Demonstrate how xsbti.Problem#quickfix could be used

This requires https://github.com/smarter/sbt/tree/quickFix

The following code should emit a Problem containing a quickfix
replacing `f _` by `() => f`:
```
class Foo {
  def f: Int = { println("hi"); 1 }
  val g = f _
}
```

TODO: This broke -rewrite since I commented out the `patch` call, ideally they
shouldn't be needed since -rewrite should be able to use the information stored
in the `Message`.

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

@@ -13,6 +13,8 @@ import scala.language.unsafeNulls
 
 import scala.annotation.threadUnsafe
 
+import rewrites.Rewrites.Patch
+
 /** ## Tips for error message generation
  *
  *  - You can use the `em` interpolator for error messages. It's defined in core.Decorators.
@@ -384,6 +386,8 @@ abstract class Message(val errorId: ErrorMessageID)(using Context) { self =>
    */
   def showAlways = false
 
+  def quickFix(using Context): java.util.List[Patch] = java.util.Collections.emptyList
+
   override def toString = msg
 }
 

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

@@ -29,6 +29,8 @@ import transform.SymUtils._
 import scala.util.matching.Regex
 import java.util.regex.Matcher.quoteReplacement
 import cc.CaptureSet.IdentityCaptRefMap
+import util.Spans._
+import rewrites.Rewrites.Patch
 
 /**  Messages
   *  ========
@@ -1796,12 +1798,19 @@ class FailureToEliminateExistential(tp: Type, tp1: Type, tp2: Type, boundSyms: L
         |are only approximated in a best-effort way."""
 }
 
-class OnlyFunctionsCanBeFollowedByUnderscore(tp: Type)(using Context)
+class OnlyFunctionsCanBeFollowedByUnderscore(tp: Type, tree: untpd.PostfixOp)(using Context)
   extends SyntaxMsg(OnlyFunctionsCanBeFollowedByUnderscoreID) {
   def msg(using Context) = i"Only function types can be followed by ${hl("_")} but the current expression has type $tp"
   def explain(using Context) =
     i"""The syntax ${hl("x _")} is no longer supported if ${hl("x")} is not a function.
         |To convert to a function value, you need to explicitly write ${hl("() => x")}"""
+  override def quickFix(using Context) =
+    val untpd.PostfixOp(qual, Ident(nme.WILDCARD)) = tree: @unchecked
+    import scala.jdk.CollectionConverters.*
+    import scala.language.unsafeNulls
+    List(
+      Patch(Span(tree.span.start), "(() => "),
+      Patch(Span(qual.span.end, tree.span.end), ")")).asJava
 }
 
 class MissingEmptyArgumentList(method: String)(using Context)

compiler/src/dotty/tools/dotc/rewrites/Rewrites.scala

@@ -15,7 +15,7 @@ import java.nio.charset.StandardCharsets.UTF_8
 object Rewrites {
   private class PatchedFiles extends mutable.HashMap[SourceFile, Patches]
 
-  private case class Patch(span: Span, replacement: String) {
+  case class Patch(span: Span, replacement: String) {
     def delta = replacement.length - (span.end - span.start)
   }
 

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

@@ -2799,11 +2799,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case closure(_, _, _) =>
       case _ =>
         val recovered = typed(qual)(using ctx.fresh.setExploreTyperState())
-        report.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(recovered.tpe.widen), tree.srcPos, from = `3.0`)
+        report.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(recovered.tpe.widen, tree), tree.srcPos, from = `3.0`)
         if (migrateTo3) {
           // Under -rewrite, patch `x _` to `(() => x)`
-          patch(Span(tree.span.start), "(() => ")
-          patch(Span(qual.span.end, tree.span.end), ")")
+          // patch(Span(tree.span.start), "(() => ")
+          // patch(Span(qual.span.end, tree.span.end), ")")
           return typed(untpd.Function(Nil, qual), pt)
         }
     }

project/Build.scala

@@ -549,6 +549,7 @@ object Build {
       libraryDependencies ++= Seq(
         "org.scala-lang.modules" % "scala-asm" % "9.4.0-scala-1", // used by the backend
         Dependencies.oldCompilerInterface, // we stick to the old version to avoid deprecation warnings
+        "org.scala-sbt" % "util-interface" % "1.8.1-SNAPSHOT",
         "org.jline" % "jline-reader" % "3.19.0",   // used by the REPL
         "org.jline" % "jline-terminal" % "3.19.0",
         "org.jline" % "jline-terminal-jna" % "3.19.0", // needed for Windows

sbt-bridge/src/dotty/tools/xsbt/DelegatingReporter.java

@@ -12,9 +12,12 @@
 import dotty.tools.dotc.reporting.Message;
 import dotty.tools.dotc.util.SourceFile;
 import dotty.tools.dotc.util.SourcePosition;
+import dotty.tools.dotc.rewrites.Rewrites.Patch;
 import xsbti.Position;
 import xsbti.Severity;
 
