Warn if there are files not covered by rules?

Desired Behavior

If there are files that look like they should be compiled (say *.ml) but they are not matched by any rule, I would like to receive a warning.

A stronger requirement may be to list all «extra files» and give an error for any unexpected file. This feature may be hidden behind a flag.

Why?

• Detect dead code.
• Make sure all tests execute.

Example

% tree
├── examples/
│   ├── .merlin
│   ├── dune
│   ├── i_am_failing.ml
│   └── i_am_succeeding.ml
. . .

% cat examples/dune
(tests
  (names i_am_succeeding)
  . . .
)

Here, a test has been written but I forgot to add it to the dune file. There is an impression of success, and a fault is missed.

Discussion

There is already an error when some extra files are present. (See #3933.) But it is not really motivated. The feature I propose here, on the opposite, has a good reason to exist.

Start by reviewing the existing extra-files error referenced in #3933 and the example rules in examples/dune. Define how Dune should identify source-like files such as .ml that match no rule, and whether reporting is a warning or a flag-controlled error. Done means the omitted test is surfaced without changing the behavior of matched files.