Add documentation for traits to relevant pages

Add trait-related keywords to syntax highlighting

Update style guide and static typing pages

Currently, the style guide uses what seem like good reasonable policies to me (notably, PascalCase adjectives for trait names, and `uses` after `extends`). We'll need to make sure this is agreed-upon as the good way to do it before merging, though

Document behavior of static variables in traits [no ci]

Document behavior of casting to traits

_extensions/gdscript.py

@@ -151,7 +151,9 @@ def innerstring_rules(ttype):
                         "namespace", # Reserved for potential future use.
                         "signal",
                         "static",
-                        "trait", # Reserved for potential future use.
+                        "trait",
+                        "trait_name",
+                        "uses",
                         "var",
                         # Other keywords.
                         "await",

tutorials/scripting/gdscript/gdscript_basics.rst

@@ -42,6 +42,9 @@ here's an example of how GDScript looks.
     # Inheritance:
     extends BaseClass
 
+    # Trait usage:
+    uses Talkable, Flammable
+
 
     # Member variables.
     var a = 5
@@ -103,6 +106,19 @@ here's an example of how GDScript looks.
         super.something(p1, p2)
 
 
+    # When traits are used, they may have to implement abstract
+    # methods defined by the trait:
+    func talk_to():
+        print("Hi, you just talked to me!")
+
+
+    # Traits can also implement methods that the concrete class
+    # can use without defining itself:
+    func yet_another_something():
+        print("I'm going to light on fire!")
+        light_on_fire()
+
+
     # Inner class
     class Something:
         var a = 10
@@ -174,7 +190,13 @@ in case you want to take a look under the hood.
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
 | extends    | Defines what class to extend with the current class.                                                                                              |
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
-| is         | Tests whether a variable extends a given class, or is of a given built-in type.                                                                   |
+| trait      | Defines an inner trait. See `Inner traits`_.                                                                                                      |
++------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
+| trait_name | Defines the script as a globally accessible trait with the specified name. See `Registering named traits`_.                                       |
++------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
+| uses       | Defines what trait(s) the current class should use.                                                                                               |
++------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
+| is         | Tests whether a variable extends a given class, uses a given trait, or is of a given built-in type.                                               |
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
 | in         | Tests whether a value is within a string, array, range, dictionary, or node. When used with ``for``, it iterates through them instead of testing. |
 +------------+---------------------------------------------------------------------------------------------------------------------------------------------------+
@@ -853,15 +875,16 @@ The GDScript static analyzer takes typed arrays into account, however array meth
 ``front()`` and ``back()`` still have the ``Variant`` return type.
 
 Typed arrays have the syntax ``Array[Type]``, where ``Type`` can be any ``Variant`` type,
-native or user class, or enum. Nested array types (like ``Array[Array[int]]``) are not supported.
+native or user class, trait, or enum. Nested array types (like ``Array[Array[int]]``) are not supported.
 
 ::
 
     var a: Array[int]
     var b: Array[Node]
     var c: Array[MyClass]
     var d: Array[MyEnum]
-    var e: Array[Variant]
+    var e: Array[MyTrait]
+    var f: Array[Variant]
 
 ``Array`` and ``Array[Variant]`` are the same thing.
 
@@ -1036,9 +1059,10 @@ Valid types are:
 
 - Built-in types (Array, Vector2, int, String, etc.).
 - Engine classes (Node, Resource, RefCounted, etc.).
-- Constant names if they contain a script resource (``MyScript`` if you declared ``const MyScript = preload("res://my_script.gd")``).
-- Other classes in the same script, respecting scope (``InnerClass.NestedClass`` if you declared ``class NestedClass`` inside the ``class InnerClass`` in the same scope).
+- Constant names if they contain a script or trait resource (``MyScript`` if you declared ``const MyScript = preload("res://my_script.gd")``).
+- Other classes or traits in the same file, respecting scope (``InnerClass.NestedClass`` if you declared ``class NestedClass`` inside the ``class InnerClass`` in the same scope).
 - Script classes declared with the ``class_name`` keyword.
+- Traits declared with the ``trait_name`` keyword.
 - Autoloads registered as singletons.
 
