Enforce strict field initialization

[dcodeIO]

As a follow-up to #1155, this PR implements strict field initialization and makes it the default, so we always get the safety guarantees we need in AOT. I expect this to be somewhat controversial because it's stricter than TS in potentially unsafe scenarios.

To give a typical example:

class Foo {
  bar: Bar;
  constructor() {
    this.bar = new Bar(this);
    // illegal because ^ `Bar#constructor` can do anything with
    // the partially initialized `this` before `.bar` is assigned
  }
}
class Bar {
  constructor(public foo: Foo) {
  }
}

must instead be written like

class Foo {
  private _bar: Bar | null; // initialized with zero
  constructor() {
    this._bar = new Bar(this);
  }
  get bar(): Bar {
    return assert(this._bar);
  }
}
...

or using definite assignment syntax to automatically inject the assert:

class Foo {
  bar!: Bar;
  constructor() {
    this.bar = new Bar(this);
    // ^ valid because `.bar` is ignored and checked at runtime
  }
}
...

essentially enforcing an actual safety check for the case where Foo#bar is being accessed too early in Bar#constructor.

What this PR doesn't do compared to TS, however, is to enforce strict initialization of fields that can be trivially initialized with zero (which is our undefined), like of type i32, f64 or a nullable type like string | null, so one can leave these uninitialized instead of adding boilerplate assignments, similar to locals or globals.

For more examples of what's allowed and what's not, see:

• Test cases
• Error test cases

• I've read the contributing guidelines
• Fix object literals

fixes #1155

[MaxGraey]

Overall looks pretty good for me

[github-actions]

🎉 This PR is included in version 0.13.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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[jedisct1]

Looks like if a constructor has the @inline attribute, fields it initializes are considered as not being initialized.

Is that intentional?

[dcodeIO]

Sounds like a bug to me. Need to check.