zeroize: Provide ZeroizeOnDrop type about crates HOT 4 CLOSED

I'm planning on adding this, but via a custom derive rather than a wrapper type. This avoids the need to thunk through things like Deref/Borrow solely for the purpose of adding a Drop impl.

I think it'd wind up looking like this:

trait ZeroizeOnDrop: Zeroize + Drop {}

#[derive(ZeroizeOnDrop)]
pub struct MyStruct { ... }

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If you're trying to use someone else's struct from another library you won't be able to leverage a custom derive, so it still seems useful to have a ZeroizeOnDrop utility struct, though it comes at the cost of making the API larger for this library.

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I think a struct ZeroizeAThingOnDrop<T: Zeroize>(T) would also be useful to have, however:

• I'd definitely like to support the custom derive, so we'd need names which clearly disambiguate trait ZeroizeOnDrop and struct ZeroizeAThingOnDrop (the latter is not intended as a serious suggestion)
• This functionality is already provided by the clear_on_drop crate. Would it make more sense to submit a PR to that crate to use zeroize, so all existing users get the benefits of zeroize for free?

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Added Zeroizing<Z> in #182

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