Proposal: Defensive trait for infallible frame operations (#10626)

* add a blueprint of how defensive traits could look like

* add something for arithmetic as well

* add some use cases in different pallets

* some build fixing

* Some new stuff and examples

* Fix deadly bug

* add more doc.

* undo faulty change to assets pallet

* Update frame/support/src/traits/misc.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* some review comments

* remove draft comment

* Fix ident test

* Fix proxy tests as well

* a few new ideas

* Fix build

* Fix doc

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/election-provider-multi-phase/src/signed.rs

@@ -25,7 +25,7 @@ use crate::{
 use codec::{Decode, Encode, HasCompact};
 use frame_support::{
 	storage::bounded_btree_map::BoundedBTreeMap,
-	traits::{Currency, Get, OnUnbalanced, ReservableCurrency},
+	traits::{defensive_prelude::*, Currency, Get, OnUnbalanced, ReservableCurrency},
 };
 use sp_arithmetic::traits::SaturatedConversion;
 use sp_npos_elections::{is_score_better, ElectionScore, NposSolution};
@@ -365,7 +365,7 @@ impl<T: Config> Pallet<T> {
 			let active_voters = raw_solution.solution.voter_count() as u32;
 			let feasibility_weight = {
 				// defensive only: at the end of signed phase, snapshot will exits.
-				let desired_targets = Self::desired_targets().unwrap_or_default();
+				let desired_targets = Self::desired_targets().defensive_unwrap_or_default();
 				T::WeightInfo::feasibility_check(voters, targets, active_voters, desired_targets)
 			};
 			// the feasibility check itself has some weight