experimental support for outputting code snippets in error messages; implements #301

see #301 for description.
currently, the feature is activated by setting the verbosity level to 2 or more

compiler/msgs.nim

@@ -506,6 +506,8 @@ var gCodegenLineInfo* = newLineInfo(int32(1), 1, 1)
 proc raiseRecoverableError*(msg: string) {.noinline, noreturn.} =
   raise newException(ERecoverableError, msg)
 
+proc sourceLine*(i: TLineInfo): PRope
+
 var
   gNotes*: TNoteKinds = {low(TNoteKind)..high(TNoteKind)} - {warnShadowIdent}
   gErrorCounter*: int = 0     # counts the number of errors
@@ -707,6 +709,11 @@ proc rawMessage*(msg: TMsgKind, arg: string) =
 var
   lastError = UnknownLineInfo()
 
+proc writeSurroundingSrc(info: TLineInfo) =
+  const indent = "  "
+  MsgWriteln(indent & info.sourceLine.data)
+  MsgWriteln(indent & repeatChar(info.col, ' ') & '^')
+
 proc liMessage(info: TLineInfo, msg: TMsgKind, arg: string, 
                eh: TErrorHandling) =
   var frmt: string
@@ -732,6 +739,8 @@ proc liMessage(info: TLineInfo, msg: TMsgKind, arg: string,
                   coordToStr(info.col), getMessageStr(msg, arg)]
   if not ignoreMsg:
     MsgWriteln(s)
+    if optPrintSurroundingSrc and msg in errMin..errMax:
+      info.writeSurroundingSrc
   handleError(msg, eh, s)
   
 proc Fatal*(info: TLineInfo, msg: TMsgKind, arg = "") = 
@@ -771,6 +780,12 @@ proc addSourceLine*(fileIdx: int32, line: string) =
 
 proc sourceLine*(i: TLineInfo): PRope =
   if i.fileIndex < 0: return nil
+  
+  if not optPreserveOrigSource and
+         fileInfos[i.fileIndex].lines.len == 0:
+    for line in lines(i.toFullPath):
+      addSourceLine i.fileIndex, line.string
+
   InternalAssert i.fileIndex < fileInfos.len and
                  i.line <= fileInfos[i.fileIndex].lines.len