When using a `do` block to call an internal iterator, if you forgot to
return a value from the body, it would tell you
error: Do-block body must return bool, but returns () here. Perhaps
you meant to write a `for`-loop?
This advice no longer applies as `for` loops are now for external
iterators. Delete this message outright and let it use the default error
message
error: mismatched types: expected `bool` but found `()`
r? @thestinger
8fff3f40f2