Fix lifter problems

[LPTK]

TODO

• Member classes are not lifted correctly: they are no longer accessible from the original owner (outer definition).
• The current lifter has these comment:

  // the current problem is that we need extra code to find which variables were really defined by a function
  // this may be resolved in the future when the IR gets explicit variable declarations

  // TODO: let declarations inside loops (also broken without class lifting)
  // I'll fix it once it's fixed in the IR since we will have more tools to determine
  // what locals belong to what block.

  We should add explicit variable declarations and simplify this code.
• Ordering problems introduced by the lifter will be solved by fixing Reorder simple classes and require let class on the others #278
• Also see all the test cases in hkmc2/shared/src/test/mlscript/backlog/Lifter.mls
• Modules are not lifted at all. In the future, we want to introduce parameterized modules, which would allow us to properly lift them. Though It's not clear if we want modules inside other definitions in the first place.
• Lifting functions with spread arguments (i.e. fun f(...rest)) is broken.
• This test does not work:

  class A(x) with
    class B(y) with
      fun getB() = x + y
    fun getA() = this.B(2).getB()
  A(1).getA()

  as B is lifted out and the selection this.B does not have a FieldSymbol, meaning we can't properly detect references to the class B. See the relevant test in the backlog for a possible lifting method.
  See Class Lifter #266 (comment)

[LPTK]

• I just noticed that the lifter generates isMut parameters all over the place even when they're clearly useless, eg:

  :sir
  fun f(x) =
    class C with
      fun y = x
    new C
  //│ ———————————————| Lowered IR |———————————————————————————————————————————————————————————————————————
  //│ let C⁰, f⁰, C$⁰;
  //│ define C$⁰ as fun C$¹(isMut, x) {
  //│   let tmp;
  //│   match isMut
  //│     true =>
  //│       set tmp = new mut C¹(x);
  //│       end
  //│     else
  //│       set tmp = new C¹(x);
  //│       end
  //│   return tmp
  //│ };
  //│ define C⁰ as class C¹ {
  //│   private val C$cap⁰;
  //│   private val x⁰;
  //│   constructor {
  //│     set x⁰ = x;
  //│     end
  //│   }
  //│   method y⁰ = fun y¹ { return x⁰ }
  //│ };
  //│ define f⁰ as fun f¹(x1) { return C$¹(false, x1) };
  //│ end
  //│ —————————————————| Output |—————————————————————————————————————————————————————————————————————————

  Why are we even generating a function in the first place, here? Why don't we just directly return new C¹(x)? @CAG2Mark
  There is also a strange useless private val C$cap⁰.

[CAG2Mark]

• I just noticed that the lifter generates isMut parameters all over the place even when they're clearly useless, eg:
  :sir
  fun f(x) =
  class C with
  fun y = x
  new C
  //│ ———————————————| Lowered IR |———————————————————————————————————————————————————————————————————————
  //│ let C⁰, f⁰, C$⁰;
  //│ define C$⁰ as fun C$¹(isMut, x) {
  //│ let tmp;
  //│ match isMut
  //│ true =>
  //│ set tmp = new mut C¹(x);
  //│ end
  //│ else
  //│ set tmp = new C¹(x);
  //│ end
  //│ return tmp
  //│ };
  //│ define C⁰ as class C¹ {
  //│ private val C$cap⁰;
  //│ private val x⁰;
  //│ constructor {
  //│ set x⁰ = x;
  //│ end
  //│ }
  //│ method y⁰ = fun y¹ { return x⁰ }
  //│ };
  //│ define f⁰ as fun f¹(x1) { return C$¹(false, x1) };
  //│ end
  //│ —————————————————| Output |—————————————————————————————————————————————————————————————————————————

  Why are we even generating a function in the first place, here? Why don't we just directly return new C¹(x)? @CAG2Mark
  There is also a strange useless private val C$cap⁰.

IIRC, initial reason for generating such a function was to support first-class classes. As it stands, lifted classes must store their extra parameters in an auxiliary parameter list that comes after the main list due to the rendering semantics, but this list should be at the front if trying to support first-class classes. Regardless of this, for classes without a parameter list, I guess it's not really needed and it can be removed.

I forgot how this made it into the rewrite, but I think it was so that Instantiate could be rewritten into a single function call rather than a sequence of assignments and function calls.

Honestly, the whole mut/non-mut thing is quite messy with the lifter, and I think Object.freeze does not even work with curried ctors.

The private val C$cap⁰ thing was something I forgot to remove when the class doesn't capture anything. I can remove it.

[LPTK]

We never want to allow lifting classes that are used as values. The lifter already complains in that case. I guess what you really mean is "using the function constructor of parameterized classes as a value". There, it's fine to generate a function. But the function should only be generated if the class is parameterized and if its function constructor is used as a value.

The private val C$cap⁰ thing was something I forgot to remove when the class doesn't capture anything. I can remove it.

👍

[LPTK]

Oh, and BTW, the constructor function of parameterized classes always constructs immutable instances, so you should never need anything like the isMut parameter.