`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.

Este issue es una discusión de diseño sobre cómo expresar una comprobación de «must use the result» en el compilador de TypeScript, usando ImmutableJS y la anotación #[must_use] de Rust como ejemplos. No se identifican archivos de implementación, tests ni puntos de entrada del compilador; primero aclara el diseño del lenguaje y el comportamiento esperado antes de localizar las áreas relevantes del checker y de los tests.