backup-buffer is a compiled Lisp function in `
Make a backup of the disk file visited by the current buffer, if appropriate.
This is normally done before saving the buffer the first time.
A backup may be done by renaming or by copying; see documentation of
variable `make-backup-files'. If it's done by renaming, then the file is
no longer accessible under its old name.
The value is non-nil after a backup was made by renaming.
It has the form (MODES EXTENDED-ATTRIBUTES BACKUPNAME).
MODES is the result of `file-modes' on the original
file; this means that the caller, after saving the buffer, should change
the modes of the new file to agree with the old modes.
EXTENDED-ATTRIBUTES is the result of `file-extended-attributes'
on the original file; this means that the caller, after saving
the buffer, should change the extended attributes of the new file
to agree with the old attributes.
BACKUPNAME is the backup file name, which is the old file renamed.