bpo-41084: Adjust message when an f-string expression causes a SyntaxError (GH-21084)

Prefix the error message with `fstring: `, when parsing an f-string expression throws a `SyntaxError`.
(cherry picked from commit 2e0a920e9eb540654c0bb2298143b00637dc5961)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/Parser/pegen/pegen.c b/Parser/pegen/pegen.c
index a1a59ae..53591d2 100644
--- a/Parser/pegen/pegen.c
+++ b/Parser/pegen/pegen.c
@@ -391,6 +391,21 @@ _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
     PyObject *tmp = NULL;
     p->error_indicator = 1;
 
+    if (p->start_rule == Py_fstring_input) {
+        const char *fstring_msg = "f-string: ";
+        Py_ssize_t len = strlen(fstring_msg) + strlen(errmsg);
+
+        char *new_errmsg = PyMem_RawMalloc(len + 1); // Lengths of both strings plus NULL character
+        if (!new_errmsg) {
+            return (void *) PyErr_NoMemory();
+        }
+
+        // Copy both strings into new buffer
+        memcpy(new_errmsg, fstring_msg, strlen(fstring_msg));
+        memcpy(new_errmsg + strlen(fstring_msg), errmsg, strlen(errmsg));
+        new_errmsg[len] = 0;
+        errmsg = new_errmsg;
+    }
     errstr = PyUnicode_FromFormatV(errmsg, va);
     if (!errstr) {
         goto error;
@@ -427,11 +442,17 @@ _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
 
     Py_DECREF(errstr);
     Py_DECREF(value);
+    if (p->start_rule == Py_fstring_input) {
+        PyMem_RawFree((void *)errmsg);
+    }
     return NULL;
 
 error:
     Py_XDECREF(errstr);
     Py_XDECREF(error_line);
+    if (p->start_rule == Py_fstring_input) {
+        PyMem_RawFree((void *)errmsg);
+    }
     return NULL;
 }