Improve Multi-line code message rendering

Improve rendering of messages with multi-line code positions. Two improvements:

 - Add a blank line between the error message proper and the code lines that follow it.
 - If there are more than 3 code lines following it, summarize them by printing
   the first and last line and a `...` in between.

Author: odersky

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

@@ -22,6 +22,11 @@ trait MessageRendering {
   import Highlight.*
   import Offsets.*
 
+  /** The maximal number of lines of code that are shown in a message after the
+   *  `^` and error message.
+   */
+  private inline val maxRenderedLinesAfterPoint = 3
+
   /** Remove ANSI coloring from `str`, useful for getting real length of
     * strings
     *
@@ -64,9 +69,21 @@ trait MessageRendering {
     val lines = linesFrom(syntax)
     val (before, after) = pos.beforeAndAfterPoint
 
+    def compress(offsetsAndLines: List[(Int, String)]): List[(Int, String)] =
+      if offsetsAndLines.isEmpty then offsetsAndLines
+      else
+        val compressedLines =
+          if offsetsAndLines.length > maxRenderedLinesAfterPoint then
+            offsetsAndLines.take(maxRenderedLinesAfterPoint - 2)
+            ++ List(
+                (offsetsAndLines(maxRenderedLinesAfterPoint - 2)._1, "..."),
+                offsetsAndLines.last)
+          else offsetsAndLines
+        compressedLines
+
     (
       before.zip(lines).map(render),
-      after.zip(lines.drop(before.length)).map(render),
+      compress(after.zip(lines.drop(before.length))).map(render),
       maxLen
     )
   }
@@ -140,7 +157,7 @@ trait MessageRendering {
     *
     * @return aligned error message
     */
-  private def errorMsg(pos: SourcePosition, msg: String)(using Context, Level, Offset): String = {
+  private def errorMsg(pos: SourcePosition, msg: String, addLine: Boolean)(using Context, Level, Offset): String = {
     val padding = msg.linesIterator.foldLeft(pos.startColumnPadding) { (pad, line) =>
       val lineLength = stripColor(line).length
       val maxPad = math.max(0, ctx.settings.pageWidth.value - offset - lineLength) - offset
@@ -149,9 +166,10 @@ trait MessageRendering {
       else pad
     }
 
-    msg.linesIterator
+    val msgStr = msg.linesIterator
       .map { line => offsetBox + (if line.isEmpty then "" else padding + line) }
       .mkString(EOL)
+    if addLine then msgStr ++ s"${EOL}$offsetBox" else msgStr
   }
 
   // file.path or munge it to normalize for testing
@@ -280,7 +298,7 @@ trait MessageRendering {
     if pos.exists && pos1.exists && pos1.source.file.exists then
       val (srcBefore, srcAfter, offset) = sourceLines(pos1)
       val marker = positionMarker(pos1)
-      val err = errorMsg(pos1, msg.message)
+      val err = errorMsg(pos1, msg.message, srcAfter.nonEmpty)
       sb.append((srcBefore ::: marker :: err :: srcAfter).mkString(EOL))
 
       if inlineStack.nonEmpty then

tests/neg-custom-args/captures/boundary-homebrew.check

@@ -10,6 +10,7 @@
    |where:    ?=> refers to a fresh root capability created in anonymous function of type (using l1²: boundary.Label[boundary.Label[Int]^]^): boundary.Label[Int]^ when checking argument to parameter body of method apply
    |          ^   refers to the universal root capability
    |          cap is a fresh root capability created in anonymous function of type (using l2: boundary.Label[Int]^'s2): Int of parameter parameter l2² of method $anonfun
+   |
 20 |      boundary.break(l2)(using l1)
 21 |      15
    |

tests/neg-custom-args/captures/boundary.check

@@ -10,6 +10,7 @@
    |
    |  where:    ^ and cap refer to the universal root capability
    |            ^²        refers to a fresh root capability classified as Control in the type of value local
+   |
  6 |      boundary[Unit]: l2 ?=>
  7 |        boundary.break(l2)(using l1) // error
  8 |      ???
@@ -47,6 +48,7 @@
    |
    |  where:    ^ and cap² refer to the universal root capability
    |            ^² and cap refer to a fresh root capability created in package <empty>
+   |
  6 |      boundary[Unit]: l2 ?=>
  7 |        boundary.break(l2)(using l1) // error
  8 |      ???

tests/neg-custom-args/captures/capt1.check

@@ -41,6 +41,7 @@
    |  Required: A
    |
    |  Note that capability x is not included in capture set {}.
+   |
 28 |    def m() = if x == null then y else y
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/check-inferred.check

