GDScript: Add C#-style property setter/getter shorthand

[dalexeev]

• Closes Add a shorthand for getter-only properties in GDScript godot-proposals#10360.
• See also a thread on #gdscript.

The following:

var position = Vector2()
var x => position.x
var y => position.y
var length => position.length()

is a shorthand for:

var position = Vector2()

var x:
    set(value):
        position.x = value
    get:
        return position.x

var y:
    set(value):
        position.y = value
    get:
        return position.y

var length:
    set(_value):
        assert(false, 'Cannot assign a value to property "length".')
    get:
        return position.length()

A valid setter is generated for "lvalue" expressions (identifiers and subscripts), for non-"lvalue" expressions the setter has an assert. It is also planned to add an analyzer error when assigning a "readonly" property (not yet implemented).

Type specifiers, type inference, and static variables are supported:

var a: int => _a
var b :=> _b
static var c => _c

This might be useful together with the new @export_tool_button annotation, as there are issues with self referenced lambdas stored in properties of RefCounted-based objects. See godotengine/godot-proposals#11147 and #97834.

Q&A

• Are local variables supported?
  • No, like other setter/getter styles, it only supports member variables. This would require a completely different implementation for local variables and is actually quite controversial if we want to add control flow optimizations in the future.
• Is it possible to have granular control for setter and getter? For example, public getter and private setter?
  • No, GDScript does not support private variables, and you can't do this with other setter/getter styles.
• Is it possible to customize the setter behavior (e.g. emit a signal)?
  • No, customizability makes language design and implementation difficult. Use the full form if the default behavior doesn't suit you. See also Allow setters and getters to take additional arguments #78991.
• Is it possible to exclude the setter generation even if the expression is an "lvalue"?
  • No. See Add support for immutable/one-time assignable variables godot-proposals#820 instead.
• Are setters/getters with the shorthand notation inlined?
  • No, although this form has special qualities that are convenient for potential inlining, it is not currently implemented. Also, removing a frame from the call stack can be inconvenient in debug versions.

TODO

• Open a proposal.
• Check non-"lvalue" identifiers/subscripts (like constants).
• Add static analysis when assigning a "readonly" property.
• Add annotation validation (e.g. @onready doesn't make sense).
• Add tests.