Revamp exception handling, from nsc

Author: dwijnand

compiler/src/dotty/tools/dotc/Driver.scala

@@ -30,18 +30,19 @@ class Driver {
 
   protected def doCompile(compiler: Compiler, files: List[AbstractFile])(using Context): Reporter =
     if files.nonEmpty then
+      var run = ctx.run
       try
-        val run = compiler.newRun
+        run = compiler.newRun
         run.compile(files)
         finish(compiler, run)
       catch
         case ex: FatalError =>
-          report.error(ex.getMessage.nn) // signals that we should fail compilation.
-        case ex: TypeError =>
-          println(s"${ex.toMessage} while compiling ${files.map(_.path).mkString(", ")}")
+          report.error(run.enrichErrorMessage(ex.getMessage.nn)) // signals that we should fail compilation.
+        case ex: TypeError if !run.enrichedErrorMessage =>
+          println(run.enrichErrorMessage(s"TypeError while compiling ${files.map(_.path).mkString(", ")}"))
           throw ex
-        case ex: Throwable =>
-          println(s"$ex while compiling ${files.map(_.path).mkString(", ")}")
+        case NonFatal(ex) if !run.enrichedErrorMessage =>
+          println(run.enrichErrorMessage(s"Exception while compiling ${files.map(_.path).mkString(", ")}"))
           throw ex
     ctx.reporter
 

compiler/src/dotty/tools/dotc/Run.scala

@@ -173,15 +173,22 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
    */
   var ccImportEncountered = false
 
+  private var myEnrichedErrorMessage = false
+
+  def enrichedErrorMessage: Boolean = myEnrichedErrorMessage
+
+  def enrichErrorMessage(errorMessage: String)(using Context): String =
+    if !enrichedErrorMessage then
+      myEnrichedErrorMessage = true
+      report.enrichErrorMessage(errorMessage)
+    else errorMessage
+
   def compile(files: List[AbstractFile]): Unit =
-    try
-      val sources = files.map(runContext.getSource(_))
-      compileSources(sources)
-    catch
-      case NonFatal(ex) =>
-        if units.nonEmpty then report.echo(i"exception occurred while compiling $units%, %")
-        else report.echo(s"exception occurred while compiling ${files.map(_.name).mkString(", ")}")
-        throw ex
+    try compileSources(files.map(runContext.getSource(_)))
+    catch case NonFatal(ex) if !enrichedErrorMessage =>
+      val files1 = if units.nonEmpty then units.map(_.source.file) else files
+      report.echo(enrichErrorMessage(s"exception occurred while compiling ${files1.map(_.path)}"))
+      throw ex
 
   /** TODO: There's a fundamental design problem here: We assemble phases using `fusePhases`
    *  when we first build the compiler. But we modify them with -Yskip, -Ystop

compiler/src/dotty/tools/dotc/core/Decorators.scala

@@ -280,10 +280,7 @@ object Decorators {
           case ex: CyclicReference => "... (caught cyclic reference) ..."
           case NonFatal(ex)
           if !ctx.mode.is(Mode.PrintShowExceptions) && !ctx.settings.YshowPrintErrors.value =>
-            val msg = ex match
-              case te: TypeError => te.toMessage.message
-              case _ => ex.getMessage
-            s"[cannot display due to $msg, raw string = $x]"
+            s"... (caught ${ex.className}) ..."
       case _ => String.valueOf(x).nn
 
     /** Returns the simple class name of `x`. */

compiler/src/dotty/tools/dotc/core/Phases.scala

@@ -322,8 +322,8 @@ object Phases {
       units.map { unit =>
         val unitCtx = ctx.fresh.setPhase(this.start).setCompilationUnit(unit).withRootImports
         try run(using unitCtx)
-        catch case ex: Throwable =>
-          println(s"$ex while running $phaseName on $unit")
+        catch case NonFatal(ex) if !ctx.run.enrichedErrorMessage =>
+          println(ctx.run.enrichErrorMessage(s"unhandled exception while running $phaseName on $unit"))
           throw ex
         unitCtx.compilationUnit
       }

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -687,6 +687,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       tp
     case tp if (tp `eq` NoType) || (tp `eq` NoPrefix) =>
       tp
+    case tp =>
+      unreachable(i"TypeErasure applied to $tp (of class ${tp.className})")
   }
 
   /** Like translucentSuperType, but issue a fatal error if it does not exist. */

compiler/src/dotty/tools/dotc/core/TypeErrors.scala

@@ -183,4 +183,3 @@ object CyclicReference:
         cyclicErrors.println(elem.toString)
     ex
 end CyclicReference
-

compiler/src/dotty/tools/dotc/report.scala

@@ -4,13 +4,12 @@ import reporting._
 import Diagnostic._
 import util.{SourcePosition, NoSourcePosition, SrcPos}
 import core._
-import Contexts._, Symbols._, Decorators._
+import Contexts._, Flags.*, Symbols._, Decorators._
 import config.SourceVersion
-import ast._
+import ast._, tpd.*
 import config.Feature.sourceVersion
 import java.lang.System.currentTimeMillis
 
-
 object report:
 
   /** For sending messages that are printed only if -verbose is set */
@@ -129,4 +128,62 @@ object report:
       case Nil => pos
     recur(pos.sourcePos, tpd.enclosingInlineds)
 
