According to its website, the Ensime project has been shut down since 2019. We
have been supporting both Ensime and Metals for [a year and a
half](https://github.com/syl20bnr/spacemacs/pull/12234#issuecomment-524916394).
Ensime's GitHub repositories, including its Emacs integration, are archived.
Meanwhile, Metals has been developed actively, regularly releasing new versions.
It has stabilized significantly, and supports an increasingly full set of
features. Scala 3 is just around the corner, and the community is poised to make
the transition smoothly and relatively quickly. Metals supports it already,
whereas of course Ensime does not and never will. In fact, Scala 2 has had
several import minor versions released since Ensime died. Now that it's 2021,
it's time to cut the baggage we are carrying around for Ensime.
Ensime seems to be finally dead, as ensime-mode is not longer
available on melpa. The same applies to ob-scala the package
which delivered scala support for org babel.
I have changed the layers default to metals and took care that
ensime is not tried to be installed until it is really selected
as a package.
In addition I have also fixed some smaller issues in the layer
which caused ensime specific settings to be forced even when
metals was selected as a backend.
I have also removed the not longer existing org-babel support
for scala as it requires ob-scala which in turn is based on ensime.
See https://github.com/hvesalai/emacs-scala-mode/issues/155 for details.
There was a edge case with the declaration of the `lsp` layer in `layers.el`
files.
The `hy` layer depends on the `python` layer which in turn depends on the `lsp`
layer if and only if the `python-backend` layer variable is set to `lsp`.
When the `hy` layer was declared first then it declares the `python` layer
without its layer variables, thus the `lsp` layer was not declared because the
`python-backend` variable was not set.
The fix is to gather all the layer dependencies and resolve them only after all
the used layers have been declared.
* new function `configuration-layer/declare-layer-dependencies`
* replace all calls to `configuration-layer/declare-layer` by the new function
except for distribution layers (we declare layer dependencies right away in
distribution layers)
