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/elections-phragmen/src/lib.rs

@@ -102,9 +102,9 @@ use codec::{Decode, Encode};
 use frame_support::{
 	dispatch::WithPostDispatchInfo,
 	traits::{
-		ChangeMembers, Contains, ContainsLengthBound, Currency, CurrencyToVote, Get,
-		InitializeMembers, LockIdentifier, LockableCurrency, OnUnbalanced, ReservableCurrency,
-		SortedMembers, StorageVersion, WithdrawReasons,
+		defensive_prelude::*, ChangeMembers, Contains, ContainsLengthBound, Currency,
+		CurrencyToVote, Get, InitializeMembers, LockIdentifier, LockableCurrency, OnUnbalanced,
+		ReservableCurrency, SortedMembers, StorageVersion, WithdrawReasons,
 	},
 	weights::Weight,
 };
@@ -1028,7 +1028,7 @@ impl<T: Config> Pallet<T> {
 				candidates_and_deposit
 					.iter()
 					.find_map(|(c, d)| if c == x { Some(*d) } else { None })
-					.unwrap_or_default()
+					.defensive_unwrap_or_default()
 			};
 			// fetch deposits from the one recorded one. This will make sure that a candidate who
 			// submitted candidacy before a change to candidacy deposit will have the correct amount