scalafix-interfaces: improve docs & be more liberal in parsing

Even though not documented, but since "major.minor" / source-compatibility is
non-standard (we need more than "binary" as all Scala 3 versions are binary-
compatible) and can be confusing, this supports using just a major version.

scalafix-interfaces/src/main/java/scalafix/interfaces/Scalafix.java

@@ -86,9 +86,10 @@ public interface Scalafix {
      * <p>
      * The custom classloader optionally provided with {@link ScalafixArguments#withToolClasspath} to compile and
      * classload external rules must have the classloader of the returned instance as ancestor to share a common
-     * loaded instance of `scalafix-core`, and therefore have been compiled against the requested Scala binary version.
+     * loaded instance of `scalafix-core`, and therefore have been compiled against the requested Scala version.
      *
-     * @param requestedScalaVersion The Scala version ("3.3.4" for example) available in the classloader of the
+     * @param requestedScalaVersion A full Scala version (i.e. "3.3.4") or a major.minor one (i.e. "3.3") to infer
+     *                              the major.minor Scala version that should be available in the classloader of the
      *                              returned instance. To be able to run advanced semantic rules using the Scala
      *                              Presentation Compiler such as ExplicitResultTypes, this must match the version
      *                              that the target classpath was built with, as provided with
@@ -106,9 +107,10 @@ static Scalafix fetchAndClassloadInstance(String scalaBinaryVersion) throws Scal
      * <p>
      * The custom classloader optionally provided with {@link ScalafixArguments#withToolClasspath} to compile and
      * classload external rules must have the classloader of the returned instance as ancestor to share a common
-     * loaded instance of `scalafix-core`, and therefore have been compiled against the requested Scala binary version.
+     * loaded instance of `scalafix-core`, and therefore have been compiled against the requested Scala version.
      *
-     * @param requestedScalaVersion The Scala version ("3.3.4" for example) available in the classloader of the
+     * @param requestedScalaVersion A full Scala version (i.e. "3.3.4") or a major.minor one (i.e. "3.3") to infer
+     *                              the major.minor Scala version that should be available in the classloader of the
      *                              returned instance. To be able to run advanced semantic rules using the Scala
      *                              Presentation Compiler such as ExplicitResultTypes, this must match the version
      *                              that the target classpath was built with, as provided with
@@ -120,17 +122,21 @@ static Scalafix fetchAndClassloadInstance(String scalaBinaryVersion) throws Scal
     static Scalafix fetchAndClassloadInstance(String requestedScalaVersion, List<Repository> repositories)
             throws ScalafixException {
 
+        String requestedScalaMajorMinorOrMajorVersion =
+            requestedScalaVersion.replaceAll("^(\\d+\\.\\d+).*", "$1");
+
         String scalaVersionKey;
-        if (requestedScalaVersion.startsWith("2.12")) {
+        if (requestedScalaMajorMinorOrMajorVersion.equals("2.12")) {
             scalaVersionKey = "scala212";
-        } else if (requestedScalaVersion.startsWith("2.13")) {
+        } else if (requestedScalaMajorMinorOrMajorVersion.equals("2.13") ||
+        requestedScalaMajorMinorOrMajorVersion.equals("2")) {
             scalaVersionKey = "scala213";
-        } else if (requestedScalaVersion.startsWith("3.0") ||
-            requestedScalaVersion.startsWith("3.1") ||
-            requestedScalaVersion.startsWith("3.2") ||
-            requestedScalaVersion.startsWith("3.3")) {
+        } else if (requestedScalaMajorMinorOrMajorVersion.equals("3.0") ||
+            requestedScalaMajorMinorOrMajorVersion.equals("3.1") ||
+            requestedScalaMajorMinorOrMajorVersion.equals("3.2") ||
+            requestedScalaMajorMinorOrMajorVersion.equals("3.3")) {
             scalaVersionKey = "scala3LTS";
-        } else if (requestedScalaVersion.startsWith("3")) {
+        } else if (requestedScalaMajorMinorOrMajorVersion.startsWith("3")) {
             scalaVersionKey = "scala3Next";
         } else {
             throw new IllegalArgumentException("Unsupported scala version " + requestedScalaVersion);

scalafix-tests/integration/src/test/scala/scalafix/tests/interfaces/ScalafixSuite.scala

@@ -54,16 +54,80 @@ class ScalafixSuite extends AnyFunSuite {
     assert(help.contains("Usage: scalafix"))
   }
 
-  test("classload Scala 3 LTS as a fallback for pre-LTS versions") {
+  test("fail to classload Scala 2.11 with full version") {
+    assertThrows[IllegalArgumentException](
+      Scalafix.fetchAndClassloadInstance("2.11.0", repositories)
+    )
+  }
+
+  test("fail to classload Scala 2.11 with minor version") {
+    assertThrows[IllegalArgumentException](
+      Scalafix.fetchAndClassloadInstance("2.11", repositories)
+    )
+  }
+
+  test("classload Scala 2.12 with full version") {
+    val scalafixAPI =
+      Scalafix.fetchAndClassloadInstance("2.12.20", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala212)
+  }
+
+  test("classload Scala 2.12 with major.minor version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("2.12", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala212)
+  }
+
+  test("classload Scala 2.13 with full version") {
+    val scalafixAPI =
+      Scalafix.fetchAndClassloadInstance("2.13.15", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala213)
+  }
+
+  test("classload Scala 2.13 with major.minor version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("2.13", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala213)
+  }
+
+  test("classload Scala 2.13 with major version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("2", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala213)
+  }
+
+  test("classload Scala 3 LTS with full pre-LTS version") {
     val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.0.0", repositories)
     assert(scalafixAPI.scalaVersion() == Versions.scala3LTS)
   }
 
-  test("classload Scala 3 Next as a fallback for post-LTS versions") {
+  test("classload Scala 3 LTS with major.minor pre-LTS version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.2", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala3LTS)
+  }
+
+  test("classload Scala 3 LTS with full LTS version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.3.4", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala3LTS)
+  }
+
+  test("classload Scala 3 LTS with major.minor LTS version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.3", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala3LTS)
+  }
+
+  test("classload Scala 3 Next with full post-LTS version") {
     val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.4.0", repositories)
     assert(scalafixAPI.scalaVersion() == Versions.scala3Next)
   }
 
+  test("classload Scala 3 Next with major.minor post-LTS version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("3.5", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala3Next)
+  }
+
+  test("classload Scala 3 Next with major version") {
+    val scalafixAPI = Scalafix.fetchAndClassloadInstance("3", repositories)
+    assert(scalafixAPI.scalaVersion() == Versions.scala3Next)
+  }
+
   test("invalid class loader") {
     val cl = new URLClassLoader(Array(), null)
     val ex = intercept[ScalafixException] {