 .. note::
@@ -1104,11 +1128,11 @@ A class member variable can be declared static::
 
     static var a
 
-Static variables belong to the class, not instances. This means that static variables
+Static variables belong to the class or trait, not instances. This means that static variables
 share values between multiple instances, unlike regular member variables.
 
-From inside a class, you can access static variables from any function, both static and non-static.
-From outside the class, you can access static variables using the class or an instance
+From inside a class/trait, you can access static variables from any function, both static and non-static.
+From outside the class/trait, you can access static variables using the class/trait or an instance
 (the second is not recommended as it is less readable).
 
 .. note::
@@ -1179,6 +1203,24 @@ A base class static variable can also be accessed via a child class::
         B.x = 3
         prints(A.x, B.x) # 3 3
 
+.. warning::
+
+    Static variables declared on traits belong to the trait *itself*, and are *not*
+    inherited by classes that use the trait::
+
+        trait HasStatic:
+            static var static_var = 3
+
+        class UsingClass:
+            uses HasStatic
+
+        func _ready():
+            # Will work
+            print(HasStatic.static_var)
+
+            # Will NOT work, as UsingClass does not get static_var from HasStatic
+            print(UsingClass.static_var)
+
 ``@static_unload`` annotation
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1208,15 +1250,15 @@ Values assigned to typed variables must have a compatible type. If it's needed t
 coerce a value to be of a certain type, in particular for object types, you can
 use the casting operator ``as``.
 
-Casting between object types results in the same object if the value is of the
-same type or a subtype of the cast type.
+Casting between object types or traits results in the same object if the value is of the
+same type or a subtype of the cast type, or if the value uses the trait.
 
 ::
 
     var my_node2D: Node2D
     my_node2D = $Sprite2D as Node2D # Works since Sprite2D is a subtype of Node2D.
 
-If the value is not a subtype, the casting operation will result in a ``null`` value.
+If the value is not a subtype or does not use the trait, the casting operation will result in a ``null`` value.
 
 ::
 
@@ -1324,7 +1366,7 @@ or ``0`` if it is the first entry in the enum. Multiple keys with the same value
 Functions
 ---------
 
-Functions always belong to a `class <Classes_>`_. The scope priority for
+Functions always belong to a `class <Classes_>`_ or a `trait <Traits_>`_. The scope priority for
 variable look-up is: local → class member → global. The ``self`` variable is
 always available and is provided as an option for accessing class members, but
 is not always required (and should *not* be sent as the function's first
@@ -1941,6 +1983,11 @@ A class (stored as a file) can inherit from:
 
 Multiple inheritance is not allowed.
 
+.. note:: 
+    Godot 4.x introduces `traits <Traits_>`_ to GDScript, which may cover many of
+    the use cases for multiple inheritance. See their section for more information
+    about how they work.
+
 Inheritance uses the ``extends`` keyword::
 
     # Inherit/extend a globally available class.
@@ -1957,7 +2004,7 @@ Inheritance uses the ``extends`` keyword::
     If inheritance is not explicitly defined, the class will default to inheriting
     :ref:`class_RefCounted`.
 
-To check if a given instance inherits from a given class,
+To check if a given instance inherits from a given class or uses a given trait,
 the ``is`` keyword can be used::
 
     # Cache the enemy class.
@@ -1969,6 +2016,15 @@ the ``is`` keyword can be used::
     if entity is Enemy:
         entity.apply_damage()
 
+    # Cache the Flammable trait.
+    const Flammable = preload("flammable.gdt")
+
+    # [...]
+
+    # Use 'is' to check usage of the trait.
+    if entity is Flammable:
+        entity.light_on_fire()
+
 To call a function in a *super class* (i.e. one ``extend``-ed in your current
 class), use the ``super`` keyword::
 
