Macro secure 

macro_rules! secure {
    ([u8; $N:literal], $expr:expr $(,)?) => { ... };
    ($ty:ty, $expr:expr $(,)?) => { ... };
    (String, $expr:expr $(,)?) => { ... };
    (Vec<u8>, $expr:expr $(,)?) => { ... };
    (heap $ty:ty, $expr:expr $(,)?) => { ... };
}

Create a secret — works for fixed-size arrays and heap types.