Add const qualifier support for function arguments in shaders

This prevents the function argument from being reassigned within
the function.

Example:

    void test(const int t) {}

drivers/gles2/shader_compiler_gles2.cpp

@@ -64,6 +64,13 @@ static String _prestr(SL::DataPrecision p_pres) {
 	return "";
 }
 
+static String _constr(bool p_is_const) {
+	if (p_is_const) {
+		return "const ";
+	}
+	return "";
+}
+
 static String _qualstr(SL::ArgumentQualifier p_qual) {
 	switch (p_qual) {
 		case SL::ARGUMENT_QUALIFIER_IN:
@@ -254,6 +261,8 @@ void ShaderCompilerGLES2::_dump_function_deps(const SL::ShaderNode *p_node, cons
 				header += ", ";
 			}
 
+			header += _constr(fnode->arguments[i].is_const);
+
 			if (fnode->arguments[i].type == SL::TYPE_STRUCT) {
 				header += _qualstr(fnode->arguments[i].qualifier) + _mkid(fnode->arguments[i].type_str) + " " + _mkid(fnode->arguments[i].name);
 			} else {
@@ -425,7 +434,7 @@ String ShaderCompilerGLES2::_dump_node_code(const SL::Node *p_node, int p_level,
 
 			for (int i = 0; i < snode->vconstants.size(); i++) {
 				String gcode;
-				gcode += "const ";
+				gcode += _constr(true);
 				if (snode->vconstants[i].type == SL::TYPE_STRUCT) {
 					gcode += _mkid(snode->vconstants[i].type_str);
 				} else {
@@ -517,9 +526,7 @@ String ShaderCompilerGLES2::_dump_node_code(const SL::Node *p_node, int p_level,
 			SL::VariableDeclarationNode *var_dec_node = (SL::VariableDeclarationNode *)p_node;
 
 			StringBuffer<> declaration;
-			if (var_dec_node->is_const) {
+			declaration += _constr(var_dec_node->is_const);
-				declaration += "const ";
-			}
 			if (var_dec_node->datatype == SL::TYPE_STRUCT) {
 				declaration += _mkid(var_dec_node->struct_name);
 			} else {

drivers/gles3/shader_compiler_gles3.cpp

@@ -213,6 +213,13 @@ static String _prestr(SL::DataPrecision p_pres) {
 	return "";
 }
 
+static String _constr(bool p_is_const) {
+	if (p_is_const) {
+		return "const ";
+	}
+	return "";
+}
+
 static String _qualstr(SL::ArgumentQualifier p_qual) {
 	switch (p_qual) {
 		case SL::ARGUMENT_QUALIFIER_IN:
@@ -373,6 +380,9 @@ void ShaderCompilerGLES3::_dump_function_deps(const SL::ShaderNode *p_node, cons
 			if (i > 0) {
 				header += ", ";
 			}
+
+			header += _constr(fnode->arguments[i].is_const);
+
 			if (fnode->arguments[i].type == SL::TYPE_STRUCT) {
 				header += _qualstr(fnode->arguments[i].qualifier) + _mkid(fnode->arguments[i].type_str) + " " + _mkid(fnode->arguments[i].name);
 			} else {
@@ -562,7 +572,7 @@ String ShaderCompilerGLES3::_dump_node_code(const SL::Node *p_node, int p_level,
 			for (int i = 0; i < pnode->vconstants.size(); i++) {
 				const SL::ShaderNode::Constant &cnode = pnode->vconstants[i];
 				String gcode;
-				gcode += "const ";
+				gcode += _constr(true);
 				if (pnode->vconstants[i].type == SL::TYPE_STRUCT) {
 					gcode += _mkid(cnode.type_str);
 				} else {
@@ -655,9 +665,7 @@ String ShaderCompilerGLES3::_dump_node_code(const SL::Node *p_node, int p_level,
 			SL::VariableDeclarationNode *vdnode = (SL::VariableDeclarationNode *)p_node;
 
