let rec x = [| x |]; 1.;;
[%%expect{|
Line _, characters 12-19:
Warning 10: this expression should have type unit.
val x : float = 1.
|}];;

let rec x = let u = [|y|] in 10. and y = 1.;;
[%%expect{|
Line _, characters 16-17:
Warning 26: unused variable u.
val x : float = 10.
val y : float = 1.
|}];;