How to set up IDEA to auto-configure Scala facet for mixed Scala/Java Maven project?

Question

How do I get IntelliJ to auto-configure the Scala facet in a Maven project with mixed Scala and Java source code?

I am using Scala Plugin Nightly for Maia Build 2099.

mvn compile and mvn test both work from the command prompt and from the Maven Projects panel in IntelliJ. However, if I try to run ScalaSpec directly in IntelliJ it presents an error dialog Cannot compile Scala files with content Please, specify compiler in Scala facet.

Project directory structure:

MixedJavaScala
│   MixedScalaJava.iml
│   pom.xml
│
└───src
    ├───main
    │   ├───java
    │   │       HelloJava.java
    │   │
    │   └───scala
    │           HelloScala.scala
    │
    └───test
        ├───java
        │       TestJava.java
        │
        └───scala
                ScalaSpec.scala

Listing of HelloJava.java:

public class HelloJava {}

Listing of HelloScala.scala:

class HelloScala

Listing of TestJava.java:

public class TestJava
{
    @org.junit.Test public void example() {}
}

Listing of ScalaSpec.scala:

class ScalaSpec extends org.specs.Specification {
  "nothing interesting should happen" in {}
}

Listing of pom.xml:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>org.scala-tools.maven-scala-plugin</groupId>
  <artifactId>MixedScalaJava</artifactId>
  <version>1.0</version>
  <name>Test for Java + Scala compilation</name>
  <description>Test for Java + Scala compilation</description>

  <dependencies>
    <dependency>
      <groupId>org.scala-tools.testing</groupId>
      <artifactId>specs_2.8.0</artifactId>
      <version>1.6.5</version>
    </dependency>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.8.1</version>
    </dependency>
    <dependency>
      <groupId>org.scala-lang</groupId>
      <artifactId>scala-library</artifactId>
      <version>2.8.0</version>
    </dependency>
  </dependencies>
  <repositories>
    <repository>
      <id>scala-tools.org</id>
      <name>Scala-tools Maven2 Repository</name>
      <url>http://scala-tools.org/repo-releases</url>
    </repository>
  </repositories>
  <pluginRepositories>
    <pluginRepository>
      <id>scala-tools.org</id>
      <name>Scala-tools Maven2 Repository</name>
      <url>http://scala-tools.org/repo-releases</url>
    </pluginRepository>
  </pluginRepositories>

  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.scala-tools</groupId>
          <artifactId>maven-scala-plugin</artifactId>
          <version>2.14.1</version>
        </plugin>
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>2.3</version>
        </plugin>
      </plugins>
    </pluginManagement>
    <plugins>
      <plugin>
        <groupId>org.scala-tools</groupId>
        <artifactId>maven-scala-plugin</artifactId>
        <executions>
          <execution>
            <id>scala-compile-first</id>
            <phase>process-resources</phase>
            <goals>
                <goal>add-source</goal>
              <goal>compile</goal>
            </goals>
          </execution>
          <execution>
            <id>scala-test-compile</id>
            <phase>process-test-resources</phase>
            <goals>
              <goal>testCompile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <executions>
          <execution>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <source>1.6</source>
          <target>1.6</target>
        </configuration>
      </plugin>
    </plugins>
  </build>

</project>

Answer 1

Add a <configuration/> node to the <plugin/> node for maven-scala-plugin right after the <executions/> node

<configuration>
  <scalaVersion>2.8.0</scalaVersion>
</configuration>

Now the Scala Plugin auto-detects the Scala compiler and auto-configures the Scala Facet. Debugging, running, make and the like all work directly through the IntelliJ interface now.

It works up to scala-maven-plugin 3.2.0.

Please note that Since version 2.7 of the plugin, the scala version to use is detected from dependency to scala-library. So it suggested to not use scalaVersion configuration.