xmake-docs/docs/codes/examples/configuration/custom_rule/basic/xmake.lua

rule("markdown")
    set_extensions(".md")
    on_build_file(function (target, sourcefile, opt)
        print("processing %s", sourcefile)
    end)

target("test")
    set_kind("phony")
    add_files("src/*.md")
    add_rules("markdown")