gh-156241: Add a no-escape fast path to the _json str escapers

Misc/NEWS.d/next/Library/2026-08-13-09-00-00.gh-issue-129711.eScFP1.rst

@@ -0,0 +1 @@
+Speed up :func:`json.dump` for strings that need no escaping.

Modules/_json.c

@@ -223,6 +223,28 @@ ascii_escape_unicode_and_size(const void *input, int kind, Py_ssize_t input_char
     return rval;
 }
 
+static PyObject *
+quote_unescaped_unicode(PyObject *pystr)
+{
+    Py_ssize_t len = PyUnicode_GET_LENGTH(pystr);
+    PyObject *rval = PyUnicode_New(len + 2, PyUnicode_MAX_CHAR_VALUE(pystr));
+    if (rval == NULL) {
+        return NULL;
+    }
+    int kind = PyUnicode_KIND(rval);
+    void *data = PyUnicode_DATA(rval);
+    PyUnicode_WRITE(kind, data, 0, '"');
+    if (PyUnicode_CopyCharacters(rval, 1, pystr, 0, len) < 0) {
+        Py_DECREF(rval);
+        return NULL;
+    }
+    PyUnicode_WRITE(kind, data, len + 1, '"');
+#ifdef Py_DEBUG
+    assert(_PyUnicode_CheckConsistency(rval, 1));
+#endif
+    return rval;
+}
+
 static PyObject *
 ascii_escape_unicode(PyObject *pystr)
 {
@@ -236,6 +258,10 @@ ascii_escape_unicode(PyObject *pystr)
         return NULL;
     }
 
+    if (output_size == input_chars + 2) {
+        return quote_unescaped_unicode(pystr);
+    }
+
     return ascii_escape_unicode_and_size(input, kind, input_chars, output_size);
 }
 
@@ -383,6 +409,10 @@ escape_unicode(PyObject *pystr)
         return NULL;
     }
 
+    if (output_size == input_chars + 2) {
+        return quote_unescaped_unicode(pystr);
+    }
+
     return escape_unicode_and_size(input, kind, maxchar, input_chars, output_size);
 }