$ cat > dune-project << EOF
  > (lang dune 2.0)
  > (using action-plugin 0.1)
  > EOF

  $ cat > dune << EOF
  > (rule
  >  (target some_file1)
  >  (action
  >   (dynamic-run ./foo1.exe)))
  > \
  > (rule
  >  (target some_file2)
  >  (action
  >   (dynamic-run ./foo2.exe)))
  > EOF

  $ cp ./bin/foo1.exe ./
  $ cp ./bin/foo2.exe ./

  $ dune build some_file1
  Error: Dependency cycle between:
     _build/default/some_file1
  [1]

  $ dune build some_file2
  Error: Dependency cycle between:
     _build/default/some_file2
  [1]