Move `SerializedSignature` into its own module

This de-clutters the code and prepares for the next step of adding
`IntoIterator`. The type is still re-exported so the change is neither
breaking nor inconvenient.

This also adds more datialed explanation of `SerializedSignature` and
why it's needed.

src/ecdsa/mod.rs

@@ -1,10 +1,12 @@
 //! Structs and functionality related to the ECDSA signature algorithm.
 
-use core::{fmt, str, ops, ptr};
+use core::{fmt, str, ptr};
 
 use crate::{Signing, Verification, Message, PublicKey, Secp256k1, SecretKey, from_hex, Error, ffi};
 use crate::ffi::CPtr;
 
+pub mod serialized_signature;
+
 #[cfg(feature = "recovery")]
 mod recovery;
 
@@ -12,6 +14,8 @@ mod recovery;
 #[cfg_attr(docsrs, doc(cfg(feature = "recovery")))]
 pub use self::recovery::{RecoveryId, RecoverableSignature};
 
+pub use serialized_signature::SerializedSignature;
+
 #[cfg(feature = "global-context")]
 use crate::SECP256K1;
 
@@ -19,13 +23,6 @@ use crate::SECP256K1;
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Signature(pub(crate) ffi::Signature);
 
-/// A DER serialized Signature
-#[derive(Copy, Clone)]
-pub struct SerializedSignature {
-    data: [u8; 72],
-    len: usize,
-}
-
 impl fmt::Debug for Signature {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Display::fmt(self, f)
@@ -39,21 +36,6 @@ impl fmt::Display for Signature {
     }
 }
 
-impl fmt::Debug for SerializedSignature {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
-    }
-}
-
-impl fmt::Display for SerializedSignature {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        for v in self.data.iter().take(self.len) {
-            write!(f, "{:02x}", v)?;
-        }
-        Ok(())
-    }
-}
-
 impl str::FromStr for Signature {
     type Err = Error;
     fn from_str(s: &str) -> Result<Signature, Error> {
@@ -65,83 +47,6 @@ impl str::FromStr for Signature {
     }
 }
 
-impl Default for SerializedSignature {
-    fn default() -> SerializedSignature {
-        SerializedSignature {
-            data: [0u8; 72],
-            len: 0,
-        }
-    }
-}
-
-impl PartialEq for SerializedSignature {
-    fn eq(&self, other: &SerializedSignature) -> bool {
-        **self == **other
-    }
-}
-
-impl AsRef<[u8]> for SerializedSignature {
-    fn as_ref(&self) -> &[u8] {
-        &*self
-    }
-}
-
-impl ops::Deref for SerializedSignature {
-    type Target = [u8];
-
-    fn deref(&self) -> &[u8] {
-        &self.data[..self.len]
-    }
-}
-
-impl Eq for SerializedSignature {}
-
-impl<'a> IntoIterator for &'a SerializedSignature {
-    type IntoIter = core::slice::Iter<'a, u8>;
-    type Item = &'a u8;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.iter()
-    }
-}
-
-impl SerializedSignature {
-    /// Get a pointer to the underlying data with the specified capacity.
-    pub(crate) fn get_data_mut_ptr(&mut self) -> *mut u8 {
-        self.data.as_mut_ptr()
-    }
-
-    /// Get the capacity of the underlying data buffer.
-    pub fn capacity(&self) -> usize {
-        self.data.len()
-    }
-
-    /// Get the len of the used data.
-    pub fn len(&self) -> usize {
-        self.len
-    }
-
-    /// Set the length of the object.
-    pub(crate) fn set_len(&mut self, len: usize) {
-        self.len = len;
-    }
-
-    /// Convert the serialized signature into the Signature struct.
-    /// (This DER deserializes it)
-    pub fn to_signature(&self) -> Result<Signature, Error> {
-        Signature::from_der(self)
-    }
-
-    /// Create a SerializedSignature from a Signature.
-    /// (this DER serializes it)
-    pub fn from_signature(sig: &Signature) -> SerializedSignature {
-        sig.serialize_der()
-    }
-
-    /// Check if the space is zero.
-    pub fn is_empty(&self) -> bool { self.len() == 0 }
-}
-
 impl Signature {
     #[inline]
     /// Converts a DER-encoded byte slice to a signature

src/ecdsa/serialized_signature.rs

@@ -0,0 +1,109 @@
+//! Implements [`SerializedSignature`] and related types.
+//!
+//! DER-serialized signatures have the issue that they can have different lengths.
+//! We want to avoid using `Vec` since that would require allocations making the code slower and
+//! 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::{fmt, ops};
+use crate::Error;
+use super::Signature;
+
+/// A DER serialized Signature
+#[derive(Copy, Clone)]
+pub struct SerializedSignature {
+    data: [u8; 72],
+    len: usize,
+}
+
+impl fmt::Debug for SerializedSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl fmt::Display for SerializedSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for v in self.data.iter().take(self.len) {
+            write!(f, "{:02x}", v)?;
+        }
+        Ok(())
+    }
+}
+
+impl Default for SerializedSignature {
+    fn default() -> SerializedSignature {
+        SerializedSignature {
+            data: [0u8; 72],
+            len: 0,
+        }
+    }
+}
+
+impl PartialEq for SerializedSignature {
+    fn eq(&self, other: &SerializedSignature) -> bool {
+        **self == **other
+    }
+}
+
+impl AsRef<[u8]> for SerializedSignature {
+    fn as_ref(&self) -> &[u8] {
+        &*self
+    }
+}
+
+impl ops::Deref for SerializedSignature {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        &self.data[..self.len]
+    }
+}
+
+impl Eq for SerializedSignature {}
+
+impl<'a> IntoIterator for &'a SerializedSignature {
+    type IntoIter = core::slice::Iter<'a, u8>;
+    type Item = &'a u8;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl SerializedSignature {
+    /// Get a pointer to the underlying data with the specified capacity.
+    pub(crate) fn get_data_mut_ptr(&mut self) -> *mut u8 {
+        self.data.as_mut_ptr()
+    }
+
+    /// Get the capacity of the underlying data buffer.
+    pub fn capacity(&self) -> usize {
+        self.data.len()
+    }
+
+    /// Get the len of the used data.
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    /// Set the length of the object.
+    pub(crate) fn set_len(&mut self, len: usize) {
+        self.len = len;
+    }
+
+    /// Convert the serialized signature into the Signature struct.
+    /// (This DER deserializes it)
+    pub fn to_signature(&self) -> Result<Signature, Error> {
+        Signature::from_der(self)
+    }
+
+    /// Create a SerializedSignature from a Signature.
+    /// (this DER serializes it)
+    pub fn from_signature(sig: &Signature) -> SerializedSignature {
+        sig.serialize_der()
+    }
+
+    /// Check if the space is zero.
+    pub fn is_empty(&self) -> bool { self.len() == 0 }
+}