improvement: Allow passing -explain to the presentation compiler

Author: tgodzik

presentation-compiler/src/main/dotty/tools/pc/DiagnosticProvider.scala

@@ -15,7 +15,6 @@ import scala.jdk.CollectionConverters.*
 import scala.meta.pc.VirtualFileParams
 
 class DiagnosticProvider(driver: InteractiveDriver, params: VirtualFileParams):
-
   def diagnostics(): List[lsp4j.Diagnostic] =
     if params.shouldReturnDiagnostics then
       val diags = driver.run(params.uri().nn, params.text().nn)
@@ -25,9 +24,13 @@ class DiagnosticProvider(driver: InteractiveDriver, params: VirtualFileParams):
 
   private def toLsp(diag: Diagnostic)(using Context): Option[lsp4j.Diagnostic] =
     Option.when(diag.pos.exists):
+      val message = 
+        if Diagnostic.shouldExplain(diag) then 
+          diag.msg.message + "\n\n# Explanation (enabled by `-explain`)\n\n" + diag.msg.explanation 
+        else diag.msg.message
       val lspDiag = lsp4j.Diagnostic(
         diag.pos.toLsp,
-        diag.msg.message,
+        message,
         toDiagnosticSeverity(diag.level),
         "presentation compiler",
       )

presentation-compiler/test/dotty/tools/pc/tests/DiagnosticProviderSuite.scala

@@ -19,7 +19,7 @@ import org.eclipse.lsp4j.CodeAction
 class DiagnosticProviderSuite extends PcAssertions {
   case class TestDiagnostic(startIndex: Int, endIndex: Int, msg: String, severity: DiagnosticSeverity)
 
-  val pc = RawScalaPresentationCompiler().newInstance("", TestResources.classpath.asJava, Nil.asJava)
+  val pc = RawScalaPresentationCompiler().newInstance("", TestResources.classpath.asJava, List("-explain").asJava)
 
   case class TestVirtualFileParams(uri: URI, text: String) extends VirtualFileParams {
     override def shouldReturnDiagnostics: Boolean = true
@@ -45,6 +45,47 @@ class DiagnosticProviderSuite extends PcAssertions {
          |  Int.maaxValue
          |""".stripMargin,
       List(TestDiagnostic(12,25, "value maaxValue is not a member of object Int - did you mean Int.MaxValue?", DiagnosticSeverity.Error))
+    )  
+    
+  @Test def error1 =
+    check(
+      """|object C:
+         |  def m(x: Int) = 1
+         |  object T extends K:
+         |    val x = m(1) // error
+         |class K:
+         |  def m(i: Int) = 2
+         |""".stripMargin,
+      List(
+        TestDiagnostic(
+        64,65,
+        """|Reference to m is ambiguous.
+           |It is both defined in object C
+           |and inherited subsequently in object T
+           |
+           |# Explanation (enabled by `-explain`)
+           |
+           |The identifier m is ambiguous because a name binding of lower precedence
+           |in an inner scope cannot shadow a binding with higher precedence in
+           |an outer scope.
+           |
+           |The precedence of the different kinds of name bindings, from highest to lowest, is:
+           | - Definitions in an enclosing scope
+           | - Inherited definitions and top-level definitions in packages
+           | - Names introduced by import of a specific name
+           | - Names introduced by wildcard import
+           | - Definitions from packages in other files
+           |Note:
+           | - As a rule, definitions take precedence over imports.
+           | - Definitions in an enclosing scope take precedence over inherited definitions,
+           |   which can result in ambiguities in nested classes.
+           | - When importing, you can avoid naming conflicts by renaming:
+           |   import scala.{m => mTick}
+           |""".stripMargin,
+        DiagnosticSeverity.Error
+        
+        )
+      )
     )
 
   @Test def warning =