Allow somehow validate the decoded output

For example, we want a custom integer type can only old number less than 100:

#[derive(Encode, Decode)]
struct UIntLessThan100(u8);

impl UIntLessThan100 {
  pub new(val: u8) -> Option<Self> { if val < 100 { Some(Self(val)) } else { None } 
  pub add(self, other: Self) -> u8 { self.0 + other.0 } // this can never overflow because we know 100 + 100 < 255
}

But this will result an invalid struct and break the invariant and causing panics.

let bad: UIntLessThan100 = Decode::decode(&mut &[255u8]).unwrap();

BoundedVec have similar issue and the solution is write a custom decode implementation. But it is tedious for large struct.

Start with the issue’s Decode example and compare it with the mentioned BoundedVec behavior and custom Decode implementations. Determine how decoded values could be validated without requiring a tedious custom implementation for large structs. Done should provide a way to preserve invariants such as UIntLessThan100 rejecting invalid encoded values.