secrecy: add SecretSlice<S> (#1214)

Adds a type alias for `SecretBox<[S]>` as a more convenient way to work
with boxed secret slices.

secrecy/src/lib.rs

@@ -36,7 +36,7 @@
 
 extern crate alloc;
 
-use alloc::{boxed::Box, string::String};
+use alloc::{boxed::Box, string::String, vec::Vec};
 use core::{
     any,
     fmt::{self, Debug},
@@ -167,6 +167,23 @@ impl<S: Zeroize + ?Sized> ExposeSecretMut<S> for SecretBox<S> {
     }
 }
 
+/// Secret slice type.
+///
+/// This is a type alias for [`SecretBox<[S]>`] which supports some helpful trait impls.
+///
+/// Notably it has a [`From<Vec<S>>`] impl which is the preferred method for construction.
+pub type SecretSlice<S> = SecretBox<[S]>;
+
+impl<S> From<Vec<S>> for SecretSlice<S>
+where
+    S: Zeroize,
+    [S]: Zeroize,
+{
+    fn from(vec: Vec<S>) -> Self {
+        Self::from(vec.into_boxed_slice())
+    }
+}
+
 /// Secret string type.
 ///
 /// This is a type alias for [`SecretBox<str>`] which supports some helpful trait impls.