Re: files.el: Patch to make project-settings optional/customizable

[Richard M. Stallman]

    Because it's not a variable, it has to be a constant.  (And a string
    literal as it was until a few days ago is even better.)
    In a few years someone will see the defvar and will want to turn it into
    a defcustom and we'll have the same discussion again.

defconst seems appropriate in this case.