gh-153568: Pause the GC while converting the AST to Python objects

Freshly converted AST nodes cannot be part of a reference cycle yet, so
collections during the conversion only pay to traverse the half-built
tree without ever freeing any of it.

Author: pablogsal

Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-16-51-51.gh-issue-153568.ast2objgc.rst

@@ -0,0 +1,2 @@
+Speed up :func:`ast.parse` by pausing the garbage collector while the AST is
+converted to Python objects.

Parser/asdl_c.py

@@ -2116,7 +2116,15 @@ class PartingShots(StaticVisitor):
     if (state == NULL) {
         return NULL;
     }
+    // Freshly converted AST nodes cannot be part of a reference cycle yet,
+    // so a garbage collection while building them cannot free anything of
+    // this tree and only pays to traverse its half-built nodes. Pause the
+    // collector while the conversion runs.
+    int gc_was_enabled = PyGC_Disable();
     PyObject *result = ast2obj_mod(state, t);
+    if (gc_was_enabled) {
+        PyGC_Enable();
+    }
 
     return result;
 }