+  private object messageRendering extends MessageRendering
+
+  // Should only be called from Run#enrichErrorMessage.
+  def enrichErrorMessage(errorMessage: String)(using Context): String = try {
+    def formatExplain(pairs: List[(String, Any)]) = pairs.map((k, v) => f"$k%20s: $v").mkString("\n")
+
+    val settings = ctx.settings.userSetSettings(ctx.settingsState).sortBy(_.name)
+    val tree     = ctx.tree
+    val sym      = tree.symbol
+    val pos      = tree.sourcePos
+    val path     = pos.source.path
+    val site     = ctx.outersIterator.map(_.owner).filter(sym => !sym.exists || sym.isClass || sym.is(Method)).next()
+
+    extension (tree: Tree) def summaryString: String = tree match
+      case Literal(const)     => s"Literal($const)"
+      case Ident(name)        => s"Ident(${name.decode})"
+      case Select(qual, name) => s"Select(${qual.summaryString}, ${name.decode})"
+      case tree: NameTree     => (if tree.isType then "type " else "") + tree.name.decode
+      case tree               => s"${tree.className}${if tree.symbol.exists then s"(${tree.symbol})" else ""}"
+
+    val info1 = formatExplain(List(
+      "while compiling"    -> ctx.compilationUnit,
+      "during phase"       -> ctx.phase.prevMega,
+      "mode"               -> ctx.mode,
+      "library version"    -> scala.util.Properties.versionString,
+      "compiler version"   -> dotty.tools.dotc.config.Properties.versionString,
+      "settings"           -> settings.map(s => if s.value == "" then s"${s.name} \"\"" else s"${s.name} ${s.value}").mkString(" "),
+    ))
+    val symbolInfos = if sym eq NoSymbol then List("symbol" -> sym) else List(
+      "symbol"             -> sym.showLocated,
+      "symbol definition"  -> s"${sym.showDcl} (a ${sym.className})",
+      "symbol package"     -> sym.enclosingPackageClass.fullName,
+      "symbol owners"      -> sym.showExtendedLocation,
+    )
+    val info2 = formatExplain(List(
+      "tree"               -> tree.summaryString,
+      "tree position"      -> (if pos.exists then s"$path:${pos.line + 1}:${pos.column}" else s"$path:<unknown>"),
+      "tree type"          -> tree.typeOpt.show,
+    ) ::: symbolInfos ::: List(
+      "call site"          -> s"${site.showLocated} in ${site.enclosingPackageClass}"
+    ))
+    val context_s = try
+      s"""  == Source file context for tree position ==
+         |
+         |${messageRendering.messageAndPos(Diagnostic.Error("", pos))}""".stripMargin
+    catch case _: Exception => "<Cannot read source file>"
+    s"""
+       |  $errorMessage
+       |
+       |  An unhandled exception was thrown in the compiler. Please file a crash
+       |  report here: https://github.com/lampepfl/dotty/issues/new/choose
+       |
+       |$info1
+       |
+       |$info2
+       |
+       |$context_s""".stripMargin
+  } catch case _: Throwable => errorMessage // don't introduce new errors trying to report errors, so swallow exceptions
 end report

compiler/src/dotty/tools/dotc/typer/ReTyper.scala

@@ -124,12 +124,10 @@ class ReTyper(nestingLevel: Int = 0) extends Typer(nestingLevel) with ReChecking
 
   override def typedUnadapted(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree =
     try super.typedUnadapted(tree, pt, locked)
-    catch {
-      case NonFatal(ex) =>
-        if ctx.phase != Phases.typerPhase && ctx.phase != Phases.inliningPhase then
-          println(i"exception while typing $tree of class ${tree.getClass} # ${tree.uniqueId}")
-        throw ex
-    }
+    catch case NonFatal(ex) if ctx.phase != Phases.typerPhase && ctx.phase != Phases.inliningPhase && !ctx.run.enrichedErrorMessage =>
+      val treeStr = tree.show(using ctx.withPhase(ctx.phase.prevMega))
+      println(ctx.run.enrichErrorMessage(s"exception while retyping $treeStr of class ${tree.className} # ${tree.uniqueId}"))
+      throw ex
 
   override def inlineExpansion(mdef: DefDef)(using Context): List[Tree] = mdef :: Nil
 

compiler/src/dotty/tools/package.scala

@@ -39,12 +39,27 @@ package object tools {
   def unreachable(x: Any = "<< this case was declared unreachable >>"): Nothing =
     throw new MatchError(x)
 
-  transparent inline def assertShort(inline assertion: Boolean, inline message: Any = null): Unit =
-    if !assertion then
-      val msg = message
-      val e = if msg == null then AssertionError() else AssertionError("assertion failed: " + msg)
-      e.setStackTrace(Array())
-      throw e
+  transparent inline def assertShort(inline assertion: Boolean, inline message: Any | Null = null): Unit =
+    if !assertion then throwAssertionShortError(message)
+
+  private def throwAssertionShortError(msg: Any): Nothing =
+    val e = AssertionError("assertion failed: " + String.valueOf(msg))
+    e.setStackTrace(Array())
+    throw e
+
+  import dotty.tools.dotc.core.Contexts.Context
+
+  transparent inline def assert(inline assertion: Boolean, inline message: Any | Null = null)(using inline ctx: Context | Null = null): Unit =
+    if !assertion then throwAssertionError(message)
+
+  // extracted from `assert` to make it as small (and inlineable) as possible
+  private def throwAssertionError(msg: Any | Null)(using ctx: Context | Null): Nothing =
+    if ctx == null || ctx.run == null then
+      throw AssertionError("assertion failed: " + String.valueOf(msg))
+    else if ctx.run == null then
+      throw AssertionError("assertion failed: " + dotc.report.enrichErrorMessage(String.valueOf(msg)))
+    else
+      throw AssertionError("assertion failed: " + ctx.run.enrichErrorMessage(String.valueOf(msg)))
 
   // Ensure this object is already classloaded, since it's only actually used
   // when handling stack overflows and every operation (including class loading)