Add package core:flags

core/flags/validation.odin

@@ -0,0 +1,37 @@
+package flags
+
+/*
+Check a flag after parsing, during the validation stage.
+
+Inputs:
+- model: A raw pointer to the data structure provided to `parse`.
+- name: The name of the flag being checked.
+- value: An `any` type that contains the value to be checked.
+- args_tag: The `args` tag from within the struct.
+
+Returns:
+- error: An error message, or an empty string if no error occurred.
+*/
+Custom_Flag_Checker :: #type proc(
+	model:    rawptr,
+	name:     string,
+	value:    any,
+	args_tag: string,
+) -> (
+	error: string,
+)
+
+@(private)
+global_custom_flag_checker: Custom_Flag_Checker
+
+/*
+Set the global custom flag checker.
+
+Note that only one can be active at a time.
+
+Inputs:
+- checker: The flag checker. Pass `nil` to disable any previously set checker.
+*/
+register_flag_checker :: proc(checker: Custom_Flag_Checker) {
+	global_custom_flag_checker = checker
+}