Offset line numbers when reporting given a magic comment

Author: Linyxus

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

@@ -11,7 +11,7 @@ import core.Decorators.*
 import printing.Highlighting.{Blue, Red, Yellow}
 import printing.SyntaxHighlighting
 import Diagnostic.*
-import util.{ SourcePosition, NoSourcePosition }
+import util.{ SourcePosition, NoSourcePosition, WrappedSourceFile }
 import util.Chars.{ LF, CR, FF, SU }
 import scala.annotation.switch
 
@@ -44,7 +44,9 @@ trait MessageRendering {
     var maxLen = Int.MinValue
     def render(offsetAndLine: (Int, String)): String = {
       val (offset1, line) = offsetAndLine
-      val lineNbr = (pos.source.offsetToLine(offset1) + 1).toString
+      val magicOffset = WrappedSourceFile.locateMagicHeader(pos.source).getOrElse(0)
+      println(i"magicOffset: $magicOffset")
+      val lineNbr = (pos.source.offsetToLine(offset1) + 1 - magicOffset).toString
       val prefix = String.format(s"%${offset - 2}s |", lineNbr)
       maxLen = math.max(maxLen, prefix.length)
       val lnum = hl(" " * math.max(0, maxLen - prefix.length - 1) + prefix)

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

@@ -7,7 +7,7 @@ import core.Contexts.*
 import collection.mutable
 import scala.annotation.tailrec
 import dotty.tools.dotc.reporting.Reporter
-import dotty.tools.dotc.util.SourcePosition;
+import dotty.tools.dotc.util.SourcePosition
 
 import java.io.OutputStreamWriter
 import java.nio.charset.StandardCharsets.UTF_8

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -61,6 +61,21 @@ object ScriptSourceFile {
   }
 }
 
+object WrappedSourceFile:
+  private val cache: mutable.HashMap[SourceFile, Int] = mutable.HashMap.empty
+  def locateMagicHeader(sourceFile: SourceFile)(using Context): Option[Int] =
+    def findOffset: Int =
+      val magicHeader = ctx.settings.XmagicOffsetHeader.value
+      if magicHeader.isEmpty then
+        -1
+      else
+        val s = new String(sourceFile.content)
+        val regex = ("(?m)^" + java.util.regex.Pattern.quote(magicHeader) + "$").r
+        val pos = regex.findFirstMatchIn(s).map(_.start).map(sourceFile.offsetToLine(_))
+        pos.getOrElse(-1)
+    val result = cache.getOrElseUpdate(sourceFile, findOffset)
+    if result >= 0 then Some(result + 1) else None
+
 class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends interfaces.SourceFile {
   import SourceFile.*