Make decoding of `compact<perthing>` saturating instead of invalid (#7062)

* make decoding of cmopact<perthing> saturating

* fix stable build

* Update primitives/arithmetic/src/per_things.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update primitives/arithmetic/src/per_things.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/primitives/arithmetic/src/per_things.rs

@@ -323,9 +323,28 @@ macro_rules! implement_per_thing {
 		///
 		#[doc = $title]
 		#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
-		#[derive(Encode, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, RuntimeDebug, CompactAs)]
+		#[derive(Encode, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, RuntimeDebug)]
 		pub struct $name($type);
 
+		/// Implementation makes any compact encoding of `PerThing::Inner` valid,
+		/// when decoding it will saturate up to `PerThing::ACCURACY`.
+		impl CompactAs for $name {
+			type As = $type;
+			fn encode_as(&self) -> &Self::As {
+				&self.0
+			}
+			fn decode_from(x: Self::As) -> Self {
+				// Saturates if `x` is more than `$max` internally. 
+				Self::from_parts(x)
+			}
+		}
+
+		impl From<codec::Compact<$name>> for $name {
+			fn from(x: codec::Compact<$name>) -> $name {
+				x.0
+			}
+		}
+
 		impl PerThing for $name {
 			type Inner = $type;
 			type Upper = $upper_type;
@@ -1166,6 +1185,17 @@ macro_rules! implement_per_thing {
 				// deconstruct is also const, hence it can be called in const rhs.
 				const C5: bool = C1.deconstruct() == 0;
 			}
+
+			#[test]
+			fn compact_decoding_saturate_when_beyond_accuracy() {
+				use num_traits::Bounded;
+				use codec::Compact;
+
+				let p = Compact::<$name>::decode(&mut &Compact(<$type>::max_value()).encode()[..])
+					.unwrap();
+				assert_eq!((p.0).0, $max);
+				assert_eq!($name::from(p), $name::max_value());
+			}
 		}
 	};
 }