+import static java.util.stream.Collectors.toList;
+
 final public class DelegatingReporter extends AbstractReporter {
   private xsbti.Reporter delegate;
 
@@ -34,7 +37,8 @@ public void printSummary(Context ctx) {
 
   public void doReport(Diagnostic dia, Context ctx) {
     Severity severity = severityOf(dia.level());
-    Position position = positionOf(dia.pos().nonInlined());
+    SourcePosition srcPosition = dia.pos().nonInlined();
+    Position position = positionOf(srcPosition);
 
     StringBuilder rendered = new StringBuilder();
     rendered.append(messageAndPos(dia, ctx));
@@ -47,8 +51,10 @@ public void doReport(Diagnostic dia, Context ctx) {
       rendered.append(explanation(message, ctx));
       messageBuilder.append(System.lineSeparator()).append(explanation(message, ctx));
     }
+    java.util.List<xsbti.TextEdit> quickFix =
+      message.quickFix(ctx).stream().map(patch -> textEditOf(patch, srcPosition.source())).collect(toList());
 
-    delegate.log(new Problem(position, messageBuilder.toString(), severity, rendered.toString(), diagnosticCode));
+    delegate.log(new Problem(position, messageBuilder.toString(), severity, rendered.toString(), diagnosticCode, quickFix));
   }
 
   private static Severity severityOf(int level) {
@@ -71,6 +77,13 @@ private static Position positionOf(SourcePosition pos) {
     }
   }
 
+  // TODO: Replace Patch#span by Patch#SourcePosition to support patches in other files?
+  private static xsbti.TextEdit textEditOf(Patch patch, SourceFile source) {
+    SourcePosition srcPos = SourcePosition.apply(source, patch.span(), dotty.tools.dotc.util.NoSourcePosition$.MODULE$);
+    Position pos = positionOf(srcPos);
+    return new TextEditBridge(pos, patch.replacement());
+  }
+
   @SuppressWarnings("unchecked")
   // [warn] sbt-bridge/src/dotty/tools/xsbt/DelegatingReporter.java:18:1: dotty$tools$dotc$reporting$UniqueMessagePositions$$positions() in dotty.tools.dotc.reporting.AbstractReporter implements dotty$tools$dotc$reporting$UniqueMessagePositions$$positions() in dotty.tools.dotc.reporting.UniqueMessagePositions
   // [warn]   return type requires unchecked conversion from scala.collection.mutable.HashMap to scala.collection.mutable.HashMap<scala.Tuple2<dotty.tools.dotc.util.SourceFile,java.lang.Integer>,dotty.tools.dotc.reporting.Diagnostic>

sbt-bridge/src/dotty/tools/xsbt/Problem.java

@@ -1,23 +1,27 @@
 package dotty.tools.xsbt;
 
+import java.util.List;
 import java.util.Optional;
 import xsbti.Position;
 import xsbti.Severity;
+import xsbti.TextEdit;
 
 final public class Problem implements xsbti.Problem {
   private final Position _position;
   private final String _message;
   private final Severity _severity;
   private final Optional<String> _rendered;
   private final String _diagnosticCode;
+  private final List<TextEdit> _quickFix;
 
-  public Problem(Position position, String message, Severity severity, String rendered, String diagnosticCode) {
+  public Problem(Position position, String message, Severity severity, String rendered, String diagnosticCode, List<TextEdit> quickFix) {
     super();
     this._position = position;
     this._message = message;
     this._severity = severity;
     this._rendered = Optional.of(rendered);
     this._diagnosticCode = diagnosticCode;
+    this._quickFix = quickFix;
   }
 
   public String category() {
@@ -40,6 +44,10 @@ public Optional<String> rendered() {
     return _rendered;
   }
 
+  public List<TextEdit> quickFix() {
+    return _quickFix;
+  }
+
   public Optional<xsbti.DiagnosticCode> diagnosticCode() {
     // We don't forward the code if it's -1 since some tools will assume that this is actually
     // the diagnostic code and show it or attempt to use it. This will ensure tools consuming

sbt-bridge/src/dotty/tools/xsbt/TextEditBridge.java

@@ -0,0 +1,35 @@
+/*
+ * Zinc - The incremental compiler for Scala.
+ * Copyright Lightbend, Inc. and Mark Harrah
+ */
+
+package dotty.tools.xsbt;
+
+import dotty.tools.dotc.util.SourceFile;
+import dotty.tools.dotc.util.SourcePosition;
+import dotty.tools.io.AbstractFile;
+import xsbti.Position;
+import xsbti.TextEdit;
+
+import java.io.File;
+import java.util.Optional;
+
+public class TextEditBridge implements TextEdit {
+  private final Position position;
+  private final String newText;
+
+  public TextEditBridge(Position position, String newText) {
+    this.position = position;
+    this.newText = newText;
+  }
+
+  @Override
+  public Position position() {
+    return position;
+  }
+
+  @Override
+  public String newText() {
+    return newText;
+  }
+}