lexer: catch concatenation of f'' and '' strings

This turns the "edge case" into a parse-time error.

Author: jepler

py/lexer.c

@@ -583,6 +583,8 @@ void mp_lexer_to_next(mp_lexer_t *lex) {
         // MP_TOKEN_END is used to indicate that this is the first string token
         lex->tok_kind = MP_TOKEN_END;
 
+        bool saw_normal = false, saw_fstring = false;
+
         // Loop to accumulate string/bytes literals
         do {
             // parse type codes
@@ -619,6 +621,17 @@ void mp_lexer_to_next(mp_lexer_t *lex) {
                 is_fstring = true;
             }
 
+            if (is_fstring) {
+                saw_fstring = true;
+            } else {
+                saw_normal = true;
+            }
+
+            if (saw_fstring && saw_normal) {
+                // Can't concatenate f-string with normal string
+                break;
+            }
+
             // Set or check token kind
             if (lex->tok_kind == MP_TOKEN_END) {
                 lex->tok_kind = kind;

tests/basics/string_pep498_fstring.py

@@ -104,10 +104,10 @@ def foo():
 x = 10
 y = 'hi'
 assert (f'h' f'i') == 'hi'
-assert (f'h' 'i') == 'hi'
-assert ('h' f'i') == 'hi'
+#assert (f'h' 'i') == 'hi'
+#assert ('h' f'i') == 'hi'
 assert f'{x:^4}' == ' 10 '
-assert ('a' 'b' f'{x}' f'str<{y:^4}>' 'd' 'e') == 'ab10str< hi >de'
+#assert ('a' 'b' f'{x}' f'str<{y:^4}>' 'd' 'e') == 'ab10str< hi >de'
 
 # Other tests
 assert f'{{{4*10}}}' == '{40}'