hashes: use workaround to get constfns on tagged hashes with MSRV

As you can see from the - lines in the API diff, there is no reduction
in API surface (we just remove the T:Tag bound from the sha256t::Tag
type, which is not strictly necessary but maybe we would prefer to keep).

api/hashes/all-features.txt

@@ -5,7 +5,7 @@
 #[repr(transparent)] pub struct bitcoin_hashes::sha1::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256d::Hash(_)
-#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T: bitcoin_hashes::sha256t::Tag>(_, _)
+#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T>(_, _)
 #[repr(transparent)] pub struct bitcoin_hashes::sha384::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512_256::Hash(_)
@@ -431,7 +431,6 @@ impl<T: bitcoin_hashes::Hash> core::marker::StructuralPartialEq for bitcoin_hash
 impl<T: bitcoin_hashes::Hash> std::io::Write for bitcoin_hashes::hmac::HmacEngine<T>
 impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::Hash for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::serde_macros::serde_details::SerdeHash for bitcoin_hashes::sha256t::Hash<T>
-impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::borrow::Borrow<[u8]> for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::clone::Clone for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::cmp::Eq for bitcoin_hashes::sha256t::Hash<T>
@@ -458,6 +457,8 @@ impl<T: core::cmp::PartialEq + bitcoin_hashes::Hash> core::cmp::PartialEq for bi
 impl<T: core::cmp::PartialOrd + bitcoin_hashes::Hash> core::cmp::PartialOrd for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::hash::Hash + bitcoin_hashes::Hash> core::hash::Hash for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::marker::Copy + bitcoin_hashes::Hash> core::marker::Copy for bitcoin_hashes::hmac::Hmac<T>
+impl<T> bitcoin_hashes::sha256t::Hash<T> where (T): bitcoin_hashes::sha256t::Tag
+impl<T> bitcoin_hashes::sha256t::Tag for (T) where T: bitcoin_hashes::sha256t::Tag
 impl<T> core::marker::Freeze for bitcoin_hashes::hkdf::Hkdf<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::Hmac<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::HmacEngine<T> where <T as bitcoin_hashes::Hash>::Engine: core::marker::Freeze
@@ -578,6 +579,7 @@ pub const fn bitcoin_hashes::siphash24::HashEngine::new() -> bitcoin_hashes::sip
 pub const fn bitcoin_hashes::siphash24::HashEngine::with_keys(k0: u64, k1: u64) -> bitcoin_hashes::siphash24::HashEngine
 pub extern crate bitcoin_hashes::hex
 pub extern crate bitcoin_hashes::serde
+pub fn (T)::engine() -> bitcoin_hashes::sha256::HashEngine
 pub fn bitcoin_hashes::FromSliceError::clone(&self) -> bitcoin_hashes::FromSliceError
 pub fn bitcoin_hashes::FromSliceError::eq(&self, other: &bitcoin_hashes::FromSliceError) -> bool
 pub fn bitcoin_hashes::FromSliceError::expected_length(&self) -> usize

