Non-nil means protect against I/O errors while saving files.
Some modes set this non-nil in particular buffers.
This feature works by writing the new contents into a temporary file
and then renaming the temporary file to replace the original.
In this way, any I/O error in writing leaves the original untouched,
and there is never any instant where the file is nonexistent.
Note that this feature forces backups to be made by copying.
Yet, at the same time, saving a precious file
breaks any hard links between it and other files.
This feature is advisory: for example, if the directory in which the
file is being saved is not writable, Emacs may ignore a non-nil value
of `file-precious-flag' and write directly into the file. See also: `break-hardlink-on-save'.