error: unexpected expression: `|x| [b; x]`
  --> $DIR/issue-90878-2.rs:1:7
   |
LL |  #![l=|x|[b;x ]]
   |       ^^^^^^^^^

error: cannot find attribute `l` in this scope
  --> $DIR/issue-90878-2.rs:1:5
   |
LL |  #![l=|x|[b;x ]]
   |     ^

error: aborting due to 2 previous errors