Introduce -Xpatmat-analysis-timeout

Author: dwijnand

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

@@ -62,6 +62,30 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
    */
   @volatile var isCancelled = false
 
+  /** The timeout for pattern match exhaustivity analysis, in ms.
+   *  When the timeout is reached, it is reduced for the next analysis (see "backoff").
+   *  When the timeout is not reached, it is recovered (up to the original, see "recover").
+   * */
+  private var myExhaustivityAnalysisTimeout: Int =
+    Int.MinValue // sentinel value; means whatever is set in command line option
+
+  def exhaustivityAnalysisTimeout: Int =
+    if myExhaustivityAnalysisTimeout == Int.MinValue
+    then ctx.settings.XpatmatAnalysisTimeout.value
+    else myExhaustivityAnalysisTimeout
+
+  /** Exponentially back off, by halving on every timeout, to a minimum 100 ms. */
+  def backoffExhaustivityAnalysisTimeout(): Unit =
+    myExhaustivityAnalysisTimeout =
+      (exhaustivityAnalysisTimeout / 2)
+        .max(100)
+
+  /** Recover slowly, by 1.5 times, up to the original value. */
+  def recoverExhaustivityAnalysisTimeout(): Unit =
+    myExhaustivityAnalysisTimeout =
+      (exhaustivityAnalysisTimeout * 1.5).toInt
+        .min(ictx.settings.XpatmatAnalysisTimeout.value)
+
   private var compiling = false
 
   private var myUnits: List[CompilationUnit] = Nil

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -335,6 +335,7 @@ private sealed trait XSettings:
   val XverifySignatures: Setting[Boolean] = BooleanSetting(AdvancedSetting, "Xverify-signatures", "Verify generic signatures in generated bytecode.")
   val XignoreScala2Macros: Setting[Boolean] = BooleanSetting(AdvancedSetting, "Xignore-scala2-macros", "Ignore errors when compiling code that calls Scala2 macros, these will fail at runtime.")
   val XimportSuggestionTimeout: Setting[Int] = IntSetting(AdvancedSetting, "Ximport-suggestion-timeout", "Timeout (in ms) for searching for import suggestions when errors are reported.", 8000)
+  val XpatmatAnalysisTimeout: Setting[Int] = IntSetting(AdvancedSetting, "Xpatmat-analysis-timeout", "Timeout (in ms) for match analysis.", 8 * 1000) // 8s
   val Xsemanticdb: Setting[Boolean] = BooleanSetting(AdvancedSetting, "Xsemanticdb", "Store information in SemanticDB.", aliases = List("-Ysemanticdb"))
   val XuncheckedJavaOutputVersion: Setting[String] = ChoiceSetting(AdvancedSetting, "Xunchecked-java-output-version", "target", "Emit bytecode for the specified version of the Java platform. This might produce bytecode that will break at runtime. Corresponds to -target flag in javac. When on JDK 9+, consider -java-output-version as a safer alternative.", ScalaSettingsProperties.supportedTargetVersions, "", aliases = List("-Xtarget", "--Xtarget"))
   val XcheckMacros: Setting[Boolean] = BooleanSetting(AdvancedSetting, "Xcheck-macros", "Check some invariants of macro generated code while expanding macros", aliases = List("--Xcheck-macros"))

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -115,13 +115,15 @@ object Formatting {
       given Show[Char]                                = ShowAny
       given Show[Boolean]                             = ShowAny
       given Show[Integer]                             = ShowAny
+      given Show[Long]                                = ShowAny
       given Show[String]                              = ShowAny
       given Show[Class[?]]                            = ShowAny
       given Show[Throwable]                           = ShowAny
       given Show[StringBuffer]                        = ShowAny
       given Show[CompilationUnit]                     = ShowAny
       given Show[Phases.Phase]                        = ShowAny
       given Show[TyperState]                          = ShowAny
+      given Show[Unit]                                = ShowAny
       given Show[config.ScalaVersion]                 = ShowAny
       given Show[io.AbstractFile]                     = ShowAny
       given Show[parsing.Scanners.Scanner]            = ShowAny

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

@@ -96,6 +96,7 @@ trait TraceSyntax:
                         (op: => T)(using Context): T =
     if ctx.mode.is(Mode.Printing) || !isForced && (printer eq Printers.noPrinter) then op
     else
+      val start = System.nanoTime
       // Avoid evaluating question multiple time, since each evaluation
       // may cause some extra logging output.
       val q = question
@@ -109,7 +110,13 @@ trait TraceSyntax:
       def finalize(msg: String) =
         if !finalized then
           ctx.base.indent -= 1
-          doLog(s"$margin$msg")
+          val stop = System.nanoTime
+          val diffNs = stop - start
+          val diffS = (diffNs / 1000 / 1000).toInt / 1000.0
+          if diffS > 0.1 then
+            doLog(s"$margin$msg (${"%.2f".format(diffS)} s)")
+          else
+            doLog(s"$margin$msg")
           finalized = true
       try
         doLog(s"$margin$leading")

