Diagnostic: only compute the stack trace when needed

Diagnostic extends Exception, but the stack trace is only used for debugging
purposes in combination with ThrowingReporter when using either -Ycheck,
-Ytest-pickler or scala3-staging.
We make this explicit by adding Reporter#computeStackTraces which is queried
in the constructor of Diagnostic.

Author: smarter

compiler/src/dotty/tools/dotc/reporting/Diagnostic.scala

@@ -13,6 +13,8 @@ import java.util.Optional
 import scala.util.chaining._
 import core.Decorators.toMessage
 
+import scala.annotation.constructorOnly
+
 object Diagnostic:
 
   def shouldExplain(dia: Diagnostic)(using Context): Boolean =
@@ -24,8 +26,8 @@ object Diagnostic:
   class Error(
     msg: Message,
     pos: SourcePosition
-  ) extends Diagnostic(msg, pos, ERROR):
-    def this(str: => String, pos: SourcePosition) = this(str.toMessage, pos)
+  )(using @constructorOnly ictx: Context) extends Diagnostic(msg, pos, ERROR):
+    def this(str: => String, pos: SourcePosition)(using Context) = this(str.toMessage, pos)
 
   /** A sticky error is an error that should not be hidden by backtracking and
    *  trying some alternative path. Typically, errors issued after catching
@@ -34,62 +36,68 @@ object Diagnostic:
   class StickyError(
     msg: Message,
     pos: SourcePosition
-  ) extends Error(msg, pos)
+  )(using @constructorOnly ictx: Context) extends Error(msg, pos)
 
   class Warning(
     msg: Message,
     pos: SourcePosition
-  ) extends Diagnostic(msg, pos, WARNING) {
-    def toError: Error = new Error(msg, pos).tap(e => if isVerbose then e.setVerbose())
-    def toInfo: Info = new Info(msg, pos).tap(e => if isVerbose then e.setVerbose())
+  )(using @constructorOnly ictx: Context) extends Diagnostic(msg, pos, WARNING) {
+    def toError(using Context): Error = new Error(msg, pos).tap(e => if isVerbose then e.setVerbose())
+    def toInfo(using Context): Info = new Info(msg, pos).tap(e => if isVerbose then e.setVerbose())
     def isSummarizedConditional(using Context): Boolean = false
   }
 
   class Info(
     msg: Message,
     pos: SourcePosition
-  ) extends Diagnostic(msg, pos, INFO):
-    def this(str: => String, pos: SourcePosition) = this(str.toMessage, pos)
+  )(using @constructorOnly ictx: Context) extends Diagnostic(msg, pos, INFO):
+    def this(str: => String, pos: SourcePosition)(using Context) = this(str.toMessage, pos)
 
   abstract class ConditionalWarning(
     msg: Message,
     pos: SourcePosition
-  ) extends Warning(msg, pos) {
+  )(using @constructorOnly ictx: Context) extends Warning(msg, pos) {
     def enablingOption(using Context): Setting[Boolean]
     override def isSummarizedConditional(using Context): Boolean = !enablingOption.value
   }
 
   class FeatureWarning(
     msg: Message,
     pos: SourcePosition
-  ) extends ConditionalWarning(msg, pos) {
+  )(using @constructorOnly ictx: Context) extends ConditionalWarning(msg, pos) {
     def enablingOption(using Context): Setting[Boolean] = ctx.settings.feature
   }
 
   class UncheckedWarning(
     msg: Message,
     pos: SourcePosition
-  ) extends ConditionalWarning(msg, pos) {
+  )(using @constructorOnly ictx: Context) extends ConditionalWarning(msg, pos) {
     def enablingOption(using Context): Setting[Boolean] = ctx.settings.unchecked
   }
 
   class DeprecationWarning(
     msg: Message,
     pos: SourcePosition
-  ) extends ConditionalWarning(msg, pos) {
+  )(using @constructorOnly ictx: Context) extends ConditionalWarning(msg, pos) {
     def enablingOption(using Context): Setting[Boolean] = ctx.settings.deprecation
   }
 
