Fix #2063: Print tabs in message padding

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

@@ -77,33 +77,30 @@ trait MessageRendering {
   /** The column markers aligned under the error */
   def columnMarker(pos: SourcePosition, offset: Int)(implicit ctx: Context): String = {
     val prefix = " " * (offset - 1)
-    val whitespace = " " * pos.startColumn
+    val padding = pos.startColumnPadding
     val carets = Red {
       if (pos.startLine == pos.endLine)
         "^" * math.max(1, pos.endColumn - pos.startColumn)
       else "^"
     }
-
-    s"$prefix|$whitespace${carets.show}"
+    s"$prefix|$padding${carets.show}"
   }
 
   /** The error message (`msg`) aligned under `pos`
     *
     * @return aligned error message
     */
   def errorMsg(pos: SourcePosition, msg: String, offset: Int)(implicit ctx: Context): String = {
-    val leastWhitespace = msg.lines.foldLeft(Int.MaxValue) { (minPad, line) =>
+    val padding = msg.lines.foldLeft(pos.startColumnPadding) { (pad, line) =>
       val lineLength = stripColor(line).length
-      val currPad = math.min(
-        math.max(0, ctx.settings.pageWidth.value - offset - lineLength),
-        offset + pos.startColumn
-      )
+      val maxPad = math.max(0, ctx.settings.pageWidth.value - offset - lineLength) - offset
 
-      math.min(currPad, minPad)
+      if (maxPad < pad.length) " " * maxPad
+      else pad
     }
 
     msg.lines
-      .map { line => " " * (offset - 1) + "|" + (" " * (leastWhitespace - offset)) + line}
+      .map { line => " " * (offset - 1) + "|" + padding + line}
       .mkString(sys.props("line.separator"))
   }
 

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

@@ -136,6 +136,17 @@ case class SourceFile(file: AbstractFile, content: Array[Char]) extends interfac
     col
   }
 
+  /** The padding of the column corresponding to `offset`, includes tabs */
+  def startColumnPadding(offset: Int): String = {
+    var idx = startOfLine(offset)
+    val pad = new StringBuilder
+    while (idx != offset) {
+      pad.append(if (idx < length && content(idx) == '\t') '\t' else ' ')
+      idx += 1
+    }
+    pad.result()
+  }
+
   override def toString = file.toString
 }
 

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

@@ -39,6 +39,7 @@ extends interfaces.SourcePosition {
   def start: Int = pos.start
   def startLine: Int = source.offsetToLine(start)
   def startColumn: Int = source.column(start)
+  def startColumnPadding: String = source.startColumnPadding(start)
 
   def end: Int = pos.end
   def endLine: Int = source.offsetToLine(end)

tests/repl/i2063.check

@@ -0,0 +1,22 @@
+scala> 	class Foo extends Bar // with one tab
+-- [E006] Unbound Identifier Error: <console>:4:18 -----------------------------
+4 |	class Foo extends Bar // with one tab
+  |	                  ^^^
+  |	                  not found: type Bar
+
+longer explanation available when compiling with `-explain`
+scala>                                                                               class Foo extends Bar // with spaces
+-- [E006] Unbound Identifier Error: <console>:4:96 -----------------------------
+4 |                                                                              class Foo extends Bar // with spaces
+  |                                                                                                ^^^
+  |                                                       not found: type Bar
+
+longer explanation available when compiling with `-explain`
+scala>                                                                                  class Foo extends Bar // with tabs
+-- [E006] Unbound Identifier Error: <console>:4:99 -----------------------------
+4 |                                                                                 class Foo extends Bar // with tabs
+  |                                                                                                   ^^^
+  |                                                       not found: type Bar
+
+longer explanation available when compiling with `-explain`
+scala> :quit