bpo-40246: Revert reporting of invalid string prefixes (GH-19888)

Due to backwards compatibility concerns regarding keywords immediately followed by a string without whitespace between them (like in `bg="#d00" if clear else"#fca"`) will fail to parse,
commit 41d5b94af4 has to be reverted.

Include/errcode.h

@@ -31,7 +31,6 @@ extern "C" {
 #define E_LINECONT      25      /* Unexpected characters after a line continuation */
 #define E_IDENTIFIER    26      /* Invalid characters in identifier */
 #define E_BADSINGLE     27      /* Ill-formed single statement input */
-#define E_BADPREFIX     28      /* Bad string prefixes */
 
 #ifdef __cplusplus
 }

Lib/test/test_fstring.py

@@ -864,7 +864,7 @@ non-important content
                              "Bf''",
                              "BF''",]
         double_quote_cases = [case.replace("'", '"') for case in single_quote_cases]
-        self.assertAllRaise(SyntaxError, 'invalid string prefix',
+        self.assertAllRaise(SyntaxError, 'unexpected EOF while parsing',
                             single_quote_cases + double_quote_cases)
 
     def test_leading_trailing_spaces(self):

Misc/NEWS.d/next/Core and Builtins/2020-05-03-23-28-11.bpo-40246.c1D7x8.rst

@@ -0,0 +1 @@
+Reporting a specialised error message for invalid string prefixes, which was introduced in :issue:`40246`, is being reverted due to backwards compatibility concerns for strings that immediately follow a reserved keyword without whitespace between them. Constructs like `bg="#d00" if clear else"#fca"` were failing to parse, which is not an acceptable breakage on such short notice.

Parser/pegen/pegen.c

@@ -334,9 +334,6 @@ tokenizer_error(Parser *p)
         case E_IDENTIFIER:
             msg = "invalid character in identifier";
             break;
-        case E_BADPREFIX:
-            RAISE_SYNTAX_ERROR("invalid string prefix");
-            return -1;
         case E_EOFS:
             RAISE_SYNTAX_ERROR("EOF while scanning triple-quoted string literal");
             return -1;

Parser/tokenizer.c

@@ -1396,10 +1396,6 @@ tok_get(struct tok_state *tok, const char **p_start, const char **p_end)
         *p_start = tok->start;
         *p_end = tok->cur;
 
-        if (c == '"' || c == '\'') {
-            tok->done = E_BADPREFIX;
-            return ERRORTOKEN;
-        }
         /* async/await parsing block. */
         if (tok->cur - tok->start == 5 && tok->start[0] == 'a') {
             /* May be an 'async' or 'await' token.  For Python 3.7 or

Python/pythonrun.c

@@ -1609,9 +1609,6 @@ err_input(perrdetail *err)
     case E_BADSINGLE:
         msg = "multiple statements found while compiling a single statement";
         break;
-    case E_BADPREFIX:
-        msg = "invalid string prefix";
-        break;
     default:
         fprintf(stderr, "error=%d\n", err->error);
         msg = "unknown parsing error";