Allow enabled_if in executable(s) stanza

[voodoos]

Fix #1690

This PR add support for enabled_if in executable(s) stanza.

Any attempt to build an executable whose enable_if field evaluates to false will raise a "Don't know how to build" error.

I am unsure whether or not that behaviour is the correct one. Other options are:

• do nothing
• warn the user that the enable_if field is not satisfied.

[rgrinberg]

Any attempt to build an executable whose enable_if field evaluates to false will raise a "Don't know how to build" error.

Isn't this the same for libraries with enabled_if that evaluates to false?

[voodoos]

Yes it is indeed the case.

Current behaviour is: nothing for aliases, "don't know" for rules and libraries, and a more precise error when depending on a disabled library.

From enabled_if/run.t (which is missing the case "explicitly build a disabled library"):

This alias is disabled, building it should do nothing:
  $ dune build @x

This one is enabled:
  $ dune build @y
  Building alias y

This rule is disabled, trying to build a should fail:
  $ dune build a
  Error: Don't know how to build a
  Hint: did you mean b?
  [1]

This one is enabled:
  $ dune build b
  Building file b

Test the enabled_if field for libraries:

  $ dune build main.exe
  File "dune", line 33, characters 12-15:
  33 |  (libraries foo))
                   ^^^
  Error: Library "foo" in _build/default is hidden (unsatisfied 'enabled_if').
  Hint: try: dune external-lib-deps --missing main.exe
  [1]

[rgrinberg]

i think this is sufficient for now. thanks.

[ghost]

Did you check what happens if the executable is public? From a quick glance over the path, it seems to me that Dune would try to build the executable and fail.

[voodoos]

I can confirm that attempting to build @install a project including disabled public executables will trigger a don't know how to build error.

What do you think should be the correct behaviour ?

[ghost]

I think we should exclude executables with unsatified enabled_if clauses from installation. And while reading this code I found a very embarrassing bug: #3163

Bascially, you should interpret the enabled_if in the keep_if function that #3163 modifies.