Support OrganizeImports.removeUnused in Scala 3

Once this PR scala/scala3#17835
has merged and released, scalafix-organize-imports should be
able to run OrganizeImports.removeUnused based on the
diagnostics information in SemanticDB emit from Scala3 compiler.

In order to make OrganizeImports rule to work with Scala 3,
this commit added a few adjustments to the rule.

docs/rules/OrganizeImports.md

@@ -52,15 +52,11 @@ do not rewrite import statements in ways that conflict with
 
 Known limitations:
 
-1.  The [`removeUnused`](OrganizeImports.md#removeunused) option must be
-    explicitly set to `false` - the rule currently doesn’t remove unused
-    imports as it is currently not supported by the compiler.
-
-2.  Usage of [deprecated package
+1.  Usage of [deprecated package
     objects](http://dotty.epfl.ch/docs/reference/dropped-features/package-objects.html)
     may result in incorrect imports.
 
-3.  The
+2.  The
     [`groupExplicitlyImportedImplicitsSeparately`](OrganizeImports.md#groupexplicitlyimportedimplicitsseparately)
     option has no effect.
 
@@ -1279,12 +1275,9 @@ Remove unused imports.
 > using Scala compilation diagnostics information, and the compilation phase
 > happens before Scalafix rules get applied.
 
-> The `removeUnused` option is currently not supported for source files
-> compiled with Scala 3, as the [compiler cannot issue warnings for unused
-> imports
-> yet](https://docs.scala-lang.org/scala3/guides/migration/options-lookup.html#warning-settings).
-> As a result, you must set `removeUnused` to `false` when running the
-> rule on source files compiled with Scala 3.
+> The `removeUnused` option is not supported for source files compiled with
+> early versions of Scala 3 as these do not export SemanticDB diagnostics for
+> unused imports. You must compile with Scala 3.4.0 or later to use it.
 
 ### Value type
 
@@ -1299,7 +1292,7 @@ Boolean
 ```conf
 OrganizeImports {
   groups = ["javax?\\.", "scala.", "*"]
-  removeUnused = true // not supported in Scala 3
+  removeUnused = true
 }
 ```
 

scalafix-rules/src/main/scala/scalafix/internal/rule/OrganizeImports.scala

@@ -96,7 +96,8 @@ class OrganizeImports(
 
   private def fixWithImplicitDialect(implicit doc: SemanticDocument): Patch = {
     unusedImporteePositions ++= doc.diagnostics.collect {
-      case d if d.message == "Unused import" => d.position
+      // Scala2 says "Unused import" while Scala3 says "unused import"
+      case d if d.message.toLowerCase == "unused import" => d.position
     }
 
     val (globalImports, localImports) = collectImports(doc.tree)
@@ -112,8 +113,18 @@ class OrganizeImports(
     diagnostics.map(Patch.lint).asPatch + globalImportsPatch + localImportsPatch
   }
 
-  private def isUnused(importee: Importee): Boolean =
-    unusedImporteePositions contains positionOf(importee)
+  private def isUnused(importee: Importee): Boolean = {
+    // positionOf returns the position of `bar` for `import foo.{bar => baz}`
+    // this position matches with the diagnostics from Scala2,
+    // but Scala3 diagnostics has a position for `bar => baz`, which doesn't match with
+    // the return value of `positionOf`.
+    // We could adjust the behavior of `positionOf` based on Scala version,
+    // but this implementation just checking the unusedImporteePosition includes the importee pos, for simplicity.
+    val pos = positionOf(importee)
+    unusedImporteePositions.exists(unused =>
+      unused.start <= pos.start && pos.end <= unused.end
+    )
+  }
 
   private def organizeGlobalImports(
       imports: Seq[Import]
@@ -839,7 +850,9 @@ object OrganizeImports {
       scalacOptions: List[String],
       scalaVersion: String
   ): Configured[Rule] = {
-    val hasCompilerSupport = scalaVersion.startsWith("2")
+    val hasCompilerSupport =
+      Seq("3.0", "3.1", "3.2", "3.3")
+        .forall(v => !scalaVersion.startsWith(v))
 
     val hasWarnUnused = hasCompilerSupport && {
       val warnUnusedPrefix = Set("-Wunused", "-Ywarn-unused")
@@ -887,17 +900,17 @@ object OrganizeImports {
       )
     else if (hasCompilerSupport)
       Configured.error(
-        "The Scala compiler option \"-Ywarn-unused\" is required to use OrganizeImports with"
+        "A Scala compiler option is required to use OrganizeImports with"
           + " \"OrganizeImports.removeUnused\" set to true. To fix this problem, update your"
-          + " build to use at least one Scala compiler option like -Ywarn-unused-import (2.11"
-          + " only), -Ywarn-unused, -Xlint:unused (2.12.2 or above) or -Wunused (2.13 only)."
+          + " build to add `-Ywarn-unused` (2.12), `-Wunused:imports` (2.13), or"
+          + " `-Wunused:import` (3.4+)."
       )
     else
       Configured.error(
-        "\"OrganizeImports.removeUnused\" is not supported on Scala 3 as the compiler is"
-          + " not providing enough information. Run the rule with"
-          + " \"OrganizeImports.removeUnused\" set to false to organize imports while keeping"
-          + " potentially unused imports."
+        "\"OrganizeImports.removeUnused\"" + s"is not supported on $scalaVersion as the compiler is"
+          + " not providing enough information. Please upgrade the Scala compiler to 3.4.0 or greater."
+          + " Otherwise, run the rule with \"OrganizeImports.removeUnused\" set to false"
+          + " to organize imports while keeping potentially unused imports."
       )
   }