Merge rust-bitcoin/rust-bitcoin#1761: Feature (hashes): Add from_bytes_ref and from_bytes_mut to all Hash types

91863c8a78 Feature (hashes): Add from_bytes_ref and from_bytes_mut to all Hash types (junderw)

Pull request description:

  Closes #1756

  I added tests to one of the Hash types, let me know if this seems reasonable as a test and I'll add it for all the other Hash types.

ACKs for top commit:
  apoelstra:
    ACK 91863c8a78
  tcharding:
    ACK 91863c8a78

Tree-SHA512: e1fc278eab4c13b04462636d424b05c599d8f75d76ac8c50cff195085af5ab71aee8a74386d58a457a7ab528226ff4686b3fd0c0019530835709ac1c9577281e

hashes/src/internal_macros.rs

@@ -84,6 +84,20 @@ macro_rules! hash_trait_impls {
             pub fn backward_hex(&self) -> impl '_ + core::fmt::LowerHex + core::fmt::UpperHex {
                 internals::hex::display::DisplayArray::<_, [u8; $bits / 8 * 2]>::new(self.0.iter().rev())
             }
+
+            /// Zero cost conversion between a fixed length byte array shared reference and
+            /// a shared reference to this Hash type.
+            pub fn from_bytes_ref(bytes: &[u8; $bits / 8]) -> &Self {
+                // Safety: Sound because Self is #[repr(transparent)] containing [u8; $bits / 8]
+                unsafe { &*(bytes as *const _ as *const Self) }
+            }
+
+            /// Zero cost conversion between a fixed length byte array exclusive reference and
+            /// an exclusive reference to this Hash type.
+            pub fn from_bytes_mut(bytes: &mut [u8; $bits / 8]) -> &mut Self {
+                // Safety: Sound because Self is #[repr(transparent)] containing [u8; $bits / 8]
+                unsafe { &mut *(bytes as *mut _ as *mut Self) }
+            }
         }
 
         impl<$($gen: $gent),*> str::FromStr for Hash<$($gen),*> {

hashes/src/ripemd160.rs

@@ -405,6 +405,8 @@ mod tests {
     #[test]
     #[cfg(feature = "alloc")]
     fn test() {
+        use std::convert::TryFrom;
+
         use crate::{ripemd160, Hash, HashEngine};
 
         #[derive(Clone)]
@@ -465,12 +467,24 @@ mod tests {
             },
         ];
 
-        for test in tests {
+        for mut test in tests {
             // Hash through high-level API, check hex encoding/decoding
             let hash = ripemd160::Hash::hash(test.input.as_bytes());
             assert_eq!(hash, test.output_str.parse::<ripemd160::Hash>().expect("parse hex"));
             assert_eq!(&hash[..], &test.output[..]);
             assert_eq!(&hash.to_string(), &test.output_str);
+            assert_eq!(
+                ripemd160::Hash::from_bytes_ref(
+                    <&[u8; 20]>::try_from(&*test.output).expect("known length")
+                ),
+                &hash
+            );
+            assert_eq!(
+                ripemd160::Hash::from_bytes_mut(
+                    <&mut [u8; 20]>::try_from(&mut *test.output).expect("known length")
+                ),
+                &hash
+            );
 
             // Hash through engine, checking that we can input byte by byte
             let mut engine = ripemd160::Hash::engine();