Merge #658: Serialized signture improvements

62c839c Implement conversion traits (Martin Habovstiak)
dc3eab7 Implement `Borrow<[u8]>`, `PartialEq<[u8]>`, `Hash` (Martin Habovstiak)
7dac91d Deprecate `capacity` and `is_empty` (Martin Habovstiak)

Pull request description:

  This deprecates methods returning constants and impls a few traits.

ACKs for top commit:
  apoelstra:
    ACK 62c839c

Tree-SHA512: 724a08af7dc915e166e3efcdc4be681a53ae14a55d9cbd62dd4c5240fa8c0f13110498d03ebb0edc1d56f969901f978aa33bae9df19376957ff7f51698ed9535

src/ecdsa/serialized_signature.rs

@@ -7,6 +7,8 @@
 //! unable to run on platforms without allocator. We implement a special type to encapsulate
 //! serialized signatures and since it's a bit more complicated it has its own module.
 
+use core::borrow::Borrow;
+use core::convert::TryFrom;
 use core::{fmt, ops};
 
 pub use into_iter::IntoIter;
@@ -41,11 +43,30 @@ impl PartialEq for SerializedSignature {
     fn eq(&self, other: &SerializedSignature) -> bool { **self == **other }
 }
 
+impl PartialEq<[u8]> for SerializedSignature {
+    #[inline]
+    fn eq(&self, other: &[u8]) -> bool { **self == *other }
+}
+
+impl PartialEq<SerializedSignature> for [u8] {
+    #[inline]
+    fn eq(&self, other: &SerializedSignature) -> bool { *self == **other }
+}
+
+impl core::hash::Hash for SerializedSignature {
+    fn hash<H: core::hash::Hasher>(&self, state: &mut H) { (**self).hash(state) }
+}
+
 impl AsRef<[u8]> for SerializedSignature {
     #[inline]
     fn as_ref(&self) -> &[u8] { self }
 }
 
+impl Borrow<[u8]> for SerializedSignature {
+    #[inline]
+    fn borrow(&self) -> &[u8] { self }
+}
+
 impl ops::Deref for SerializedSignature {
     type Target = [u8];
 
@@ -71,6 +92,28 @@ impl<'a> IntoIterator for &'a SerializedSignature {
     fn into_iter(self) -> Self::IntoIter { self.iter() }
 }
 
+impl From<Signature> for SerializedSignature {
+    fn from(value: Signature) -> Self { Self::from_signature(&value) }
+}
+
+impl<'a> From<&'a Signature> for SerializedSignature {
+    fn from(value: &'a Signature) -> Self { Self::from_signature(value) }
+}
+
+impl TryFrom<SerializedSignature> for Signature {
+    type Error = Error;
+
+    fn try_from(value: SerializedSignature) -> Result<Self, Self::Error> { value.to_signature() }
+}
+
+impl<'a> TryFrom<&'a SerializedSignature> for Signature {
+    type Error = Error;
+
+    fn try_from(value: &'a SerializedSignature) -> Result<Self, Self::Error> {
+        value.to_signature()
+    }
+}
+
 impl SerializedSignature {
     /// Creates `SerializedSignature` from data and length.
     ///
@@ -84,6 +127,7 @@ impl SerializedSignature {
     }
 
     /// Get the capacity of the underlying data buffer.
+    #[deprecated = "This always returns 72"]
     #[inline]
     pub fn capacity(&self) -> usize { self.data.len() }
 
@@ -106,6 +150,7 @@ impl SerializedSignature {
     pub fn from_signature(sig: &Signature) -> SerializedSignature { sig.serialize_der() }
 
     /// Check if the space is zero.
+    #[deprecated = "This always returns false"]
     #[inline]
     pub fn is_empty(&self) -> bool { self.len() == 0 }
 }