fix(compiler): no error when trying to reassign to a let variable

[yoav-steinberg]

now this:

let x = 9;
x = 10;

produces something like: Error at ../../examples/tests/invalid/reassing_to_nonreassignable.w:2:1 | Variable x is not reassignable

Also correctly import reassignability (let/var) from JSII class properties.

see #1186

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Monada Contribution License.

[yoav-steinberg]

Note that this is currently failing because one of our tests attempt to write to an argument passed to a function, but there's currently no way of specifying if an argument is reassignable:

let f = (x: num) => {
  x = x + 1; // This write fails because `x` is non reassignable
};

Should we suppor the following syntax to fix this? @eladb @Chriscbr WDYT?

let f = (var x: num) => {
  x = x + 1;
};

See related TypeScript and Kotlin discussions

[Chriscbr]

@yoav-steinberg I'd be okay adding that syntax later if there is demand for it, though I feel like it's generally safer for parameters to not be reassignable by default.

[Chriscbr]

One suggestion to reduce some verbosity - otherwise LGTM. I like the var argument idea as well 👍

[Chriscbr]

Also, can you update the spec to mention this feature as well? We should keep it in sync 👍

[yoav-steinberg]

Update: fixed everything related to to function arguments not being var.
Also added support for modifying non-var class fields in init so we can initialize classes (this also caused test failures).
Also added support (and tests) for checking reassignablity of class fields.
Additionally added support for mutable JSII class fields by marking them as reassignable on import.

[eladb]

@yoav-steinberg is this ready to be merged?

[yoav-steinberg]

@yoav-steinberg is this ready to be merged?

Not yet, I woke up this morning having a feeling this might break:

resource R {
  f: num;
  init() {
    if true {
      this.f = 1; // Access a non reassignable field from constructor but not in upper most scope. Need to make sure this works!
    }
  }
}

[monadabot]

Congrats! 🚀 This was released in Wing 0.4.121.