decode-coding-inserted-region is a compiled Lisp function in `mule.el'.

(decode-coding-inserted-region FROM TO FILENAME &optional VISIT BEG END REPLACE)

Decode the region between FROM and TO as if it is read from file FILENAME.
The idea is that the text between FROM and TO was just inserted somehow.
Optional arguments VISIT, BEG, END, and REPLACE are the same as those
of the function `insert-file-contents'.
Part of the job of this function is setting `buffer-undo-list' appropriately.