> Having a static type system is orthogonal to banning mutation, isn't it?

It is, but effect types let you have mutation with some guarantees that
the mutations don't step on each other's toes.

> If you don't see it that drastic, feel free to suggest different names
> that you think would better carry the meaning of the "perform" form. I
> just defy calling the form "unsafe". There's nothing unsafe about it
> in the usual sense of unsafety. (For, otherwise, every procedure
> evaluation and every "let" form would be unsafe, as well as would be
> "map", etc.) The name should not be too complicated because people
> should be encouraged to use the form whenever it carries the semantic
> content better than "begin".

Since those basic features can be out-of-order in Scheme you have a
point. It's too late to change the fundamentals in RnRS.

How about the name "independently"? That describes what it does while
not being too short or too long.

(define (set-car+cdr! p x y)
   (assert (pair? p))
   (independently
     (set-car! p x)
     (set-cdr! p y)))

(let ((bv (make-bytevector 36)))
   (independently
     (bytevector-u32-native-set! bv 32 offset)
     (bytevector-u32-native-set! bv 28 (xid-id shmseg))
     (bytevector-u8-set! bv 27 0)
     (bytevector-u8-set! bv 26 (if send-event 1 0))
     (bytevector-u8-set! bv 25 format)
     (bytevector-u8-set! bv 24 depth)
     (bytevector-s16-native-set! bv 22 dst-y)
     (bytevector-s16-native-set! bv 20 dst-x)
     (bytevector-u16-native-set! bv 18 src-height)
     (bytevector-u16-native-set! bv 16 src-width)
     (bytevector-u16-native-set! bv 14 src-y)
     (bytevector-u16-native-set! bv 12 src-x)
     (bytevector-u16-native-set! bv 10 total-height)
     (bytevector-u16-native-set! bv 8 total-width)
     (bytevector-u32-native-set! bv 4 (xid-id gc))
     (bytevector-u32-native-set! bv 0 (xid-id drawable)))
   bv)