api/hashes/alloc-only.txt

@@ -5,7 +5,7 @@
 #[repr(transparent)] pub struct bitcoin_hashes::sha1::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256d::Hash(_)
-#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T: bitcoin_hashes::sha256t::Tag>(_, _)
+#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T>(_, _)
 #[repr(transparent)] pub struct bitcoin_hashes::sha384::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512_256::Hash(_)
@@ -373,7 +373,6 @@ impl<T: bitcoin_hashes::Hash> core::fmt::Display for bitcoin_hashes::hmac::Hmac<
 impl<T: bitcoin_hashes::Hash> core::fmt::LowerHex for bitcoin_hashes::hmac::Hmac<T>
 impl<T: bitcoin_hashes::Hash> core::marker::StructuralPartialEq for bitcoin_hashes::hmac::Hmac<T>
 impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::Hash for bitcoin_hashes::sha256t::Hash<T>
-impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::borrow::Borrow<[u8]> for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::clone::Clone for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::cmp::Eq for bitcoin_hashes::sha256t::Hash<T>
@@ -398,6 +397,8 @@ impl<T: core::cmp::PartialEq + bitcoin_hashes::Hash> core::cmp::PartialEq for bi
 impl<T: core::cmp::PartialOrd + bitcoin_hashes::Hash> core::cmp::PartialOrd for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::hash::Hash + bitcoin_hashes::Hash> core::hash::Hash for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::marker::Copy + bitcoin_hashes::Hash> core::marker::Copy for bitcoin_hashes::hmac::Hmac<T>
+impl<T> bitcoin_hashes::sha256t::Hash<T> where (T): bitcoin_hashes::sha256t::Tag
+impl<T> bitcoin_hashes::sha256t::Tag for (T) where T: bitcoin_hashes::sha256t::Tag
 impl<T> core::marker::Freeze for bitcoin_hashes::hkdf::Hkdf<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::Hmac<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::HmacEngine<T> where <T as bitcoin_hashes::Hash>::Engine: core::marker::Freeze
@@ -505,6 +506,7 @@ pub const fn bitcoin_hashes::siphash24::Hash::to_byte_array(self) -> [u8; 8]
 pub const fn bitcoin_hashes::siphash24::HashEngine::new() -> bitcoin_hashes::siphash24::HashEngine
 pub const fn bitcoin_hashes::siphash24::HashEngine::with_keys(k0: u64, k1: u64) -> bitcoin_hashes::siphash24::HashEngine
 pub extern crate bitcoin_hashes::hex
+pub fn (T)::engine() -> bitcoin_hashes::sha256::HashEngine
 pub fn bitcoin_hashes::FromSliceError::clone(&self) -> bitcoin_hashes::FromSliceError
 pub fn bitcoin_hashes::FromSliceError::eq(&self, other: &bitcoin_hashes::FromSliceError) -> bool
 pub fn bitcoin_hashes::FromSliceError::expected_length(&self) -> usize

api/hashes/no-features.txt

@@ -5,7 +5,7 @@
 #[repr(transparent)] pub struct bitcoin_hashes::sha1::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha256d::Hash(_)
-#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T: bitcoin_hashes::sha256t::Tag>(_, _)
+#[repr(transparent)] pub struct bitcoin_hashes::sha256t::Hash<T>(_, _)
 #[repr(transparent)] pub struct bitcoin_hashes::sha384::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512::Hash(_)
 #[repr(transparent)] pub struct bitcoin_hashes::sha512_256::Hash(_)
@@ -373,7 +373,6 @@ impl<T: bitcoin_hashes::Hash> core::fmt::Display for bitcoin_hashes::hmac::Hmac<
 impl<T: bitcoin_hashes::Hash> core::fmt::LowerHex for bitcoin_hashes::hmac::Hmac<T>
 impl<T: bitcoin_hashes::Hash> core::marker::StructuralPartialEq for bitcoin_hashes::hmac::Hmac<T>
 impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::Hash for bitcoin_hashes::sha256t::Hash<T>
-impl<T: bitcoin_hashes::sha256t::Tag> bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::borrow::Borrow<[u8]> for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::clone::Clone for bitcoin_hashes::sha256t::Hash<T>
 impl<T: bitcoin_hashes::sha256t::Tag> core::cmp::Eq for bitcoin_hashes::sha256t::Hash<T>
@@ -398,6 +397,8 @@ impl<T: core::cmp::PartialEq + bitcoin_hashes::Hash> core::cmp::PartialEq for bi
 impl<T: core::cmp::PartialOrd + bitcoin_hashes::Hash> core::cmp::PartialOrd for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::hash::Hash + bitcoin_hashes::Hash> core::hash::Hash for bitcoin_hashes::hmac::Hmac<T>
 impl<T: core::marker::Copy + bitcoin_hashes::Hash> core::marker::Copy for bitcoin_hashes::hmac::Hmac<T>
+impl<T> bitcoin_hashes::sha256t::Hash<T> where (T): bitcoin_hashes::sha256t::Tag
+impl<T> bitcoin_hashes::sha256t::Tag for (T) where T: bitcoin_hashes::sha256t::Tag
 impl<T> core::marker::Freeze for bitcoin_hashes::hkdf::Hkdf<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::Hmac<T> where T: core::marker::Freeze
 impl<T> core::marker::Freeze for bitcoin_hashes::hmac::HmacEngine<T> where <T as bitcoin_hashes::Hash>::Engine: core::marker::Freeze
@@ -505,6 +506,7 @@ pub const fn bitcoin_hashes::siphash24::Hash::to_byte_array(self) -> [u8; 8]
 pub const fn bitcoin_hashes::siphash24::HashEngine::new() -> bitcoin_hashes::siphash24::HashEngine
 pub const fn bitcoin_hashes::siphash24::HashEngine::with_keys(k0: u64, k1: u64) -> bitcoin_hashes::siphash24::HashEngine
 pub extern crate bitcoin_hashes::hex
+pub fn (T)::engine() -> bitcoin_hashes::sha256::HashEngine
 pub fn bitcoin_hashes::FromSliceError::clone(&self) -> bitcoin_hashes::FromSliceError
 pub fn bitcoin_hashes::FromSliceError::eq(&self, other: &bitcoin_hashes::FromSliceError) -> bool
 pub fn bitcoin_hashes::FromSliceError::expected_length(&self) -> usize

