This PR was checked on a sample project : the buildTarget/resources is successfully called during the project import and Intellij Idea can rightly define the resources folders :

I created an example project to show that in some case, the build client lacks some information about the resource file.

The build file is:

val hello = project.in(file("."))
  .settings(
    scalaVersion := "2.13.6",
    Compile / resourceGenerators += Def.task {
      val greeting = (Compile / resourceManaged).value / "assets" / "greeting.txt"
      IO.write(greeting, "Hello, World!")
      Seq(greeting)
    }
  )

This generates a assets/greeting.txt resource file.

The main method read the content of that resource and prints it:

package hello

import scala.io.Source

object Main extends App {
  Source.fromResource("assets/greeting.txt", getClass.getClassLoader)
    .getLines()
    .foreach(println)
}

Note that the resource is assets/greeting.txt and not greeting.txt.

Running the main to check that it works as expected:

sbt:hello> run
[info] running hello.Main 
Hello, World!
[success] Total time: 0 s, completed Jun 18, 2021 10:45:21 AM

When importing the project in IntelliJ the buildTarget/resources response is:

{
  "items": [
    {
      "target": {
        "uri": "file:/home/piquerez/sbt/sbt-6552/#hello/Test"
      },
      "resources": [
        "file:/home/piquerez/sbt/sbt-6552/src/test/resources"
      ]
    },
    {
      "target": {
        "uri": "file:/home/piquerez/sbt/sbt-6552/#hello/Compile"
      },
      "resources": [
        "file:/home/piquerez/sbt/sbt-6552/src/main/resources",
        "file:/home/piquerez/sbt/sbt-6552/target/scala-2.13/resource_managed/main/assets/greeting.txt"
      ]
    }
  ]
}

Here Intellij does not know that the resource file is supposed to be assets/greeting.txt rather than greeting.txt. Or, in other words, that the resource folder should be file:/home/piquerez/sbt/sbt-6552/target/scala-2.13/resource_managed/main/ instead of file:/home/piquerez/sbt/sbt-6552/target/scala-2.13/resource_managed/main/assets/.

Here IntelliJ assumes that the resource folder is file:/home/piquerez/sbt/sbt-6552/target/scala-2.13/resource_managed/main/assets/ which is wrong:

Reproduction repo: https://github.com/adpi2/sbt-6552

I think the solution for now is to send back the managed resource folders rather than the managed resource files:

val id = bspTargetIdentifier.value 
val unmanagedDirs = unmanagedResourceDirectories.value
val managedDirs = managedResourceDirectories.value
ResourcesItem(id, unmanagedDirs.toVector.map(_.toURI) ++ managedDirs.toVector.map(_.toURI))

Or more simply

val id = bspTargetIdentifier.value 
val resourcDirs = resourceDirectories.value
ResourcesItem(id, resourcDirs.toVector.map(_.toURI))

One drawback of this solution is that it does not trigger the resource generation (of the managed resources). We can handle that with:

val id = bspTargetIdentifier.value 
val resourcDirs = resourceDirectories.value
// trigger resource generation
val _ = managedResources.value
ResourcesItem(id, resourcDirs.toVector.map(_.toURI))

You're right, the main problem comes from the lack of resources kind, like we have with sources

I also tried with BSP importing a project with a src/main/resources but no src/test/resources : I end up with the test folder itself being marked as a resource !

I think it comes from BspResolverLogic that creates either a resource from a file or its own parent

Edit : I pushed a modification that adds an extra ending slash when missing on resource directories URIs.

Thanks for the contribution. I'll trust @adpi2's review and hit merge since it's been approved.