error[E0596]: cannot borrow `foo` as mutable, as it is not declared as mutable
  --> $DIR/span-covering-argument-1.rs:5:14
   |
LL |             let $s = 0;
   |                 -- help: consider changing this to be mutable: `mut foo`
LL |             *&mut $s = 0;
   |              ^^^^^^^ cannot borrow as mutable
...
LL |     bad!(foo whatever);
   |     ------------------ in this macro invocation
   |
   = note: this error originates in the macro `bad` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.