fix(flask): use rustup snap

Fixes #1743

charmcraft/extensions/gunicorn.py

@@ -158,7 +158,13 @@ def _get_root_snippet(self) -> dict[str, Any]:
                 "grafana-dashboard": {"interface": "grafana_dashboard"},
             },
             "config": {"options": {**self._WEBSERVER_OPTIONS, **self.options}},
-            "parts": {"charm": {"plugin": "charm", "source": "."}},
+            "parts": {
+                "charm": {
+                    "plugin": "charm",
+                    "source": ".",
+                    "build-snaps": ["rustup"],  # Needed to build pydantic.
+                }
+            }
         }
 
     @override
@@ -225,12 +231,6 @@ def is_experimental(base: tuple[str, ...] | None) -> bool:  # noqa: ARG004
         """Check if the extension is in an experimental state."""
         return False
 
-    @override
-    def get_parts_snippet(self) -> dict[str, Any]:
-        """Return the parts to add to parts."""
-        # rust is needed to build pydantic-core, a dependency of flask.
-        return {"flask-framework/rust-deps": {"plugin": "nil", "build-packages": ["cargo"]}}
-
 
 class DjangoFramework(_GunicornBase):
     """Extension for 12-factor Django applications."""

tests/spread/commands/init-flask-framework/task.yaml

@@ -1,9 +1,11 @@
 summary: test charmcraft init with flask-framework profile
+priority: 500  # This builds pydantic, so do it early
+kill-timeout: 45m  # Because it builds pydantic, it takes a long time.
+systems:
+  # We only need to run this test once, and it takes a long time.
+  - ubuntu-22.04-64
 
 execute: |
-  unset CHARMCRAFT_STORE_API_URL
-  unset CHARMCRAFT_UPLOAD_URL
-  unset CHARMCRAFT_REGISTRY_URL
   mkdir -p test-init
   cd test-init
   charmcraft init --profile flask-framework