Performance optimization: only compute stack traces when needed (#16616)

I noticed that `Throwable#fillInStackTrace` (which is called from the
constructor of Throwable) showed up while profiling the compiler. This
is because several data structures we use (`Diagnostic` and `TypeError`)
extend `Exception`. Since the stack trace is only used for debugging in
some rare cases and is expensive to compute, it's worth avoiding it
whenever possible. This can be done either by passing `false` to the
`writableStackTrace` constructor parameter, by overriding
`Throwable#fillInStackTrace` or by extending
`scala.util.control.NoStackTrace`.

I ended up not touching `Diagnostic` in this PR because it will be
changed in #16566 to not extend `Exception`.

Author: smarter

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

@@ -16,6 +16,7 @@ import core.Decorators._
 import config.{SourceVersion, Feature}
 import StdNames.nme
 import scala.annotation.internal.sharable
+import scala.util.control.NoStackTrace
 import transform.MacroAnnotations
 
 class CompilationUnit protected (val source: SourceFile) {
@@ -105,7 +106,7 @@ class CompilationUnit protected (val source: SourceFile) {
 
 object CompilationUnit {
 
-  class SuspendException extends Exception
+  class SuspendException extends Exception with NoStackTrace
 
   /** Make a compilation unit for top class `clsd` with the contents of the `unpickled` tree */
   def apply(clsd: ClassDenotation, unpickled: Tree, forceTrees: Boolean)(using Context): CompilationUnit =

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

@@ -12,10 +12,22 @@ import Denotations._
 import Decorators._
 import reporting._
 import ast.untpd
-import config.Printers.cyclicErrors
+import config.Printers.{cyclicErrors, noPrinter}
+
+import scala.annotation.constructorOnly
 
 abstract class TypeError(using creationContext: Context) extends Exception(""):
 
+  /** Will the stack trace of this exception be filled in?
+   *  This is expensive and only useful for debugging purposes.
+   */
+  def computeStackTrace: Boolean =
+    ctx.debug || (cyclicErrors != noPrinter && this.isInstanceOf[CyclicReference] && !(ctx.mode is Mode.CheckCyclic))
+
+  override def fillInStackTrace(): Throwable =
+    if computeStackTrace then super.fillInStackTrace().nn
+    else this
+
   /** Convert to message. This takes an additional Context, so that we
    *  use the context when the message is first produced, i.e. when the TypeError
    *  is handled. This makes a difference for CyclicErrors since we need to know
@@ -164,7 +176,7 @@ class CyclicReference private (val denot: SymDenotation)(using Context) extends
 object CyclicReference:
   def apply(denot: SymDenotation)(using Context): CyclicReference =
     val ex = new CyclicReference(denot)
-    if !(ctx.mode is Mode.CheckCyclic) || ctx.settings.Ydebug.value then
+    if ex.computeStackTrace then
       cyclicErrors.println(s"Cyclic reference involving! $denot")
       val sts = ex.getStackTrace.asInstanceOf[Array[StackTraceElement]]
       for (elem <- sts take 200)