 			String declaration;
-			if (vdnode->is_const) {
+			declaration += _constr(vdnode->is_const);
-				declaration += "const ";
-			}
 			if (vdnode->datatype == SL::TYPE_STRUCT) {
 				declaration += _mkid(vdnode->struct_name);
 			} else {

servers/visual/shader_language.cpp

@@ -953,6 +953,9 @@ bool ShaderLanguage::_find_identifier(const BlockNode *p_block, const Map<String
 				if (r_struct_name) {
 					*r_struct_name = function->arguments[i].type_str;
 				}
+				if (r_is_const) {
+					*r_is_const = function->arguments[i].is_const;
+				}
 				return true;
 			}
 		}
@@ -3436,11 +3439,13 @@ ShaderLanguage::Node *ShaderLanguage::_parse_expression(BlockNode *p_block, cons
 							for (int i = 0; i < call_function->arguments.size(); i++) {
 								int argidx = i + 1;
 								if (argidx < func->arguments.size()) {
-									if (call_function->arguments[i].qualifier == ArgumentQualifier::ARGUMENT_QUALIFIER_OUT || call_function->arguments[i].qualifier == ArgumentQualifier::ARGUMENT_QUALIFIER_INOUT) {
+									if (call_function->arguments[i].is_const || call_function->arguments[i].qualifier == ArgumentQualifier::ARGUMENT_QUALIFIER_OUT || call_function->arguments[i].qualifier == ArgumentQualifier::ARGUMENT_QUALIFIER_INOUT) {
 										bool error = false;
 										Node *n = func->arguments[argidx];
 										if (n->type == Node::TYPE_CONSTANT || n->type == Node::TYPE_OPERATOR) {
-											error = true;
+											if (!call_function->arguments[i].is_const) {
+												error = true;
+											}
 										} else if (n->type == Node::TYPE_ARRAY) {
 											ArrayNode *an = static_cast<ArrayNode *>(n);
 											if (an->call_expression != nullptr || an->is_const) {
@@ -6482,15 +6487,29 @@ Error ShaderLanguage::_parse_shader(const Map<StringName, FunctionInfo> &p_funct
 						break;
 					}
 
+					bool is_const = false;
+					if (tk.type == TK_CONST) {
+						is_const = true;
+						tk = _get_token();
+					}
+
 					ArgumentQualifier qualifier = ARGUMENT_QUALIFIER_IN;
 
 					if (tk.type == TK_ARG_IN) {
 						qualifier = ARGUMENT_QUALIFIER_IN;
 						tk = _get_token();
 					} else if (tk.type == TK_ARG_OUT) {
+						if (is_const) {
+							_set_error("'out' qualifier cannot be used within a function parameter declared with 'const'.");
+							return ERR_PARSE_ERROR;
+						}
 						qualifier = ARGUMENT_QUALIFIER_OUT;
 						tk = _get_token();
 					} else if (tk.type == TK_ARG_INOUT) {
+						if (is_const) {
+							_set_error("'inout' qualifier cannot be used within a function parameter declared with 'const'.");
+							return ERR_PARSE_ERROR;
+						}
 						qualifier = ARGUMENT_QUALIFIER_INOUT;
 						tk = _get_token();
 					}
@@ -6568,6 +6587,7 @@ Error ShaderLanguage::_parse_shader(const Map<StringName, FunctionInfo> &p_funct
 					arg.type_str = param_struct_name;
 					arg.precision = pprecision;
 					arg.qualifier = qualifier;
+					arg.is_const = is_const;
 
 					func_node->arguments.push_back(arg);
 
@@ -6977,6 +6997,10 @@ Error ShaderLanguage::complete(const String &p_code, const Map<StringName, Funct
 						if (j == completion_argument) {
 							calltip += CharType(0xFFFF);
 						}
+
+						if (shader->functions[i].function->arguments[j].is_const) {
+							calltip += "const ";
+						}
 					}
 
 					if (shader->functions[i].function->arguments.size()) {

servers/visual/shader_language.h

@@ -554,6 +554,7 @@ public:
 			DataType type;
 			StringName type_str;
 			DataPrecision precision;
+			bool is_const;
 		};
 
 		StringName name;