How to create new SBT configuration with its own Main? - sbt

Question

I'm trying to create a new Configuration in an SBT Scala project with its own main class. Here are the basic requirements:
Production application code is located in <project root>/src/main/scala/com/example/Main.scala:
package com.example
object Main extends App {
println("Hello from Main!")
}
The main class I'm trying to run should be located in <project root>/src/qa/scala/com/example/QAMain.scala:
package com.example
object QAMain extends App {
println("Hello from QA!")
}
(As suggested by the path, the actual use-case for this is a version of the application for QA to run that bypasses certain time-consuming operations.)
This main class should be runnable by executing sbt qa:run in the project root directory.
(Nice to have): The classpath of the running application should not contain any of the test classes defined under src/test.
Here's a build.sbt that I feel ought to work, but doesn't:
lazy val QA = config("qa").extend(Compile)
lazy val root = project.in(file("."))
.configs(QA)
.settings(
(sourceDirectories in (QA, compile)) += baseDirectory.value / "src" / "qa",
(mainClass in (QA, run)) := Some("com.example.QAMain"),
(mainClass in Compile) := Some("com.example.Main")
)
Unfortunately, the result is
> sbt qa:run
...
[info] Running playground.Main
Hello from Main!
> sbt "qa:runMain com.example.QAMain"
...
[info] Running com.example.QAMain
[error] (run-main-0) java.lang.ClassNotFoundException: com.example.QAMain
[error] java.lang.ClassNotFoundException: com.example.QAMain
[error] at java.lang.ClassLoader.findClass(ClassLoader.java:530)
[error] at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
[error] Nonzero exit code: 1
[error] (Compile / runMain) Nonzero exit code: 1
That last line is interesting, because it looks like SBT is running the task scoped to the Compile configuration instead of my custom QA configuration. sbt inspect confirms this.
My assumption has been that since a configuration's compile task uses the sourceDirectory setting, that an override of that setting will force an override of any task downstream of that setting. This assumption might be wrong in a couple of different ways:
sourceDirectory may not be upstream of compile, but maybe some other setting is that I could change;
compile might need to be explicitly overridden anyway.

Answer 1

It's not clear which-all settings were upstream of compile, but there's apparently enough of them that you need to use sbt.Defaults:
lazy val QA = config("qa").extend(Compile)
lazy val root = project.in(file("."))
.configs(QA)
.settings(
inConfig(QA)(Defaults.compileSettings) : _*
)
This achieves the desired behavior. To test this, add the following classes:
<project root>/src/main/scala/com/example/Water.scala
package com.example
class Water {
def drink(): Unit = {
println("Cool and refreshing")
}
}
<project root>/src/qa/scala/com/example/Poison.scala
package com.example
class Poison {
def drink(): Unit = {
println("You have died of dysentery.")
}
}
Then SBT is happy to build an instance of Water into QAMain, but will not find Poison to build into Main.