digest: add FixedOutputDirty trait + finalize_into*

digest/src/dyn_digest.rs

@@ -34,7 +34,7 @@ impl<D: Update + FixedOutput + Reset + Clone + 'static> DynDigest for D {
     }
 
     fn finalize_reset(&mut self) -> Box<[u8]> {
-        let res = self.clone().finalize_fixed().to_vec().into_boxed_slice();
+        let res = self.finalize_fixed_reset().to_vec().into_boxed_slice();
         Reset::reset(self);
         res
     }

digest/src/lib.rs

@@ -79,8 +79,64 @@ pub trait FixedOutput {
     /// Output size for fixed output digest
     type OutputSize: ArrayLength<u8>;
 
-    /// Retrieve result and consume hasher instance.
-    fn finalize_fixed(self) -> GenericArray<u8, Self::OutputSize>;
+    /// Write result into provided array and consume the hasher instance.
+    fn finalize_into(self, out: &mut GenericArray<u8, Self::OutputSize>);
+
+    /// Write result into provided array and reset the hasher instance.
+    fn finalize_into_reset(&mut self, out: &mut GenericArray<u8, Self::OutputSize>);
+
+    /// Retrieve result and consume the hasher instance.
+    #[inline]
+    fn finalize_fixed(self) -> GenericArray<u8, Self::OutputSize>
+    where
+        Self: Sized,
+    {
+        let mut out = Default::default();
+        self.finalize_into(&mut out);
+        out
+    }
+
+    /// Retrieve result and reset the hasher instance.
+    #[inline]
+    fn finalize_fixed_reset(&mut self) -> GenericArray<u8, Self::OutputSize> {
+        let mut out = Default::default();
+        self.finalize_into_reset(&mut out);
+        out
+    }
+}
+
+/// Trait for fixed-output digest implementations to use to retrieve the
+/// hash output.
+///
+/// Usage of this trait in user code is discouraged. Instead use the
+/// [`FixedOutput::finalize_fixed`] or [`FixedOutput::finalize_fixed_reset`]
+/// methods.
+///
+/// Types which impl this trait along with [`Reset`] will receive a blanket
+/// impl of [`FixedOutput`].
+pub trait FixedOutputDirty {
+    /// Output size for fixed output digest
+    type OutputSize: ArrayLength<u8>;
+
+    /// Retrieve result into provided buffer and leave hasher in a dirty state.
+    ///
+    /// Implementations should panic if this is called twice without resetting.
+    fn finalize_into_dirty(&mut self, out: &mut GenericArray<u8, Self::OutputSize>);
+}
+
+impl<D: FixedOutputDirty + Reset> FixedOutput for D {
+    type OutputSize = D::OutputSize;
+
+    #[inline]
+    fn finalize_into(mut self, out: &mut GenericArray<u8, Self::OutputSize>) {
+        self.finalize_into_dirty(out);
+    }
+
+    #[inline]
+    fn finalize_into_reset(&mut self, out: &mut GenericArray<u8, Self::OutputSize>) {
+        self.finalize_into_dirty(out);
+        self.reset();
+    }
 }
 
 /// Trait for returning digest result with the variable size