The new dir install feature does not work on existing directories

[kit-ty-kate]

Using dune master (7659e30 + #6119), the following project:

$ cat dune-project
(lang dune 3.5)
$ cat dune
(install
 (dirs (existing-dir as target-dir))
 (section lib_root))
$ mkdir existing-dir
$ touch existing-dir/a existing-dir/b

fails with:

Error: No rule found for existing-dir
-> required by alias all
-> required by alias default

From #5097 description, it doesn't seem like existing-dir needs to have a target (what would it even be?) or that opam requires it to be generated somehow.

Also, as an aside, renaming target-dir to test makes dune change its error to:

Error: No rule found for existing-dir
-> required by alias all
-> required by alias default
File "dune", line 2, characters 8-20:
2 |  (dirs (existing-dir as test))
            ^^^^^^^^^^^^
Error: This rule defines a target "test" whose name conflicts with an
internal directory used by Dune. Please use a different name.

I'm not sure to understand why test would be a forbidden target in lib_root.

[emillon]

As per last week's dev meeting, this is a new feature so it's ok to have this limitation, so I'm moving this to 3.6.0. A better error message would be good and can be added in 3.5.0.

[emillon]

I don't think we'll have a fix for this in 3.7 so I'm moving the milestone.

[rgrinberg]

I remember trying to implement this before and realized it wasn't that easy. The main issue is that the current directory loading code has special handling for source directories so implementing this feature would require some deep changes to the rule loading.