[Godot] Implement access modifiers, private and protected, to GDScript to protect a class member from being accessed externally.

[Lazy-Rabbit-2001]

This is my first contribution to redot. If there is any mistake, please let me know and I'm here to hear that. :)

Trying implementing redot-proposal-#7.

This pr provides RDScript users a way to protect their members accessibility, and prevent any illegal access from external classes.
We will have @private and @protected annotations in RDScript soon:

Introduction of new annotations

@private

@private annotation allows a member (currently a constant, a variable or a method) to be accessible only within the current class scope:

# In Class A:
@private static var a = 1
# In Class B:
var b = A.a # Error: Could not access external class member "a" because it is private.
# In Class C that inherits from A:
var c = A.a # Error: Could not access external class member "a" because it is private.

@protected

@protected annotation allows a member (currently a constant, a variable or a method) to be accessible only within the current class scope, or the scope of derived classes:

# In Class A:
@protected static var a = 1
# In Class B:
var b = A.a # Error: Could not access external class member "a" because it is protected but accessed from a class that is not derived from "A".
# In Class C that inherits from A:
var c = A.a # OK

Common features:

• A member modified with @private or @protected will hide its documentation from auto-generated doc API, even if you use ## to document it (except @export_*-ed properties and signals).
• These two annotations CANNOT modify local members.

Remaining WIP

• Support protection for signals. (RDScript)
• Move protection in static analyzer into reduce_identifier()(√) and reduce_call(). (RDScriptAnalyzer)
• Make set() (Set() in C#) and get() (Get() in C#) unable to access external protected members. (RDScriptCompiler & RDScriptVM)
• Hide RDScript autocompletion hint of private and protected members, or making private and protected members display in different colors in auto completion hint.
• Discussion about if @private and @protected should be keywords or annotations. Allow @export_* work with @private and @protected

Remaining WIP after being converted into Review

• Footprint for private/protected signals and exported variables in their auto-generated documentations. (RDDoc)
• Not sure whether an invalid access should pop an error or a warning. (Warning / error system)
• Unit tests in .gd and .out files. (Unit test)

Tracking after this PR gets merged:

• More optimizations (Such as speeding up, which needs other contributors' help as I'm just a freshman in C++ programming)

Closes Redot-Engine/redot-proposals#7

[Spartan322]

It is simply GDScript, also what is the reasoning for closing your Godot PR?

[filipworksdev]

This is great! Has anyone tested this ?

Also does this work with autocomplete?

[Lazy-Rabbit-2001]

This is great! Has anyone tested this ?

Also does this work with autocomplete?

Autocomplete, or intelliguess, is planned (in the check list of the top thread). But according to original proposal, it seems that the original proposer wanted to make a ordered list of the autocomplete.
My idea is to hide unaccessible members in a external class that is going to get access to these members. But i've no idea which one will be more preferred. Needs discussion on this.

[Lazy-Rabbit-2001]

Btw, currently a C# script can painlessly access a private or protected member in GDScript, but I haven't checked runtime and vm how C#'s Get() work so that I may track the flow to check its root code and find a solution...