IIUC, the very first option (designated initialisers with custom structs per function) is how Zig does it, and it seems to work well enough there. It's verbose,

IIUC, the very first option (designated initialisers with custom structs per function) is how Zig does it, and it seems to work well enough there. It's verbose, so I wouldn't use it for everything, but it doesn't seem all that unreasonable.