Derive Macro MaxEncodedLen 

#[derive(MaxEncodedLen)]
{
    // Attributes available to this derive:
    #[max_encoded_len_mod]
}

Available on crate feature max-encoded-len only.

Derive parity_scale_codec::MaxEncodedLen for struct and enum.

Top level attribute

By default the macro will try to bound the types needed to implement MaxEncodedLen, but the bounds can be specified manually with the top level attribute:

#[codec(mel_bound(T: MaxEncodedLen))]