error[E0275]: overflow evaluating the requirement `Option<_>: Sized`
  --> $DIR/mutual-recursion-issue-75860.rs:11:5
   |
LL |     iso(left, right)
   |     ^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`mutual_recursion_issue_75860`)
note: required by a bound in `Option`
  --> $SRC_DIR/core/src/option.rs:LL:COL

error: aborting due to previous error

For more information about this error, try `rustc --explain E0275`.