bpo-40334: Fix error location upon parsing an invalid string literal (GH-19962)

When parsing a string with an invalid escape, the old parser used to
point to the beginning of the invalid string. This commit changes the new
parser to match that behaviour, since it's currently pointing to the
end of the string (or to be more precise, to the beginning of the next
token).

4 files changed, 30 insertions, 23 deletions

diff --git a/Parser/pegen/parse_string.c b/Parser/pegen/parse_string.c
index d96303d..ca4b733 100644
--- a/Parser/pegen/parse_string.c
+++ b/Parser/pegen/parse_string.c
@@ -12,7 +12,7 @@
 // file (like "_PyPegen_raise_syntax_error").
 
 static int
-warn_invalid_escape_sequence(Parser *p, unsigned char first_invalid_escape_char)
+warn_invalid_escape_sequence(Parser *p, unsigned char first_invalid_escape_char, Token *t)
 {
     PyObject *msg =
         PyUnicode_FromFormat("invalid escape sequence \\%c", first_invalid_escape_char);
@@ -20,11 +20,16 @@ warn_invalid_escape_sequence(Parser *p, unsigned char first_invalid_escape_char)
         return -1;
     }
     if (PyErr_WarnExplicitObject(PyExc_DeprecationWarning, msg, p->tok->filename,
-                                 p->tok->lineno, NULL, NULL) < 0) {
+                                 t->lineno, NULL, NULL) < 0) {
         if (PyErr_ExceptionMatches(PyExc_DeprecationWarning)) {
             /* Replace the DeprecationWarning exception with a SyntaxError
                to get a more accurate error report */
             PyErr_Clear();
+
+            /* This is needed, in order for the SyntaxError to point to the token t,
+               since _PyPegen_raise_error uses p->tokens[p->fill - 1] for the
+               error location, if p->known_err_token is not set. */
+            p->known_err_token = t;
             RAISE_SYNTAX_ERROR("invalid escape sequence \\%c", first_invalid_escape_char);
         }
         Py_DECREF(msg);
@@ -47,7 +52,7 @@ decode_utf8(const char **sPtr, const char *end)
 }
 
 static PyObject *
-decode_unicode_with_escapes(Parser *parser, const char *s, size_t len)
+decode_unicode_with_escapes(Parser *parser, const char *s, size_t len, Token *t)
 {
     PyObject *v, *u;
     char *buf;
@@ -110,7 +115,7 @@ decode_unicode_with_escapes(Parser *parser, const char *s, size_t len)
     v = _PyUnicode_DecodeUnicodeEscape(s, len, NULL, &first_invalid_escape);
 
     if (v != NULL && first_invalid_escape != NULL) {
-        if (warn_invalid_escape_sequence(parser, *first_invalid_escape) < 0) {
+        if (warn_invalid_escape_sequence(parser, *first_invalid_escape, t) < 0) {
             /* We have not decref u before because first_invalid_escape points
                inside u. */
             Py_XDECREF(u);
@@ -123,7 +128,7 @@ decode_unicode_with_escapes(Parser *parser, const char *s, size_t len)
 }
 
 static PyObject *
-decode_bytes_with_escapes(Parser *p, const char *s, Py_ssize_t len)
+decode_bytes_with_escapes(Parser *p, const char *s, Py_ssize_t len, Token *t)
 {
     const char *first_invalid_escape;
     PyObject *result = _PyBytes_DecodeEscape(s, len, NULL, &first_invalid_escape);
@@ -132,7 +137,7 @@ decode_bytes_with_escapes(Parser *p, const char *s, Py_ssize_t len)
     }
 
