Check for type recursion without boxing (#3407)

testsuite/tests/letrec-check/unboxed.ml

@@ -23,14 +23,17 @@ Line 2, characters 12-19:
 Error: This kind of expression is not allowed as right-hand side of "let rec"
 |}];;
 
+(* This test was made to error by disallowing singleton recursive unboxed types.
+   We keep it in case these are re-allowed, in which case it should error with:
+   [This kind of expression is not allowed as right-hand side of "let rec"] *)
 type r = A of r [@@unboxed]
 let rec y = A y;;
 [%%expect{|
-type r = A of r [@@unboxed]
-Line 2, characters 12-15:
-2 | let rec y = A y;;
-                ^^^
-Error: This kind of expression is not allowed as right-hand side of "let rec"
+Line 1, characters 0-27:
+1 | type r = A of r [@@unboxed]
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: The definition of "r" is recursive without boxing:
+         "r" contains "r"
 |}];;
 
 (* This test is not allowed if 'a' is unboxed, but should be accepted

testsuite/tests/typing-layouts-products/letrec.ml

@@ -6,14 +6,17 @@
  }
 *)
 
+(* This test was made to error by disallowing singleton recursive unboxed types.
+   We keep it in case these are re-allowed, in which case it should error with:
+   [This kind of expression is not allowed as right-hand side of "let rec"] *)
 type t : value = #{ t : t }
 let rec t = #{ t = t }
 [%%expect{|
-type t = #{ t : t; }
-Line 2, characters 12-22:
-2 | let rec t = #{ t = t }
-                ^^^^^^^^^^
-Error: This kind of expression is not allowed as right-hand side of "let rec"
+Line 1, characters 0-27:
+1 | type t : value = #{ t : t }
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: The definition of "t" is recursive without boxing:
+         "t" contains "t"
 |}]
 
 type bx = { bx : ubx }