Ensure signatures with thisArg or source observable are deprecated in predicates and projection functions

[benlesh]

Just what it says. filter(fn, obj) is more readable as filter(fn.bind(obj)), because the former doesn't really tell us what obj is for. This exists in various parts of the library. We also have signatures where the source observable is provided in the predicate. That is supremely silly, as I can't even think of a use case there, and a simple closure would have worked (for example, every has both an thisArg and a source).

[OliverJAsh]

+100. I just shipped a bug to production because I accidentally passed an operator as thisArg. Very easy mistake to make:

import { Observable } from 'rxjs';
import { map, shareReplay } from 'rxjs/operators';

declare const ob$: Observable<number>;
// Oops, shareReplay is in the wrong place:
ob$.pipe(map((x) => x + 1, shareReplay()));
// Should have been:
ob$.pipe(map((x) => x + 1), shareReplay());

Even with the improved typings in RxJS v7, it's still possible to make this mistake.

[benlesh]

Core team approval.