feat: impl Deref and DerefMut without exposing Secret

I'm not sure what the usecase for this is.
Originally I thought it was required for #58 but I was wrong.

It was fun implementing this with a bit of unsafe but I don't know if I'll merge it.

Cargo.toml

@@ -23,6 +23,7 @@ serde = ["dep:serde"]
 zeroize = ["dep:zeroize"]
 
 [dependencies]
+bytemuck = { version = "1.15", default-features = false }
 fake = { version = "2.5", optional = true, default-features = false }
 rand = { version = "0.8", optional = true, default-features = false }
 serde = { version = "1.0", optional = true, default-features = false }

src/lib.rs

@@ -1,7 +1,6 @@
 #![doc = include_str!("../README.md")]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![cfg_attr(not(feature = "std"), no_std)]
-#![forbid(unsafe_code)]
 
 #[cfg(feature = "std")]
 mod error;

src/ops.rs

@@ -1,6 +1,9 @@
 use crate::Secret;
 
-use core::ops;
+use core::{
+    ops,
+    ops::{Deref, DerefMut},
+};
 
 macro_rules! ops {
     { ($type:tt, $trait:ident, $method:ident), $($tt:tt)* } => {
@@ -69,3 +72,35 @@ ops! {
     (unary, Neg, neg),
     (unary, Not, not),
 }
+
+use bytemuck::TransparentWrapper;
+
+/// We introduce this private wrapper around `Secret` to safely implement [TransparentWrapper] without
+/// inadvertently providing a way for `Secret`s to be exposed without using `Secret::expose_secret`
+#[repr(transparent)]
+struct Wrapper<T: ?Sized>(Secret<T>);
+
+// SAFETY: `Secret` and `Wrapper` both contains only a single field and are `#[repr(transparent)]`.
+// This meets the documented requirements [bytemuck::TransparentWrapper] as long as we
+// do not override any of its methods.
+unsafe impl<T: ?Sized> bytemuck::TransparentWrapper<T> for Wrapper<T> {}
+
+impl<T> Deref for Secret<T>
+where
+    T: Deref,
+{
+    type Target = Secret<T::Target>;
+
+    fn deref(&self) -> &Self::Target {
+        &Wrapper::wrap_ref(self.0.deref()).0
+    }
+}
+
+impl<T> DerefMut for Secret<T>
+where
+    T: DerefMut,
+{
+    fn deref_mut(&mut self) -> &mut Secret<T::Target> {
+        &mut Wrapper::wrap_mut(self.0.deref_mut()).0
+    }
+}