Struggles building Langkit with Alire

Bonsoir, hope that you're all doing well.

I am trying to evaluate langkit, aiming to simplify
the development of an LSP and a set of linters/formaters for a DSL. I struggled
getting it to build, here is a lengthy summary of what I encountered and where I stopped.

The documentation recommends GNAT Community 2021.
As I was surprised that this was the latest release of GNAT, I found the blogs
mentioning alire being the recommended way to get gnat, so I gave it a try that way, on macOS.

I did a manual setup of alire for langkit and added a dependency to
langkit_support, trying to avoid having to build it.

I would have a hard time bringing the entire Ada toolchain to our devenvs,
and aimed to build as little as possible, hopefully requesting a small set to
be released (in pip using a bdist ? <3) if the eval was successful.

In the created alire.toml :

[[depends-on]]
langkit_support = "^24.0.0"

I realized that default compilation with manage.py was still trying to compile
langkit_support, I found the --no-langkit-support option to bypass it.

Suggestion : Separating langkit_support out of the dependency list might be justified given
that it's expected to be built.

Moving forward I was stumbling upon an error caused by xmlada_shared.gpr not
found from the xmlada lib. I was using the one brought in by langkit_support.

xmlada_unicode.gprxmlada_unicode.gpr:24:06: imported project file "../xmlada_shared.gpr" not found
xmlada_unicode.gpr:24:06: imported by "/Users/bourgeoisclement/.local/share/alire/builds/xmlada_24.0.0_ae5a015b/b0131ab91b91aa28704f2c86cc19d71588851835808f51b4132600c1599b3c69/unicode/xmlada_unicode.gpr"
xmlada_unicode.gpr:24:06: imported by "/Users/bourgeoisclement/.local/share/alire/builds/xmlada_24.0.0_ae5a015b/b0131ab91b91aa28704f2c86cc19d71588851835808f51b4132600c1599b3c69/distrib/xmlada.gpr"
xmlada_unicode.gpr:24:06: imported by "/Users/bourgeoisclement/.local/share/alire/builds/libgpr_24.0.0_d9c96bda/9098d495aeffac6e11d3d93c6b8c5c10b8181054d2fc132f3bf2a0f70303aa83/gpr/gpr.gpr"
xmlada_unicode.gpr:24:06: imported by "/Users/bourgeoisclement/.local/share/alire/builds/gnatcoll_24.0.0_11c512d1/47cf7d7e8a2b7b3c0e9536d64ef2252d7e51940df5bfe09d9251db5741773a95/gnatcoll.gpr"
xmlada_unicode.gpr:24:06: imported by "/Users/bourgeoisclement/repos/langkit/contrib/lkt/build/liblktlang.gpr"
xmlada_unicode.gpr:29:25: unknown package or project "Xmlada_Shared"
xmlada_unicode.gpr:29:25: wrong expression kind for attribute "library_kind"

That most likely is a problem to report to xmlada, and the way xmlada_24.0.0 is
packaged for alire.
I fixed it by running ./configure in the xmlada GPR_PROJECT_PATH location to generate the
Xmlada_Shared.gpr file. Not langkit fault I know :)

Next I ran into this :

python manage.py make --no-langkit-support

Generating source for libpythonlang...
Generating source for liblktlang...
Generation complete!
Building the generated source code
Building for config (BuildMode.prod, LibraryType.static)
gpr.gpr:26:04: value "static" is illegal for typed string "bld"

I figured out that this was caused by a conflict in the supported GPR_BUILD
values in gpr.gpr, I forced that to be "production" in the gpr file, not sure which
part of the coupling should be the source of truth, but I wanted to move
forward.

After the above, the prettier_ada dependency reported being missing, that was not mentionned in the
intro, so worth updating, I assume that it's pretty recent.
I cloned the repository, setup an alire project there as well with vss and got that solved.

Finally I blocked on :

   [Ada]          libpythonlang_support-adalog-main_support.ads
/Users/bourgeoisclement/repos/langkit/contrib/python/build/src/libpythonlang_support-adalog-solver.adb:1595:36: error: "Free" is not a visible entity of "Libpythonlang_AdaSAT"

   compilation of libpythonlang_support-adalog-main_support.ads failed

gprbuild: *** compilation phase failed
Build failed: error while running gprbuild -p -j4
-P/Users/bourgeoisclement/repos/langkit/contrib/python/build/libpythonlang.gpr
-XBUILD_MODE=prod -XLIBRARY_TYPE=static -XGPR_BUILD=static -XXMLADA_BUILD=static
-XLIBPYTHONLANG_WARNINGS=true -gnatef:

I assume that it was foolish to expect langkit_support to work on master, so I
went to 24.0 but it failed similarly.

Got this error on the release branch that did not stop the build from progressing
but still sharing it for visiblity :

Generating source for liblktlang...
/Users/bourgeoisclement/repos/langkit/contrib/lkt/language/parser.py:1248:1: Then expression should have a default value provided, in cases where the type of the provided function's return type (here Int) does not have a default null value.
Errors, exiting
Generation complete!

Retried without trying to use the langkit_support from alire, building it directly, but got a similar
error :

   [Ada]          langkit_support-adalog-solver.adb
langkit_support-adalog-solver.adb:1595:22: error: "Free" is not a visible entity of "AdaSAT"

   compilation of langkit_support-adalog-solver.adb failed

Should I use an older gnat ? I was using 13.2.0.

Final note, manage.py seems to run twice, always retrying after a failure, not a big deal but that was surprising.

Let me know some of my notes are unclear or if you need more context.