@@ -6,6 +6,7 @@
    |
    |              Inferred type          : () ->{Counter.this.count} Unit
    |              Externally visible type: () -> Unit
+   |
 19 |    count.put(count.get + 1)
 -- Error: tests/neg-custom-args/captures/check-inferred.scala:20:13 ----------------------------------------------------
 20 |  val decr = () => // error
@@ -15,6 +16,7 @@
    |
    |              Inferred type          : () ->{Counter.this.count} Unit
    |              Externally visible type: () -> Unit
+   |
 21 |    count.put(count.get - 1)
 -- Error: tests/neg-custom-args/captures/check-inferred.scala:24:14 ----------------------------------------------------
 24 |  val count = Ref(): Object^ // error // error

tests/neg-custom-args/captures/class-caps.check

@@ -5,6 +5,7 @@
    |                                       Required: (Int, Int) -> Int
    |
    |                                       Note that capability Test.this.console is not included in capture set {}.
+   |
 19 |    log(s"adding a ($a) to b ($b)")(using console)
 20 |    a + b
    |
@@ -16,6 +17,7 @@
    |                                      Required: (Int, Int) -> Int
    |
    |                                      Note that capability Test1.this.console is not included in capture set {}.
+   |
 29 |    log(s"adding a ($a) to b ($b)")(using console)
 30 |    a + b
    |

tests/neg-custom-args/captures/classifiers-secondclass.check

@@ -7,6 +7,7 @@
    |Note that capability f.write is not included in capture set {cap.only[Read]}.
    |
    |where:    cap is a fresh root capability created in anonymous function of type (f²: Levels.File^): Unit when checking argument to parameter op of method parReduce
+   |
 42 |        f.write(42)         // the error stems from here
 43 |        a + b + f.read()    // ok
    |
@@ -20,6 +21,7 @@
    |Note that capability f.write is not included in capture set {cap.only[Read]}.
    |
    |where:    cap is a fresh root capability created in anonymous function of type (g²: Levels.File^): Unit when checking argument to parameter op of method parReduce
+   |
 54 |          f.write(42)         // the error stems from here
 55 |          a + b + f.read() + g.read() // ok
    |
@@ -33,6 +35,7 @@
    |Note that capability g.write is not included in capture set {cap.only[Read]}.
    |
    |where:    cap is a fresh root capability created in anonymous function of type (g²: Levels.File^): Unit when checking argument to parameter op of method parReduce
+   |
 57 |          g.write(42)         // the error stems from here
 58 |          0
    |

tests/neg-custom-args/captures/delayedRunops.check

@@ -5,6 +5,7 @@
    |    Required: () -> Unit
    |
    |    Note that capability ops* is not included in capture set {}.
+   |
 13 |      val ops1 = ops
 14 |      runOps(ops1)
    |
@@ -16,6 +17,7 @@
    |    Required: () -> Unit
    |
    |    Note that capability ops* is not included in capture set {}.
+   |
 25 |      val ops1: List[() ->{ops*} Unit] = ops
 26 |      runOps(ops1)
    |

tests/neg-custom-args/captures/effect-swaps-explicit.check

@@ -6,6 +6,7 @@
    |      Required: Result[Future[T], Nothing]
    |
    |      Note that capability fr is not included in capture set {}.
+   |
 65 |          fr.await.ok
    |--------------------------------------------------------------------------------------------------------------------
    |Inline stack trace
@@ -27,6 +28,7 @@
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
+   |
 70 |            fr.await.ok
    |
    | longer explanation available when compiling with `-explain`
@@ -40,6 +42,7 @@
    |
    |where:    ?=> refers to a fresh root capability created in method fail5 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
+   |
 74 |          Future: fut ?=>
 75 |            fr.await.ok
    |

tests/neg-custom-args/captures/effect-swaps.check

@@ -6,6 +6,7 @@
    |      Required: Result[Future[T], Nothing]
    |
    |      Note that capability fr is not included in capture set {}.
+   |
 65 |          fr.await.ok
    |--------------------------------------------------------------------------------------------------------------------
    |Inline stack trace
@@ -27,6 +28,7 @@
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
+   |
 70 |            fr.await.ok
    |
    | longer explanation available when compiling with `-explain`
@@ -40,6 +42,7 @@
    |
    |where:    ?=> refers to a fresh root capability created in method fail5 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
+   |
 74 |          Future: fut ?=>
 75 |            fr.await.ok
    |