cl-struct-slot-offset is a compiled Lisp function in `cl-macs.el'.

(cl-struct-slot-offset STRUCT-TYPE SLOT-NAME)

Return the offset of slot SLOT-NAME in STRUCT-TYPE.
The returned zero-based slot index is relative to the start of
the structure data type and is adjusted for any structure name
and :initial-offset slots. Signal error if struct STRUCT-TYPE
does not contain SLOT-NAME.