Removed arrayvec and added a different serialize function for no-std

Cargo.toml

@@ -19,9 +19,6 @@ features = [ "rand", "serde" ]
 all-features = true
 
 
-[dependencies]
-arrayvec = { version = "0.4.10", default-features = false }
-
 [build-dependencies]
 cc = "=1.0.26"
 

src/lib.rs

@@ -143,9 +143,6 @@
 #[cfg(any(test, feature = "rand"))] use rand::Rng;
 #[cfg(any(test, feature = "std"))] extern crate core;
 
-extern crate arrayvec;
-
-use arrayvec::ArrayVec;
 use core::{fmt, ptr, str};
 
 #[macro_use]
@@ -338,10 +335,11 @@ impl Signature {
         &mut self.0 as *mut _
     }
 
+    #[cfg(feature = "std")]
     #[inline]
     /// Serializes the signature in DER format
-    pub fn serialize_der(&self) -> ArrayVec<[u8; 72]> {
-        let mut ret = ArrayVec::<[_; 72]>::new();
+    pub fn serialize_der(&self) -> Vec<u8>{
+        let mut ret = Vec::with_capacity(72);
         let mut len: usize = ret.capacity() as usize;
         unsafe {
             let err = ffi::secp256k1_ecdsa_signature_serialize_der(
@@ -351,12 +349,30 @@ impl Signature {
                 self.as_ptr(),
             );
             debug_assert!(err == 1);
-            // ret.set_len(len as usize);
             ret.set_len(len as usize);
         }
         ret
     }
 
+    #[inline]
+    /// Serializes the signature in DER format without allocating memory
+    /// The signature can be anywhere from 8 bytes to 72 bytes
+    /// So the function needs a buffer that is equal or larger than 72 bytes 
+    /// It will write into that buffer and return a slice containing only the signature
+    pub fn serialize_der_no_alloc<'a>(&self, buf: &'a mut [u8]) -> &'a [u8] {
+        let mut len: usize = buf.len();
+        unsafe {
+            let err = ffi::secp256k1_ecdsa_signature_serialize_der(
+                ffi::secp256k1_context_no_precomp,
+                buf.as_mut_ptr(),
+                &mut len,
+                self.as_ptr(),
+            );
+            debug_assert!(err == 1);
+        }
+        &buf[..len]
+    }
+
     #[inline]
     /// Serializes the signature in compact format
     pub fn serialize_compact(&self) -> [u8; 64] {
@@ -458,7 +474,8 @@ impl From<ffi::RecoverableSignature> for RecoverableSignature {
 #[cfg(feature = "serde")]
 impl ::serde::Serialize for Signature {
     fn serialize<S: ::serde::Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
-        s.serialize_bytes(&self.serialize_der())
+            let mut buf = [0u8; 72];
+            s.serialize_bytes(&self.serialize_der_no_alloc(&mut buf))
     }
 }
 
@@ -915,6 +932,34 @@ mod tests {
          }
     }
 
+    #[test]
+    fn signature_serialize_no_alloc_roundtrip() {
+        let mut s = Secp256k1::new();
+        s.randomize(&mut thread_rng());
+
+        let mut msg = [0; 32];
+        for _ in 0..100 {
+            thread_rng().fill_bytes(&mut msg);
+            let msg = Message::from_slice(&msg).unwrap();
+
+            let (sk, _) = s.generate_keypair(&mut thread_rng());
+            let sig1 = s.sign(&msg, &sk);
+            let mut buf = vec![0u8; 172];
+            let der = sig1.serialize_der_no_alloc(&mut buf);
+            let sig2 = Signature::from_der(&der[..]).unwrap();
+            assert_eq!(sig1, sig2);
+
+            let compact = sig1.serialize_compact();
+            let sig2 = Signature::from_compact(&compact[..]).unwrap();
+            assert_eq!(sig1, sig2);
+
+            assert!(Signature::from_compact(&der[..]).is_err());
+            assert!(Signature::from_compact(&compact[0..4]).is_err());
+            assert!(Signature::from_der(&compact[..]).is_err());
+            assert!(Signature::from_der(&der[0..4]).is_err());
+         }
+    }
+
     #[test]
     fn signature_display() {
         let hex_str = "3046022100839c1fbc5304de944f697c9f4b1d01d1faeba32d751c0f7acb21ac8a0f436a72022100e89bd46bb3a5a62adc679f659b7ce876d83ee297c7a5587b2011c4fcc72eab45";