fix parser error message

Fixes #4000

src/parse_tree.cpp

@@ -29,13 +29,13 @@ static bool production_is_empty(const production_element_t *production) {
 /// Returns a string description of this parse error.
 wcstring parse_error_t::describe_with_prefix(const wcstring &src, const wcstring &prefix,
                                              bool is_interactive, bool skip_caret) const {
+    if (skip_caret || source_start >= src.size() || source_start + source_length > src.size()) {
+        return L"";
+    }
+
     wcstring result = prefix;
     result.append(this->text);
 
-    if (skip_caret || source_start >= src.size() || source_start + source_length > src.size()) {
-        return result;
-    }
-
     // Locate the beginning of this line of source.
     size_t line_start = 0;
 
@@ -64,15 +64,12 @@ wcstring parse_error_t::describe_with_prefix(const wcstring &src, const wcstring
     // Don't include the caret and line if we're interactive this is the first line, because
     // then it's obvious.
     bool interactive_skip_caret = is_interactive && source_start == 0;
-
     if (interactive_skip_caret) {
         return result;
     }
 
     // Append the line of text.
-    if (!result.empty()) {
+    result.push_back(L'\n');
-        result.push_back(L'\n');
-    }
     result.append(src, line_start, line_end - line_start);
 
     // Append the caret line. The input source may include tabs; for that reason we

src/parser.h

@@ -3,6 +3,7 @@
 #define FISH_PARSER_H
 
 #include <stddef.h>
+#include <unistd.h>
 
 #include <csignal>
 #include <list>