Drop the unused init@ field from classes with a self identifier #5284

F# adds a hidden init@<line> field to a class whose constructor has a self identifier (type P() as this or new () as self). .NET uses it to throw InvalidOperationException when this is used before the object is fully initialised. The check is a FailInit() call inside the member bodies, so M throws when called from P's constructor:

type P() as this =
    let x = this.M()
    member _.M() = 1
    member _.X = x

Fable emits the field and sets it, but never reads it. The typed tree Fable gets from FCS doesn't contain the FailInit() check, so there is nothing to translate. On JS:

export class P {
    constructor() {
        const this$ = new FSharpRef(defaultOf());
        this$.contents = this;
        this.x = (P__M(this$.contents) | 0);
        this["init@3"] = 1;
    }
}

export function P__M(_) {
    return 1;
}

With an explicit constructor (#5235), the field is also a constructor parameter:

export class E {
    constructor(Count, init$00408$002D1) {
        this.Count = (Count | 0);
        this["init@8-1"] = (init$00408$002D1 | 0);
    }
}

export function E_$ctor_Z524259A4(a) {
    const self = new FSharpRef(defaultOf());
    let this$_1;
    const this$ = new E(a, 0);
    self.contents = this$;
    this$["init@8-1"] = 1;
    // ...
}

Python emits the same shape (self.init_004039 = init_004039).

Since Fable can't use the field, we could drop it from the output: the class fields, the constructor parameter, the argument in the record construction and the Set. That saves a property per object and a write per construction.

Something to check when doing it: #5235 uses the init@ field of a precompiled class to detect that its constructor has a self identifier, because precompiled libraries don't keep constructor bodies. Removing it from the Fable AST must not remove that signal.