(proclaim '(declaration foo))

(sb-cltl2:define-declaration foo (declaration-specifier environment)
  (declare (ignore environment))
  (format *trace-output* "~&Declaring with declaration specifier ~S~%" declaration-specifier)
  (values :declare `(foo . ,(second declaration-specifier))))

(defmacro get-foo (&environment env)
  (format *trace-output* "~&Retrieved declaration information ~S~%"
          (sb-cltl2:declaration-information 'foo env))
  nil)

(compile nil '(lambda ()
                (declare (foo :hello))
                (get-foo)))
;; Declaring with declaration specifier (FOO :HELLO)
;; Retrieved declaration information :HELLO