Add handling of escaped backslashes in expanded string (hsutter#464)

The current change iterate over entire string end removes extra backslashes that are used to indicate escape sequence. Closes hsutter#461

Author: filipsajdak

source/lex.h

@@ -379,13 +379,15 @@ auto expand_string_literal(
  "\"", // end sequence
  string_parts::on_both_ends}; // add opening and closing sequence to generated string
 
+ bool escape = false;
  // Now we're on the first character of the string itself
  for (
  ;
- pos < length && (text[pos] != '"' || (text[pos-1] == '\\' && pos>=2 && text[pos-2] != '\\'));
+ pos < length && !(!escape && text[pos] == '"');
  ++pos
  )
  {
+ escape = (text[pos] == '\\' && !escape);
  // Find the next )$
  if (
  text[pos] == '$'
@@ -426,7 +428,13 @@ auto expand_string_literal(
 
  // Then put interpolated chunk into ret
  auto chunk = std::string{text.substr(open, pos - open)};
- replace_all(chunk, "\\\"", "\"");
+ { // unescape chunk string
+ auto last_it = std::copy_if(std::begin(chunk), std::end(chunk), std::begin(chunk), [escape = false](const auto& e) mutable {
+ escape = !escape && e == '\\';
+ return !escape;
+ });
+ chunk.erase(last_it, std::end(chunk));
+ }
  parts.add_code("cpp2::to_string" + chunk);
 
  current_start = pos+1;