Diagnostics: if a line is longer than 4096 characters, don't print it.

Specifically, don't print snippets, caret diagnostics, or ranges for
lines over 4096 characters. We copy the line around a few times in our
diagnostics machinery, and we have to print a caret line that's just as
long. This uses a lot of memory just to create a poor user experience as
we print out a line much too long for anyone to read...or spend extra
energy trying to fit it to -fmessage-length.

<rdar://problem/13106850>

llvm-svn: 173976

clang/lib/Frontend/TextDiagnostic.cpp

@@ -1096,18 +1096,26 @@ void TextDiagnostic::emitSnippetAndCaret(
 
   unsigned LineNo = SM.getLineNumber(FID, FileOffset);
   unsigned ColNo = SM.getColumnNumber(FID, FileOffset);
+  
+  // Arbitrarily stop showing snippets when the line is too long.
+  static const unsigned MaxLineLength = 4096;
+  if (ColNo > MaxLineLength)
+    return;
 
   // Rewind from the current position to the start of the line.
   const char *TokPtr = BufStart+FileOffset;
   const char *LineStart = TokPtr-ColNo+1; // Column # is 1-based.
 
-
   // Compute the line end.  Scan forward from the error position to the end of
   // the line.
   const char *LineEnd = TokPtr;
   while (*LineEnd != '\n' && *LineEnd != '\r' && *LineEnd != '\0')
     ++LineEnd;
 
+  // Arbitrarily stop showing snippets when the line is too long.
+  if (LineEnd - LineStart > MaxLineLength)
+    return;
+
   // Copy the line of code into an std::string for ease of manipulation.
   std::string SourceLine(LineStart, LineEnd);