Removed `Default` from `SerializedSignature`

`Default` was pointless, so it was replaced with internal
`from_raw_parts` method which also checks the length.

This commit also documents changes to `SerializedSignature`.

Closes #454

CHANGELOG.md

@@ -9,6 +9,7 @@ The major change in this version is the increase of the Minimum Supported Rust V
 * Key tweaking methods renamed and refactored to use a more [functional-style](https://github.com/rust-bitcoin/rust-secp256k1/pull/406), they now accept a [new Scalar](https://github.com/rust-bitcoin/rust-secp256k1/pull/445) type instead of raw slices
 * Update [`rand` dependency to 0.8](https://github.com/rust-bitcoin/rust-secp256k1/pull/331)
 * `KeyPair::from_secret_key` [borrows SecretKey](https://github.com/rust-bitcoin/rust-secp256k1/pull/430) instead of taking ownership
+* `SerializedSignature` no longer implements `Default`
 
 ## New features/APIs
 
@@ -18,6 +19,7 @@ The major change in this version is the increase of the Minimum Supported Rust V
 * [Implemented `TryFrom` for `Parity`](https://github.com/rust-bitcoin/rust-secp256k1/pull/409)
 * The [alloc feature](https://github.com/rust-bitcoin/rust-secp256k1/pull/331) can be used on targets with allocators without a standard library
 * `SharedSecret` can be created from a slice, parsed from a hex string, or [(de)serialized using serde](https://github.com/rust-bitcoin/rust-secp256k1/pull/418)
+* `SerializedSignature` implements `IntoIterator` (both owned and shared reference)
 * We now [derive `std::hash::Hash` for `Signature`](https://github.com/rust-bitcoin/rust-secp256k1/pull/441)
 
 ## Other improvements

src/ecdsa/mod.rs

@@ -157,20 +157,18 @@ impl Signature {
     #[inline]
     /// Serializes the signature in DER format
     pub fn serialize_der(&self) -> SerializedSignature {
-        let mut ret = SerializedSignature::default();
-        let mut len: usize = ret.capacity();
+        let mut data = [0u8; serialized_signature::MAX_LEN];
+        let mut len: usize = serialized_signature::MAX_LEN;
         unsafe {
             let err = ffi::secp256k1_ecdsa_signature_serialize_der(
                 ffi::secp256k1_context_no_precomp,
-                ret.get_data_mut_ptr(),
+                data.as_mut_ptr(),
                 &mut len,
                 self.as_c_ptr(),
             );
             debug_assert!(err == 1);
-            assert!(len <= serialized_signature::MAX_LEN, "libsecp256k1 set length to {} but the maximum is {}", len, serialized_signature::MAX_LEN);
-            ret.set_len(len);
+            SerializedSignature::from_raw_parts(data, len)
         }
-        ret
     }
 
     #[inline]

src/ecdsa/serialized_signature.rs

@@ -35,16 +35,6 @@ impl fmt::Display for SerializedSignature {
     }
 }
 
-impl Default for SerializedSignature {
-    #[inline]
-    fn default() -> SerializedSignature {
-        SerializedSignature {
-            data: [0u8; MAX_LEN],
-            len: 0,
-        }
-    }
-}
-
 impl PartialEq for SerializedSignature {
     #[inline]
     fn eq(&self, other: &SerializedSignature) -> bool {
@@ -91,10 +81,18 @@ impl<'a> IntoIterator for &'a SerializedSignature {
 }
 
 impl SerializedSignature {
-    /// Get a pointer to the underlying data with the specified capacity.
+    /// Creates `SerializedSignature` from data and length.
+    ///
+    /// ## Panics
+    ///
+    /// If `len` > `MAX_LEN`
     #[inline]
-    pub(crate) fn get_data_mut_ptr(&mut self) -> *mut u8 {
-        self.data.as_mut_ptr()
+    pub(crate) fn from_raw_parts(data: [u8; MAX_LEN], len: usize) -> Self {
+        assert!(len <= MAX_LEN, "attempt to set length to {} but the maximum is {}", len, MAX_LEN);
+        SerializedSignature {
+            data,
+            len,
+        }
     }
 
     /// Get the capacity of the underlying data buffer.
@@ -111,7 +109,7 @@ impl SerializedSignature {
 
     /// Set the length of the object.
     #[inline]
-    pub(crate) fn set_len(&mut self, len: usize) {
+    pub(crate) fn set_len_unchecked(&mut self, len: usize) {
         self.len = len;
     }
 
@@ -218,7 +216,7 @@ mod into_iter {
             // reach this
             let new_len = self.signature.len() - 1;
             let byte = self.signature[new_len];
-            self.signature.set_len(new_len);
+            self.signature.set_len_unchecked(new_len);
             Some(byte)
         }
     }