   class MigrationWarning(
     msg: Message,
     pos: SourcePosition
-  ) extends Warning(msg, pos)
+  )(using @constructorOnly ictx: Context) extends Warning(msg, pos)
 
 class Diagnostic(
   val msg: Message,
   val pos: SourcePosition,
   val level: Int
-) extends Exception with interfaces.Diagnostic:
+)(using @constructorOnly ictx: Context) extends Exception(
+    // These are just the default values
+    /*message =*/ null, /*cause =*/ null, /*enableSuppression =*/ true,
+
+    // Performance optimization: don't compute the stack trace unless requested by the reporter.
+    /*writableStackTrace =*/ ictx.reporter.computeStackTraces
+  ) with interfaces.Diagnostic:
   private var verbose: Boolean = false
   def isVerbose: Boolean = verbose
   def setVerbose(): this.type =

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -265,4 +265,9 @@ abstract class Reporter extends interfaces.ReporterResult {
 
   /** If this reporter buffers messages, all buffered messages, otherwise Nil */
   def pendingMessages(using Context): List[Diagnostic] = Nil
+
+  /** If true, Diagnostics created when `ctx.reporter eq this` will have their
+   *  stack traces filled in.
+   */
+  def computeStackTraces: Boolean = false
 }

compiler/src/dotty/tools/dotc/reporting/ThrowingReporter.scala

@@ -8,8 +8,10 @@ import Diagnostic.Error
 /**
  * This class implements a Reporter that throws all errors and sends warnings and other
  * info to the underlying reporter.
+ *
+ * @param computeStackTraces If true, the thrown errors will have their stack traces filled in.
  */
-class ThrowingReporter(reportInfo: Reporter) extends Reporter {
+class ThrowingReporter(reportInfo: Reporter, override val computeStackTraces: Boolean) extends Reporter {
   def doReport(dia: Diagnostic)(using Context): Unit = dia match {
     case _: Error => throw dia
     case _ => reportInfo.doReport(dia)

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -119,7 +119,7 @@ class Pickler extends Phase {
       testUnpickler(
         using ctx2
             .setPeriod(Period(ctx.runId + 1, ctx.base.typerPhase.id))
-            .setReporter(new ThrowingReporter(ctx.reporter))
+            .setReporter(new ThrowingReporter(ctx.reporter, computeStackTraces = true))
             .addMode(Mode.ReadPositions)
             .addMode(Mode.PrintShowExceptions))
     result

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -135,7 +135,7 @@ class TreeChecker extends Phase with SymTransformer {
     val checkingCtx = ctx
         .fresh
         .addMode(Mode.ImplicitsEnabled)
-        .setReporter(new ThrowingReporter(ctx.reporter))
+        .setReporter(new ThrowingReporter(ctx.reporter, computeStackTraces = true))
 
     val checker = inContext(ctx) {
       new Checker(previousPhases(phasesToRun.toList))

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

@@ -2390,7 +2390,9 @@ trait Applications extends Compatibility {
     val methodRefTree = ref(methodRef, needLoad = false)
     val truncatedSym = methodRef.symbol.asTerm.copy(info = truncateExtension(methodRef.info))
     val truncatedRefTree = untpd.TypedSplice(ref(truncatedSym)).withSpan(receiver.span)
-    val newCtx = ctx.fresh.setNewScope.setReporter(new reporting.ThrowingReporter(ctx.reporter))
+    val newCtx = ctx.fresh.setNewScope.setReporter(
+      // No need to compute stack traces since we discard all exceptions below.
+      new reporting.ThrowingReporter(ctx.reporter, computeStackTraces = false))
 
     try
       val appliedTree = inContext(newCtx) {

staging/src/scala/quoted/staging/QuoteDriver.scala

@@ -66,6 +66,6 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver:
   private def setCompilerSettings(ctx: FreshContext, settings: Compiler.Settings): ctx.type =
     // An error in the generated code is a bug in the compiler
     // Setting the throwing reporter however will report any exception
-    ctx.setReporter(new ThrowingReporter(ctx.reporter))
+    ctx.setReporter(new ThrowingReporter(ctx.reporter, computeStackTraces = true))
 
 end QuoteDriver