Into version for SecretString expose_secret

[tisonkun]

I found that we have a rewrite also. When do we release 0.9 for secrecy?

For this issue, I found:

impl<S> ExposeSecret<S> for Secret<S>
where
    S: Zeroize,
{
    fn expose_secret(&self) -> &S {
        &self.inner_secret
    }
}

But perhaps I'd like to move the inner secret in some case, and thus an into version can help to avoid copy.

[tony-iqlusion]

into version can help to avoid copy

Since Rust doesn't have placement by return, an API which returns a value will always make a copy

[tisonkun]

@tony-iqlusion but copy the pointer and length of a string (into_inner(self) -> String) is still different to copy the whole payload (self.expose_secret().to_string())?

[tony-iqlusion]

It might be okay for select heap-backed types, namely SecretString and SecretSlice, though it seems a little odd given the goal of the crate of encapsulating and owning/protecting secrets to hand off ownership