+
+  protected _error(error: any): void {
+    this.parent.notifyError(error);
+    this.unsubscribe();
+  }
+
+  protected _complete(): void {
+    this.parent.notifyComplete(this);
+    this.unsubscribe();
+  }
+}
+
+export class SimpleOuterSubscriber<T, R> extends Subscriber<T> implements ISimpleOuterSubscriber<R> {
+  notifyNext(innerValue: R): void {
+    this.destination.next(innerValue);
+  }
+
+  notifyError(err: any): void {
+    this.destination.error(err);
+  }
+
+  notifyComplete(): void {
+    this.destination.complete();
+  }
+}
+
+/**
+ * DO NOT USE (formerly "OuterSubscriber")
+ * TODO: We want to refactor this and remove it. It is retaining values it shouldn't for long
+ * periods of time.
+ */
+export class ComplexOuterSubscriber<T, R> extends Subscriber<T> {
+  /**
+   * @param _outerValue Used by: bufferToggle, delayWhen, windowToggle
+   * @param innerValue Used by: subclass default, combineLatest, race, bufferToggle, windowToggle, withLatestFrom
+   * @param _outerIndex Used by: combineLatest, race, withLatestFrom
+   * @param _innerSub Used by: delayWhen
+   */
+  notifyNext(_outerValue: T, innerValue: R, _outerIndex: number, _innerSub: ComplexInnerSubscriber<T, R>): void {
+    this.destination.next(innerValue);
+  }
+
+  notifyError(error: any): void {
+    this.destination.error(error);
+  }
+
+  /**
+   * @param _innerSub Used by: race, bufferToggle, delayWhen, windowToggle, windowWhen
+   */
+  notifyComplete(_innerSub: ComplexInnerSubscriber<T, R>): void {
+    this.destination.complete();
+  }
+}
+
+export function subscribeToResult2(result: any, innerSubscriber: Subscriber<any>): Subscription | undefined {
+  if (innerSubscriber.closed) {
+    return undefined;
+  }
+  if (result instanceof Observable) {
+    return result.subscribe(innerSubscriber);
+  }
+  return subscribeTo(result)(innerSubscriber) as Subscription;
+}