@@ -2121,6 +2177,136 @@ class resource is done by calling the ``new`` function on the class object::
         var a = MyClass.new()
         a.some_function()
 
+Traits
+------
+
+Traits are collections of behaviors and attributes that classes can use to guarantee
+functionality to themselves and other objects that may be attempting to use them.
+
+Like classes, by default all ``.gdt`` files are unnamed traits, and you must reference
+them using a relative or absolute path.
+::
+    # Use the trait 'interactable.gdt'.
+    uses "res://path/to/interactable.gdt"
+
+Note that traits on their own *cannot* be instantiated the same way that classes can.
+
+.. _doc_gdscript_basics_trait_name:
+
+Registering named traits
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Traits can be given a global name by using the ``trait_name`` keyword.
+::
+    trait_name MyTrait
+    
+Using traits in a class
+~~~~~~~~~~~~~~~~~~~~~~~
+
+For a class to use a trait, use the ``uses`` keyword:
+::
+    class_name MyScript
+    uses MyTrait
+
+
+Traits may also extend classes. If a trait extends a class, then any class
+that uses that trait must also have that class as an ancestor.
+::
+    # movable.gdt
+    trait_name Movable
+    extends PhysicsBody2D
+
+    # character.gd
+    class_name Character
+    extends CharacterBody2D
+    uses Movable  # Allowed, since CharacterBody2D inherits from PhysicsBody2D.
+
+The ``is`` keyword can be used to determine if a given instance uses a particular trait.
+::
+    if entity is Movable:
+        entity.move()
+
+If a trait provides a method signature, but no body, then the using class must implement
+a body for the method.
+::
+    # explosive.gdt
+    trait_name Explosive
+    
+    func explode()  # Body is not defined here, so it must be defined in each class that uses it.
+
+
+    # exploding_barrel.gd
+    class_name ExplodingBarrel
+    extends Sprite2D
+    uses Explosive
+
+    func explode():  # If this definition of Explosive.explode isn't provided, we will get an error.
+        print("Kaboom!")
+        queue_free()
+
+If a trait provides a method signature *and* a body, then the using class inherits it by default
+and doesn't need to provide its own implementation. It still can override the trait's
+implementation if desired, but the parameter count must stay the same, and the parameter and return
+types must be compatible.
+::
+    # damageable.gdt
+    trait_name Damageable
+
+    func take_damage():
+        print("Ouch!")
+
+
+    # invincible_npc.gd
+    class_name InvincibleNPC
+    extends Sprite2D
+    uses Damageable
+
+    # Allowed, and will run instead of Damageable's original take_damage method.
+    func take_damage():
+        print("You can't hurt me!")
+
+..
+    TODO: Confirm these behaviors
+
+Other class members have similar rules:
+
+- Variables and constants can be overridden, as long as the type is compatible and the value is changed.
+- Signals can be overriden, as long as the parameter count is maintained and the parameter types are compatible.
+- Named enums can be overriden and have new enum values.
+
+.. _doc_gdscript_basics_inner_traits:
+
+Inner traits
+~~~~~~~~~~~~
+
+Like inner classes, a class or trait file may contain inner traits, defined with the ``trait``
+keyword. Unlike inner classes, they cannot be instantiated directly, but their name can be
+referenced for using or checking use of themselves.
+::
+    # An inner trait in this class file.
+    trait SomeInnerTrait:
+        func do_something():
+            print("I did something!")
+
+
+    # An inner class in this class file, which uses the inner trait.    
+    class SomeInnerClass:
+        uses SomeInnerTrait
+
+
+    func _init():
+        var c = SomeInnerClass.new()
+        if c is SomeInnerTrait:
+            c.do_something()
+
+.. _doc_gdscript_basics_traits_as_resources:
+
+Traits as resources
+~~~~~~~~~~~~~~~~~~~
+
+Traits stored as files are treated as :ref:`GDTraits <class_GDTrait>`, and must
+be loaded similarly to classes (see `Classes as resources`_).
+
 Exports
 -------