Implementation: Given an optional plugin, conditionally discard deps

[abeldekat]

Currently, when a plugin is marked with "enabled = false", lazy.nvim investigates the dependencies of that plugin,and disables them when appropriate.
Such behaviour is not implemented for a plugin marked with "optional = true".

Any feedback and discussion about the topic is much appreciated as I would like to create a pull request:
My Implementation

Example

Consider a stock LazyVim installation with "extras/lang/python" added to "config/lazy.lua"
The resulting installation includes nvim-dap-python and its keys, but without nvim-dap installed.

Pressing <leader> d P c leads to an error.
The UI of which-key shows "prefix" as description for "d".

The cause

In "extras/lang/python", nvim-dap-python is a dependency of nvim-dap.
When "extras/dap/core" is not added to "config/lazy.lua", nvim-dap becomes completely optional and will not be installed.
Currently, the dependency of nvim-dap, nvim-dap-python, does get installed.

Note: This pattern applies to multiple configs in extras.

Is it a problem?

If the user wants to use "extras/lang/python" without "extras/dap/core", the following options are available:

• the user accepts a plugin he has no benefit of.
  The plugin will never load, unless the aforementioned keys are pressed, resulting in an error.
• the user explicitly disables nvim-dap-python
• the maintainer moves nvim-dap to its own python specific config.
  The user now needs to include both python configs for complete functionality.

Implementation

This is relatively straightforward, reusing the existing algorithm doing the disabling of explicitly disabled plugins.

• The user might see some unexpected plugins to be cleaned in the UI of lazy.nvim.
• Performance is not expected to degrade. I did not observe a decline.

References

• discussion dependency inheritance
• Optional keyword added

[folke]

Looks good! Would be great if you could submit a PR!

[abeldekat]

Thanks! I submitted the PR.

[mehalter]

Would it make sense to do something similar with dependencies in general? Like if I have a plugin that what it is added needs to turn on some options for another plugin. For example adding a telescope module plugin that enables it in the telescope opts table. If I set the telescope module plugin to enabled = false, ideally the options that it sets for telescope in it's dependencies table should no longer be applied even if telescope is still installed separately. Is this possible to do with the current setup as well? Similar to not installing dependencies for disabled plugins.

[mehalter]

Especially now that if there is no other reference to the plugin it gets fully removed, it seems that the best thing would be to just fully not consider dependencies for disabled plugins. That way it's predictable always whether or not something is being applied. It's a bit strange that the two cases are "if there's a reference to a disabled plugins dependencies fully consider it and the options are applied", but if there isn't then "remove the plugin completely". Seems to be more predictable to think "disabled plugin dependencies are not used in any way at all"

Thanks for this discussion and let me know what you think :) There also may be a way to achieve this behavior already that I'm glossing over!