Can't build Eclipse with a plugin

Partly to educate myself about Nix and partly thinking maybe it can be useful for a wider public and may be included in nixpkgs at some point, I’m trying to create a derivation for Eclipse with a 3rd-party plugin not currently available in nixpkgs. The plugin is called SASyLF, it implements an educational proof assistant.

I’m heavily basing on pkgs/applications/editors/eclipse. The plugin I’m interested in provides a JAR that can be put in the dropins directory – that’s the only supported way. The catch is that the current utilities to get plugins are not ready for a plugin with a single JAR. There are two related utilities in pkgs/applications/editors/eclipse/plugins.nix:

  • buildEclipsePlugin – requires two JARs: one with a plugin and one with feature;
  • buildEclipseUpdateSite – requires an “update site” with a plugin

The latter is not applicable at all. The former is almost applicable, I just need to avoid providing the feature JAR. Sidenote: it’s completely legit for an Eclipse plugin to not have a feature, from what I understand. At least, SASyLF manages without one.

So, I copied the code for buildEclipsePlugin, changed it a bit to accept just the plugin jar and created the following default.nix:

with import <nixpkgs> {};
rec {

  # Helper for the case where we have only plugin JAR.
  buildEclipsePluginNoFeature =
    { name, srcPlugin ? null, srcPlugins ? [], ... } @ attrs:

      assert srcPlugin == null -> srcPlugins != [];
      assert srcPlugin != null -> srcPlugins == [];

        srcs = if (srcPlugin != null) then [ srcPlugin ] else srcPlugins;
        eclipses.plugins.buildEclipsePluginBase (attrs // {
          buildCommand = ''
            mkdir -p $dropinDir
            for plugin in ${toString srcs}; do
              cp -v $plugin $dropinDir/$(stripHash $plugin)

  sasylf = buildEclipsePluginNoFeature rec {
    pname = "sasylf";
    version = "1.5.0";
    name = "${pname}-${version}";
    srcPlugin = fetchurl {
      url = "";
      sha256 = "1cchhvhi0g2i94x5j8gc1abags6lznzh9n5qhykkm5bv19l0w0i8";

  eclipseSasylf = eclipses.eclipseWithPlugins {
    eclipse = eclipses.eclipse-platform;
    plugins = [sasylf];

Unfortunately, when I build it, I get two independent results directories (result and result-2): one with the dropins directory, and one with Eclipse:

$ tree -l
├── default.nix
├── result -> /nix/store/ngyb1w90zppxh9nfq1sg67m01scxspkg-eclipse-platform-with-plugins-4.27
│   ├── bin
│   │   └── eclipse
│   ├── etc
│   │   └── eclipse.ini
│   └── share -> /nix/store/vbkzgvz8n5vasb5ljn2l0005g9bzl9hq-eclipse-platform-4.27/share
│       ├── applications
│       │   └── Eclipse.desktop
│       └── pixmaps
│           └── eclipse.xpm -> /nix/store/vbkzgvz8n5vasb5ljn2l0005g9bzl9hq-eclipse-platform-4.27/eclipse/icon.xpm
└── result-2 -> /nix/store/lv44g67izf8pnjdns8w4c5mx14zkn7g3-eclipse-plugin-sasylf-1.5.0
    └── eclipse
        └── dropins
            └── org.sasylf_1.5.0.v20201210.jar

They should all be in one directory! In that case, I think, it will work exactly right. Can someone explain me why I end up with two independent directories?

Other ideas about how to implement it and, possibly, upstream it to nixpkgs, are very welcome!