diff? error: Rule failed to generate the following targets

[anton-trunov]

Please consider the following self-contained example in the form of a dune file:

(executable
 (name print)
 (modules print))

(rule
 (with-stdout-to print.ml
  (echo "let () = print_endline \"MESSAGE\"")))

(rule
 (alias runtest)
 (action
  (progn
   (with-stdout-to output.expected (echo "MESSAGE\n"))
   (with-stdout-to output.actual (run ./print.exe))
   (diff? output.expected output.actual))))

When I run dune runtest it generates the following error message which I find confusing:

$ dune runtest
File "dune", line 9, characters 0-195:
 9 | (rule
10 |  (alias runtest)
11 |  (action
12 |    (progn
13 |     (with-stdout-to output.expected (echo "MESSAGE\n"))
14 |     (with-stdout-to output.actual (run ./print.exe))
15 |     (diff? output.expected output.actual))))
Error: Rule failed to generate the following targets:
- output.actual

According to the manual,

(diff? ) is similar to (diff ) except that should be produced by a part of the same action rather than be a dependency, is optional and will be consumed by diff?.

In this case (please correct me if I'm wrong) is produced as a part of the same action, so I would expect no error. Changing (diff? ... to (diff ... works as expected. Perhaps it's a documentation issue?

[ghost]

The documentation looks correct; it states that diff? will consume output.actual. That's why it doesn't appear as a target. However, the problem is that dune targets inference doesn't detect this case. i.e. it should know that diff? will delete output.actual and so should consider output.actual as a target.

[ghost]

If you'd like to take a stab at fixing this issue, the code to change is in the Infer sub-module in src/dune/action_unexpanded.ml. In the | Diff branch of the infer function, if optional is true we need to remove file2 from the set of targets.

Otherwise, one of us will look at this at some point.

[greedy]

I was bitten by this bug too so I took a stab at a fix.

[ghost]

This has been fixed.