Implement Closure

[HerrCai0907]

Feature suggestion

#798 and #1240 has discussed lots of thing about closure, but maybe there are some outdated information.
So I want to start to discuss it in some new page.

Project State

1. function object will be compiled as static object in data area.
2. function object store function index in table and null env.

High Level Requirement For Closure

1. Every function object from the same FunctionExpression will have different env.
2. When call function object, function can use (read / write) the variable in env and upper function env.
3. (Optional) If function is not a closure function, supporting closure has no or small influence this function in the perspective of code size and spend.

What should we do to support closure

For better code review, I think maybe we can support it step by step. I will post everything we need to implement closure. Please correct my mistakes and omissions.

let upper_function = () => {
  let variable_in_upper_function: i32 = 0;
  let current_function = () => {
    let variable_in_current_function: i32 = 0; // used in lower function.
    variable_in_upper_function = 1; // variable declared in upper function.
    let lower_function = () => {
      variable_in_current_function = 2;
    }
  }
)

code gen

• create function instance for closure function expression. feat: support first class function via builtin function #2752
• get upper function env.
• read/write variable which used by lower function in env instead of local variable.

semantic

• analyze which function expression should enable closure supporting.
• analyze which local variable is used in lower function.
• analyze which identifier is declared in upper function. (already support in resolver.ts)

[alienself]

Any progress on this @HerrCai0907 ?

I personally truly wish we had threads and closure support, I am looking at the AssemblyScript since a few years now, waiting for those features to land to be able to use it more often and I feel like I am not alone.

This project is super exciting and yet we are missing core features for the language to be more broadly used.