Fix #2063: Print tabs in message padding

Author: nicolasstucki

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

@@ -77,14 +77,13 @@ 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`
@@ -101,9 +100,12 @@ trait MessageRendering {
 
       math.min(currPad, minPad)
     }
+    val padding =
+      if (leastWhitespace - offset == pos.startColumn) pos.startColumnPadding
+      else " " * (leastWhitespace - offset)
 
     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 StringBuffer("")
+    while (idx != offset) {
+      pad.append(if (idx < length && content(idx) == '\t') '\t' else ' ')
+      idx += 1
+    }
+    pad.toString
+  }
+
   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 +1,8 @@
+scala> 	class Foo extends Bar
+-- [E006] Unbound Identifier Error: <console>:4:18 -----------------------------
+4 |	class Foo extends Bar
+  |	                  ^^^
+  |	                  not found: type Bar
+
+longer explanation available when compiling with `-explain`
+scala> :quit