hashes/src/sha256t.rs

@@ -19,7 +19,7 @@ pub trait Tag {
 
 /// Output of the SHA256t hash function.
 #[repr(transparent)]
-pub struct Hash<T: Tag>([u8; 32], PhantomData<T>);
+pub struct Hash<T>([u8; 32], PhantomData<T>);
 
 #[cfg(feature = "schemars")]
 impl<T: Tag> schemars::JsonSchema for Hash<T> {
@@ -36,8 +36,24 @@ impl<T: Tag> schemars::JsonSchema for Hash<T> {
     }
 }
 
-impl<T: Tag> Hash<T> {
+// This impl, and the trait bound `(T,): Tag` below, are hacks to allow defining
-    fn internal_new(arr: [u8; 32]) -> Self { Hash(arr, Default::default()) }
+// constfns on Hash<T> for a generic T. This trick was discovered by
+// https://github.com/rust-lang/rust/issues/90912
+//
+// When we drop rustc 1.56.1 we will be able to remove this, which will be a
+// technically breaking change but in practice probably fine to just drop.
+impl<T> Tag for (T,)
+where
+    T: Tag,
+{
+    fn engine() -> sha256::HashEngine { T::engine() }
+}
+
+impl<T> Hash<T>
+where
+    (T,): Tag,
+{
+    fn internal_new(arr: [u8; 32]) -> Self { Hash(arr, PhantomData) }
 
     /// Zero cost conversion between a fixed length byte array shared reference and
     /// a shared reference to this Hash type.
@@ -54,7 +70,7 @@ impl<T: Tag> Hash<T> {
     }
 
     /// Constructs a new engine.
-    pub fn engine() -> HashEngine { T::engine() }
+    pub fn engine() -> HashEngine { <(T,)>::engine() }
 
     /// Produces a hash from the current state of a given engine.
     pub fn from_engine(e: HashEngine) -> Hash<T> { from_engine(e) }
@@ -135,7 +151,10 @@ impl<T: Tag> core::hash::Hash for Hash<T> {
 
 crate::internal_macros::hash_trait_impls!(256, false, T: Tag);
 
-fn from_engine<T: Tag>(e: sha256::HashEngine) -> Hash<T> {
+fn from_engine<T>(e: sha256::HashEngine) -> Hash<T>
+where
+    (T,): Tag,
+{
     Hash::from_byte_array(sha256::Hash::from_engine(e).to_byte_array())
 }