GDScript: Replace abstract keyword with @abstract annotation

Co-authored-by: Danil Alexeev <[email protected]>

Author: aaronfranke

modules/gdscript/doc_classes/@GDScript.xml

@@ -295,6 +295,27 @@
 		</constant>
 	</constants>
 	<annotations>
+		<annotation name="@abstract">
+			<return type="void" />
+			<description>
+				Marks a class or a method as abstract.
+				An abstract class is a class that cannot be instantiated directly. Instead, it is meant to be inherited by other classes. Attempting to instantiate an abstract class will result in an error.
+				An abstract method is a method that has no implementation. Therefore, a newline or a semicolon is expected after the function header. This defines a contract that inheriting classes must conform to, because the method signature must be compatible when overriding.
+				Inheriting classes must either provide implementations for all abstract methods, or the inheriting class must be marked as abstract. If a class has at least one abstract method (either its own or an unimplemented inherited one), then it must also be marked as abstract. However, the reverse is not true: an abstract class is allowed to have no abstract methods.
+				[codeblock]
+				@abstract class Shape:
+					@abstract func draw()
+
+				class Circle extends Shape:
+					func draw():
+						print("Drawing a circle.")
+
+				class Square extends Shape:
+					func draw():
+						print("Drawing a square.")
+				[/codeblock]
+			</description>
+		</annotation>
 		<annotation name="@export">
 			<return type="void" />
 			<description>

modules/gdscript/gdscript.cpp

@@ -2749,7 +2749,6 @@ Vector<String> GDScriptLanguage::get_reserved_words() const {
 		"when",
 		"while",
 		// Declarations.
-		"abstract",
 		"class",
 		"class_name",
 		"const",
@@ -2915,7 +2914,7 @@ String GDScriptLanguage::get_global_class_name(const String &p_path, String *r_b
 		*r_icon_path = c->simplified_icon_path;
 	}
 	if (r_is_abstract) {
-		*r_is_abstract = false;
+		*r_is_abstract = c->is_abstract;
 	}
 	if (r_is_tool) {
 		*r_is_tool = parser.is_tool();

modules/gdscript/gdscript_analyzer.cpp

@@ -1540,12 +1540,12 @@ void GDScriptAnalyzer::resolve_class_body(GDScriptParser::ClassNode *p_class, co
 					if (member.function->is_abstract) {
 						if (base_class == p_class) {
 							const String class_name = p_class->identifier == nullptr ? p_class->fqcn.get_file() : String(p_class->identifier->name);
-							push_error(vformat(R"*(Class "%s" is not abstract but contains abstract methods. Mark the class as abstract or remove "abstract" from all methods in this class.)*", class_name), p_class);
+							push_error(vformat(R"*(Class "%s" is not abstract but contains abstract methods. Mark the class as "@abstract" or remove "@abstract" from all methods in this class.)*", class_name), p_class);
 							break;
 						} else if (!implemented_funcs.has(member.function->identifier->name)) {
 							const String class_name = p_class->identifier == nullptr ? p_class->fqcn.get_file() : String(p_class->identifier->name);
 							const String base_class_name = base_class->identifier == nullptr ? base_class->fqcn.get_file() : String(base_class->identifier->name);
-							push_error(vformat(R"*(Class "%s" must implement "%s.%s()" and other inherited abstract methods or be marked as abstract.)*", class_name, base_class_name, member.function->identifier->name), p_class);
+							push_error(vformat(R"*(Class "%s" must implement "%s.%s()" and other inherited abstract methods or be marked as "@abstract".)*", class_name, base_class_name, member.function->identifier->name), p_class);
 							break;
 						}
 					} else {
@@ -1987,6 +1987,20 @@ void GDScriptAnalyzer::resolve_function_body(GDScriptParser::FunctionNode *p_fun
 	}
 	p_function->resolved_body = true;
 
+	if (p_function->is_abstract) {
+		// Abstract functions don't have a body.
+		if (!p_function->body->statements.is_empty()) {
+			push_error(R"(Abstract function cannot have a body.)", p_function->body);
+		}
+		return;
+	} else {
+		// Non-abstract functions must have a body.
+		if (p_function->body->statements.is_empty()) {
+			push_error(R"(A function must either have a ":" followed by a body, or be marked as "@abstract".)", p_function);
+			return;
+		}
+	}
+
 	GDScriptParser::FunctionNode *previous_function = parser->current_function;
 	parser->current_function = p_function;
 
@@ -1999,7 +2013,7 @@ void GDScriptAnalyzer::resolve_function_body(GDScriptParser::FunctionNode *p_fun
 		// Use the suite inferred type if return isn't explicitly set.
 		p_function->set_datatype(p_function->body->get_datatype());
 	} else if (p_function->get_datatype().is_hard_type() && (p_function->get_datatype().kind != GDScriptParser::DataType::BUILTIN || p_function->get_datatype().builtin_type != Variant::NIL)) {
-		if (!p_function->is_abstract && !p_function->body->has_return && (p_is_lambda || p_function->identifier->name != GDScriptLanguage::get_singleton()->strings._init)) {
+		if (!p_function->body->has_return && (p_is_lambda || p_function->identifier->name != GDScriptLanguage::get_singleton()->strings._init)) {
 			push_error(R"(Not all code paths return a value.)", p_function);
 		}
 	}

modules/gdscript/gdscript_editor.cpp

@@ -1523,7 +1523,7 @@ static void _find_identifiers(const GDScriptParser::CompletionContext &p_context
 
 	static const char *_keywords_with_space[] = {
 		"and", "not", "or", "in", "as", "class", "class_name", "extends", "is", "func", "signal", "await",
-		"const", "enum", "abstract", "static", "var", "if", "elif", "else", "for", "match", "when", "while",
+		"const", "enum", "static", "var", "if", "elif", "else", "for", "match", "when", "while",
 		nullptr
 	};