     if (first_invalid_escape != NULL) {
-        if (warn_invalid_escape_sequence(p, *first_invalid_escape) < 0) {
+        if (warn_invalid_escape_sequence(p, *first_invalid_escape, t) < 0) {
             Py_DECREF(result);
             return NULL;
         }
@@ -146,9 +151,14 @@ decode_bytes_with_escapes(Parser *p, const char *s, Py_ssize_t len)
    If the string is an f-string, set *fstr and *fstrlen to the unparsed
    string object.  Return 0 if no errors occurred.  */
 int
-_PyPegen_parsestr(Parser *p, const char *s, int *bytesmode, int *rawmode, PyObject **result,
-         const char **fstr, Py_ssize_t *fstrlen)
+_PyPegen_parsestr(Parser *p, int *bytesmode, int *rawmode, PyObject **result,
+                  const char **fstr, Py_ssize_t *fstrlen, Token *t)
 {
+    const char *s = PyBytes_AsString(t->bytes);
+    if (s == NULL) {
+        return -1;
+    }
+
     size_t len;
     int quote = Py_CHARMASK(*s);
     int fmode = 0;
@@ -245,7 +255,7 @@ _PyPegen_parsestr(Parser *p, const char *s, int *bytesmode, int *rawmode, PyObje
             *result = PyBytes_FromStringAndSize(s, len);
         }
         else {
-            *result = decode_bytes_with_escapes(p, s, len);
+            *result = decode_bytes_with_escapes(p, s, len, t);
         }
     }
     else {
@@ -253,7 +263,7 @@ _PyPegen_parsestr(Parser *p, const char *s, int *bytesmode, int *rawmode, PyObje
             *result = PyUnicode_DecodeUTF8Stateful(s, len, NULL, NULL);
         }
         else {
-            *result = decode_unicode_with_escapes(p, s, len);
+            *result = decode_unicode_with_escapes(p, s, len, t);
         }
     }
     return *result == NULL ? -1 : 0;
@@ -637,7 +647,7 @@ exit:
 */
 static int
 fstring_find_literal(Parser *p, const char **str, const char *end, int raw,
-                     PyObject **literal, int recurse_lvl)
+                     PyObject **literal, int recurse_lvl, Token *t)
 {
     /* Get any literal string. It ends when we hit an un-doubled left
        brace (which isn't part of a unicode name escape such as
@@ -660,7 +670,7 @@ fstring_find_literal(Parser *p, const char **str, const char *end, int raw,
                 }
                 break;
             }
-            if (ch == '{' && warn_invalid_escape_sequence(p, ch) < 0) {
+            if (ch == '{' && warn_invalid_escape_sequence(p, ch, t) < 0) {
                 return -1;
             }
         }
@@ -704,7 +714,7 @@ done:
                                                     NULL, NULL);
         else
             *literal = decode_unicode_with_escapes(p, literal_start,
-                                                   s - literal_start);
+                                                   s - literal_start, t);
         if (!*literal)
             return -1;
     }
@@ -1041,7 +1051,7 @@ fstring_find_literal_and_expr(Parser *p, const char **str, const char *end, int
     assert(*literal == NULL && *expression == NULL);
 
     /* Get any literal string. */
-    result = fstring_find_literal(p, str, end, raw, literal, recurse_lvl);
+    result = fstring_find_literal(p, str, end, raw, literal, recurse_lvl, t);
     if (result < 0)
         goto error;
 
diff --git a/Parser/pegen/parse_string.h b/Parser/pegen/parse_string.h
index 4f2aa94..cd85bd5 100644
--- a/Parser/pegen/parse_string.h
+++ b/Parser/pegen/parse_string.h
@@ -34,8 +34,8 @@ typedef struct {
 } FstringParser;
 
 void _PyPegen_FstringParser_Init(FstringParser *);
-int _PyPegen_parsestr(Parser *, const char *, int *, int *, PyObject **,
-             const char **, Py_ssize_t *);
+int _PyPegen_parsestr(Parser *, int *, int *, PyObject **,
+                      const char **, Py_ssize_t *, Token *);
 int _PyPegen_FstringParser_ConcatFstring(Parser *, FstringParser *, const char **,
                                 const char *, int, int, Token *, Token *,
                                 Token *);
diff --git a/Parser/pegen/pegen.c b/Parser/pegen/pegen.c
index c311593..06af53b 100644
--- a/Parser/pegen/pegen.c
+++ b/Parser/pegen/pegen.c
@@ -383,7 +383,7 @@ _PyPegen_raise_error(Parser *p, PyObject *errtype, int with_col_number, const ch
     PyObject *errstr = NULL;
     PyObject *loc = NULL;
     PyObject *tmp = NULL;
-    Token *t = p->tokens[p->fill - 1];
+    Token *t = p->known_err_token != NULL ? p->known_err_token : p->tokens[p->fill - 1];
     Py_ssize_t col_number = !with_col_number;
     va_list va;
     p->error_indicator = 1;
@@ -1053,6 +1053,7 @@ _PyPegen_Parser_New(struct tok_state *tok, int start_rule, int flags,
     p->starting_col_offset = 0;
     p->flags = flags;
     p->feature_version = feature_version;
+    p->known_err_token = NULL;
 
     return p;
 }
@@ -1972,12 +1973,7 @@ _PyPegen_concatenate_strings(Parser *p, asdl_seq *strings)
         const char *fstr;
         Py_ssize_t fstrlen = -1;
 
-        char *this_str = PyBytes_AsString(t->bytes);
-        if (!this_str) {
-            goto error;
-        }
-
-        if (_PyPegen_parsestr(p, this_str, &this_bytesmode, &this_rawmode, &s, &fstr, &fstrlen) != 0) {
+        if (_PyPegen_parsestr(p, &this_bytesmode, &this_rawmode, &s, &fstr, &fstrlen, t) != 0) {
             goto error;
         }
 
diff --git a/Parser/pegen/pegen.h b/Parser/pegen/pegen.h
index cbe6f19..ffb18e4 100644
--- a/Parser/pegen/pegen.h
+++ b/Parser/pegen/pegen.h
@@ -71,6 +71,7 @@ typedef struct {
     int flags;
     int feature_version;
     growable_comment_array type_ignore_comments;
+    Token *known_err_token;
 } Parser;
 
 typedef struct {