error: detected cyclic import while importing "b.proto"
  --> a.proto:4:1
   |
 4 | import "b.proto";
   | ^^^^^^^^^^^^^^^^^ imported here
   |
  ::: b.proto:4:1
   |
 4 | import "c.proto";
   | ----------------- ...which imports "c.proto"...
   |
  ::: c.proto:4:1
   |
 4 | import "a.proto";
   | ----------------- ...which imports "a.proto", completing the cycle

error: detected cyclic import while importing "d.proto"
  --> d.proto:4:1
   |
 4 | import "d.proto";
   | ^^^^^^^^^^^^^^^^^ imported here

encountered 2 errors
