Implement cancellation

Add an `isCancelled` variable to `Run`. If this variable is true,
`typed` and `typedImplicit` calls will return immediately.

Author: odersky

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

@@ -31,6 +31,12 @@ import scala.util.control.NonFatal
 /** A compiler run. Exports various methods to compile source files */
 class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with ConstraintRunInfo {
 
+  /** If this variable is set to `true`, some core typer operations will
+   *  return immediately. Currently these early abort operatoons are
+   *  `Typer.typed` and `Implicits.typedImplicit`.
+   */
+  var isCancelled = false
+
   /** Produces the following contexts, from outermost to innermost
    *
    *    bootStrap:   A context with next available runId and a scope consisting of

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

@@ -1484,6 +1484,7 @@ trait Implicits { self: Typer =>
 
     /** Try to typecheck an implicit reference */
     def typedImplicit(cand: Candidate, contextual: Boolean)(implicit ctx: Context): SearchResult = trace(i"typed implicit ${cand.ref}, pt = $pt, implicitsEnabled == ${ctx.mode is ImplicitsEnabled}", implicits, show = true) {
+      if ctx.run.isCancelled then return NoMatchingImplicitsFailure
       record("typedImplicit")
       val ref = cand.ref
       val generated: Tree = tpd.ref(ref).withSpan(span.startPos)

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

@@ -2210,7 +2210,9 @@ class Typer extends Namer
       if (tree.source != ctx.source && tree.source.exists)
         typed(tree, pt, locked)(ctx.withSource(tree.source))
       else
-        try adapt(typedUnadapted(tree, pt, locked), pt, locked)
+        try
+          if ctx.run.isCancelled then tree.withType(WildcardType)
+          else adapt(typedUnadapted(tree, pt, locked), pt, locked)
         catch {
           case ex: TypeError =>
             errorTree(tree, ex, tree.sourcePos.focus)