Address PR review comments

rs/nns/governance/canister/canister.rs

@@ -738,7 +738,20 @@ fn update_neuron(neuron: Neuron) -> Option<GovernanceError> {
 
 #[cfg(feature = "test")]
 #[update]
-/// Internal method for calling update_neuron.
+/// Adopt the (currently open) proposal with `proposal_id` in a testing environment.
+///
+/// This function simulates a 100% YES vote rate for a given proposal.
+///
+/// ### Applicability
+///
+/// Testing with the golden state often involves open proposals that had been created
+/// before the state snapshot was taken. This function allows observing the effect that such
+/// proposals would have.
+///
+/// When possible, it is recommended to avoid using this function, instead creating a powerful
+/// neuron that gets proposals adopted in testing. Calling this function may result in a less
+/// realistic scenario, e.g., it does not guaranteed that the tally and the ballots will remain
+/// synchronized for `proposal_id`.
 fn adopt_proposal(proposal_id: ProposalIdProto) -> Option<GovernanceError> {
     debug_log("adopt_proposal");
     governance_mut()

rs/nns/governance/src/governance.rs

@@ -2305,7 +2305,7 @@ impl Governance {
 
         assert_eq!(proposal_data.status(), ProposalStatus::Open);
 
-        let Some(mut tally) = proposal_data.latest_tally else {
+        let Some(tally) = proposal_data.latest_tally.as_mut() else {
             panic!("No tally specified for proposal {}", proposal_id.id);
         };
 
@@ -2317,9 +2317,9 @@ impl Governance {
             "Bug in test-only code"
         );
 
-        proposal_data.latest_tally.replace(tally);
-
-        // Ensure that the proposal action will be processed ASAP.
+        // Ensure that the proposal action will be processed ASAP (this ensures that
+        // the precondition of `Governance.process_proposals()` holds in the post-state
+        // of this function).
         self.closest_proposal_deadline_timestamp_seconds = now;
 
         Ok(())