hashes: Use associated cost for pre-tagging

Instead of requiring users of the `Tag` trait to implement the `engine`
method we can have an associated const and provide an `engine` method
with a default implementation.

Use an associated const for the pre-tagged hash engine. Fro now keep the
`engine` trait method but have a default impl that returns the const. We
will remove it as a separate patch to assist review.

hashes/src/macros.rs

@@ -33,11 +33,7 @@ macro_rules! sha256t_tag {
         $crate::sha256t_tag_struct!($tag_vis, $tag, stringify!($hash_name), $(#[$($tag_attr)*])*);
 
         impl $crate::sha256t::Tag for $tag {
-            #[inline]
+            const MIDSTATE: $crate::sha256::Midstate = $crate::sha256t_tag_constructor!($constructor, $($tag_value)+);
-            fn engine() -> $crate::sha256::HashEngine {
-                const MIDSTATE: $crate::sha256::Midstate = $crate::sha256t_tag_constructor!($constructor, $($tag_value)+);
-                $crate::sha256::HashEngine::from_midstate(MIDSTATE)
-            }
         }
     }
 }

hashes/src/sha256t/mod.rs

@@ -11,8 +11,13 @@ type HashEngine = sha256::HashEngine;
 
 /// Trait representing a tag that can be used as a context for SHA256t hashes.
 pub trait Tag {
+    /// The [`Midstate`] after pre-tagging the hash engine.
+    const MIDSTATE: sha256::Midstate;
+
     /// Returns a hash engine that is pre-tagged and is ready to be used for the data.
-    fn engine() -> sha256::HashEngine;
+    fn engine() -> sha256::HashEngine {
+        sha256::HashEngine::from_midstate(Self::MIDSTATE)
+    }
 }
 
 /// Output of the SHA256t hash function.
@@ -190,11 +195,7 @@ mod tests {
     pub struct TestHashTag;
 
     impl sha256t::Tag for TestHashTag {
-        fn engine() -> sha256::HashEngine {
+        const MIDSTATE: sha256::Midstate = sha256::Midstate::new(TEST_MIDSTATE, 64);
-            // The TapRoot TapLeaf midstate.
-            let midstate = sha256::Midstate::new(TEST_MIDSTATE, 64);
-            sha256::HashEngine::from_midstate(midstate)
-        }
     }
 
     // We support manually implementing `Tag` and creating a tagged hash from it.

hashes/tests/regression.rs

@@ -40,10 +40,7 @@ impl_regression_test! {
 pub struct RegHashTag;
 
 impl sha256t::Tag for RegHashTag {
-    fn engine() -> sha256::HashEngine {
+    const MIDSTATE: sha256::Midstate = sha256::Midstate::new([0xab; 32], 64);
-        let midstate = sha256::Midstate::new([0xab; 32], 64);
-        sha256::HashEngine::from_midstate(midstate)
-    }
 }
 
 type RegHash = sha256t::Hash<RegHashTag>;