`Result value must be used` check

Hello.

Quite often while working with ImmutableJS data structures and other immutable libraries people forget that values are immutable:

They can accidentally write:

obj.set('name', 'newName');

Instead of:

let newObj = obj.set('name', 'newName');

These errors are hard to discover and they are very annoying. I think that this problem can't be solved by tslint, so I propose to add some sort of "you must use the result" check into the compiler. But right now I have no idea how I want it to be expressed in the language, so I create this issue mainly to start a discussion.

Rust lang, for example, solves a similar problem with #[must_use] annotation. This is not exact what we want, but it's a good example and shows that the problem is quite common.

Questa issue è una discussione di progettazione su come esprimere un controllo «must use the result» nel compilatore TypeScript, usando ImmutableJS e l’annotazione #[must_use] di Rust come esempi. Non sono identificati file di implementazione, test o punti di ingresso del compilatore; chiarisci prima la progettazione del linguaggio e il comportamento previsto prima di individuare le aree rilevanti del checker e dei test.