Improve diagnostics for non-terminated character literals

During brace matching (before lexing), add an extra diagnostic if one is found

During lexing, this would already be diagnosed - I also tweaked that message to accurately report where the missing `'` should have been (it was off by one and including the character where the `'` should have been in the suggested corrected literal, but that's the first character that can't belong in the literal so it should be excluded)

Author: hsutter

source/io.h

@@ -795,6 +795,13 @@ auto process_cpp2_line(
         prev = line[i];
     }
 
+    if (in_char_literal) {
+        errors.emplace_back(
+            source_position(lineno, ssize(line)),
+            std::string("line ended before character literal was terminated")
+        );
+    }
+
     return found_end;
 }
 

source/lex.h

@@ -1654,9 +1654,10 @@ auto lex_line(
                     if (len > 0) {
                         j += len;
                         if (peek(j) != '\'') {
+                            assert (j > 1);
                             errors.emplace_back(
                                 source_position(lineno, i),
-                                "character literal '" + std::string(&line[i+1],j)
+                                "character literal '" + std::string(&line[i+1],j-1)
                                     + "' is missing its closing '"
                             );
                         }