How to have SBT skip cross compile for a given sub-project?

Question

I've run into a couple related cases with SBT that have me stumped. Is there a way to tell SBT to skip a sub project entirely for certain scala versions when you're cross compiling?

Here are two examples where this would be useful.

1) A build with three projects A, B, and C. Both A and B are scala projects, and have 'scalaVersions ++= Seq("2.11.2", "2.10.4") in their settings. Project C is a pure-Java artifact, and thus I've excluded the Scala libraries from it's dependencies. I'd like A and B to depend on C, but ideally I'd only like to build C just once. If I use the default behavior and do "+publish" from the root aggregator project, I get two copies of C-1.0.0.jar produced, and SBT attempts to publish it twice, which is of course a no-no for a maven repository.

2) A build with multiple scala projects, but where one project should only build against a single Scala version. I've tried defining 'scalaVersions' in the settings for this project to hold only one version where the other projects have two, but again "+publish" from a root aggregator seems to ignore this and still compiles it twice, with the second time failing because it's dependencies aren't available for that Scala version. This project is a leaf node in the dependency graph, so it's a perfectly fine thing to want to do logically.

For case #2, I've thought of setting the source dirs for the 'bad' scala version to /dev/null or something similar, but that still actually runs the build and produces an empty artifact. I know I could probably go in and find all of the relevant keys and do something like

publishArtifact := if(scalaBinaryVersion.value == "2.10") false else publishArtifact.value

and then hunt down all of the other related settings/tasks (compile, compile in Test, test in Test, packageBin, etc) but that seems pretty hack-ish. Is there a 'skip' setting somewhere?

Answer 1

I wrote sbt-doge to address task aggregation across subprojects respecting their crossScalaVersions. For Java projects you might need a dummy crossScalaVersion entry.

Answer 2

The plugin sbt-doge can be used to specify a crossScalaVersion setting in each subproject.

First, add the line addSbtPlugin("com.eed3si9n" % "sbt-doge" % "0.1.5") to your projects/plugins.sbt.

To avoid the ridiculous doge syntax ("such compile", really?) you need to enablePlugins(CrossPerProjectPlugin) in your root project. With this, you can add a plus sign before your sbt commands, and they will honor the cross build settings. Just like this: + compile.