(defmacro with-replaced-function (fdef &rest body)
  (let ((oldf (gensym))
        (result (gensym))
        (name (car fdef))
        (args (cadr fdef))
        (rbody (cddr fdef)))
    `(let ((,oldf (symbol-function ',name)))
       (setf (symbol-function ',name) (lambda ,args ,@rbody))
       (let ((,result (progn ,@body)))
         (setf (symbol-function ',name) ,oldf)
         ,result))))

(defmacro show (x)
  `(format t "~a --> ~a~%"
           ',x ,x))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(defun foo (x y) (+ x y))

(defun bar (x) (foo x (* x 2)))

(show (bar 42))

(show (with-replaced-function (foo (x y) (* x y))
                              (bar 42)))

(show (bar 42))

> (defun b () 'original)
B
> (setf f #'b)
#<Compiled-function B #xC2C1546>
> (defun a () (funcall f))
A
> (a)
ORIGINAL
> (setf f #'(lambda () 'stub))
#<Anonymous Function #xC2D990E>
> (a)
STUB
> (setf f #'b)
#<Compiled-function B #xC2C1546>
> (a)
ORIGINAL

(defmacro with-fun (origfn mockfn &body body)
  `(let ((it ,origfn))
      (setf ,origfn ,mockfn)
     ,@body
      (setf ,origfn ,it)))

(ql:quickload :mockingbird)