Fix broken serde::Deserialize and FromStr impl of keyPair

Fixes #491

Author: elsirion

src/key.rs

@@ -1053,13 +1053,18 @@ impl<'a> From<&'a KeyPair> for PublicKey {
     }
 }
 
+#[cfg(feature = "alloc")]
 impl str::FromStr for KeyPair {
     type Err = Error;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let ctx = unsafe {
-            Secp256k1::from_raw_all(ffi::secp256k1_context_no_precomp as *mut ffi::Context)
-        };
+        #[cfg(feature = "global-context")]
+        let ctx = SECP256K1;
+
+        #[cfg(all(not(feature = "global-context"), feature = "alloc"))]
+        let ctx = Secp256k1::signing_only();
+
+        #[allow(clippy::needless_borrow)]
         KeyPair::from_seckey_str(&ctx, s)
     }
 }
@@ -1082,7 +1087,7 @@ impl serde::Serialize for KeyPair {
     }
 }
 
-#[cfg(feature = "serde")]
+#[cfg(all(feature = "serde", feature = "alloc"))]
 #[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
 impl<'de> serde::Deserialize<'de> for KeyPair {
     fn deserialize<D: serde::Deserializer<'de>>(d: D) -> Result<Self, D::Error> {
@@ -1093,8 +1098,14 @@ impl<'de> serde::Deserialize<'de> for KeyPair {
         } else {
             let visitor = super::serde_util::Tuple32Visitor::new(
                 "raw 32 bytes KeyPair",
-                |data| unsafe {
-                    let ctx = Secp256k1::from_raw_all(ffi::secp256k1_context_no_precomp as *mut ffi::Context);
+                |data| {
+                    #[cfg(feature = "global-context")]
+                        let ctx = SECP256K1;
+
+                    #[cfg(all(not(feature = "global-context"), feature = "alloc"))]
+                    let ctx = Secp256k1::signing_only();
+
+                    #[allow(clippy::needless_borrow)]
                     KeyPair::from_seckey_slice(&ctx, data)
                 }
             );
@@ -1630,6 +1641,7 @@ pub mod serde_keypair {
 #[cfg(test)]
 #[allow(unused_imports)]
 mod test {
+    use bitcoin_hashes::hex::ToHex;
     use super::*;
 
     use core::str::FromStr;
@@ -2431,6 +2443,16 @@ mod test {
         assert_tokens(&pk.readable(), &[Token::Str(PK_STR)]);
         assert_tokens(&pk.readable(), &[Token::String(PK_STR)]);
     }
+
+    #[test]
+    #[cfg(feature = "global-context")]
+    fn test_keypair_from_str() {
+        let ctx = crate::Secp256k1::new();
+        let keypair = KeyPair::new(&ctx, &mut thread_rng());
+        let msg = keypair.secret_key().secret_bytes().to_hex();
+        let parsed_key: KeyPair = msg.parse().unwrap();
+        assert_eq!(parsed_key, keypair);
+    }
 }
 
 #[cfg(bench)]