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -111,6 +111,20 @@ case class Prod(tp: Type, unappTp: TermRef, params: List[Space]) extends Space
 case class Or(spaces: Seq[Space]) extends Space
 
 object SpaceEngine {
+  val DeadlineKey = new Property.Key[Long]
+
+  def setupDeadline(using Context): Context =
+    val timeout = ctx.run.nn.exhaustivityAnalysisTimeout
+    if timeout >= 0 then
+      ctx.fresh.setProperty(DeadlineKey, System.currentTimeMillis() + timeout)
+    else
+      ctx
+
+  def isPastDeadline(using Context): Boolean =
+    ctx.property(DeadlineKey) match
+      case Some(deadline) => System.currentTimeMillis() > deadline
+      case _              => false
+
   def simplify(space: Space)(using Context): Space           = space.simplify
   def isSubspace(a: Space, b: Space)(using Context): Boolean = a.isSubspace(b)
   def canDecompose(typ: Typ)(using Context): Boolean         = typ.canDecompose
@@ -590,6 +604,7 @@ object SpaceEngine {
 
   /** Whether the extractor covers the given type */
   def covers(unapp: TermRef, scrutineeTp: Type, argLen: Int)(using Context): Boolean = trace(i"covers($unapp, $scrutineeTp, $argLen)") {
+    !isPastDeadline && (
     SpaceEngine.isIrrefutable(unapp, argLen)
     || unapp.symbol == defn.TypeTest_unapply && {
       val AppliedType(_, _ :: tp :: Nil) = unapp.prefix.widen.dealias: @unchecked
@@ -599,6 +614,7 @@ object SpaceEngine {
       val AppliedType(_, tp :: Nil) = unapp.prefix.widen.dealias: @unchecked
       scrutineeTp <:< tp
     }
+    )
   }
 
   /** Decompose a type into subspaces -- assume the type can be decomposed */
@@ -666,13 +682,16 @@ object SpaceEngine {
 
         val parts = children.map { sym =>
           val sym1 = if (sym.is(ModuleClass)) sym.sourceModule else sym
-          val refined = trace(i"refineUsingParent($tp, $sym1, $mixins)")(TypeOps.refineUsingParent(tp, sym1, mixins))
+          val refined =
+            if isPastDeadline then NoType
+            else TypeOps.refineUsingParent(tp, sym1, mixins)
 
           def inhabited(tp: Type): Boolean = tp.dealias match
             case AndType(tp1, tp2) => !TypeComparer.provablyDisjoint(tp1, tp2)
             case OrType(tp1, tp2) => inhabited(tp1) || inhabited(tp2)
             case tp: RefinedType => inhabited(tp.parent)
             case tp: TypeRef => inhabited(tp.prefix)
+            case NoType => false
             case _ => true
 
           if inhabited(refined) then refined
@@ -862,6 +881,21 @@ object SpaceEngine {
     case _                                          => tp
   })
 
+  def checkExhaustivityInDeadline(m: Match)(using Context): Unit = {
+    inContext(setupDeadline):
+      checkExhaustivity(m)
+      if isPastDeadline then
+        ctx.run.nn.backoffExhaustivityAnalysisTimeout()
+        val setting = ctx.settings.XpatmatAnalysisTimeout
+        report.warning(
+          em"""Match analysis requires more time than allowed. You can try:
+              |  * doubling the timeout: ${setting.name}:${setting.value * 2}
+              |  * disabling the timeout: ${setting.name}:-1
+              |  * adding `: @unchecked` to the scrutinee""", m.srcPos)
+      else
+        ctx.run.nn.recoverExhaustivityAnalysisTimeout()
+  }
+
   def checkExhaustivity(m: Match)(using Context): Unit = trace(i"checkExhaustivity($m)") {
     val selTyp = toUnderlying(m.selector.tpe.stripUnsafeNulls()).dealias
     val targetSpace = trace(i"targetSpace($selTyp)")(project(selTyp))
@@ -880,7 +914,8 @@ object SpaceEngine {
 
     if uncovered.nonEmpty then
       val deduped = dedup(uncovered)
-      report.warning(PatternMatchExhaustivity(deduped, m), m.selector)
+      if !isPastDeadline then
+        report.warning(PatternMatchExhaustivity(deduped, m), m.selector)
   }
 
   private def reachabilityCheckable(sel: Tree)(using Context): Boolean =
@@ -929,7 +964,8 @@ object SpaceEngine {
         then {
           val nullOnly = isNullable && i == len - 1 && isWildcardArg(pat)
           val msg = if nullOnly then MatchCaseOnlyNullWarning() else MatchCaseUnreachable()
-          report.warning(msg, pat.srcPos)
+          if !isPastDeadline then
+            report.warning(msg, pat.srcPos)
         }
         deferred = Nil
       }
@@ -941,6 +977,6 @@ object SpaceEngine {
   }
 
   def checkMatch(m: Match)(using Context): Unit =
-    if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
+    if exhaustivityCheckable(m.selector) then checkExhaustivityInDeadline(m)
     if reachabilityCheckable(m.selector) then checkReachability(m)
 }

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

@@ -264,7 +264,7 @@ trait ImportSuggestions:
   end importSuggestions
 
   /** Reduce next timeout for import suggestions by the amount of time it took
-   *  for current search, but but never less than to half of the previous budget.
+   *  for current search, but never less than to half of the previous budget.
    */
   private def reduceTimeBudget(used: Int)(using Context) =
     val run = ctx.run.nn