The question is more like where else would you put your backups?

No, it is not.

  #[allow(unused_assignments)]

Thank you. This works!

It doesn’t work, at least, on rustc 1.75.

Clippy didn’t tell anything about the macro.

warning: dereferencing a tuple pattern where every element takes a reference
  --> src/lib.rs:13:9
   |
13 |         &Some(ref cons_rc) => {
   |         ^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrowed_reference
   = note: `#[warn(clippy::needless_borrowed_reference)]` on by default
help: try removing the `&` and `ref` parts
   |
13 -         &Some(ref cons_rc) => {
13 +         Some(cons_rc) => {
   |

To put #[allow(this_linting_rule)] like this:

    [ $x:expr, $( $y:expr ),* ] => {
	#[allow(unused_assignments)]
	{

I got error[E0658]: attributes on expressions are experimental.

To put it like this:

#[macro_export]
#[allow(unused_assignments)]
macro_rules! list {
    () => {
	None

It doesn’t work.