Fix warnings

1) SECP256k1_BUILD is already set, so setting to Some("") avoids the
duplicate warning
2) Misc rust unused warnings in various feature flag combinations

Author: sanket1729

secp256k1-zkp-sys/build.rs

@@ -38,7 +38,7 @@ fn main() {
         .include("depend/secp256k1/include")
         .include("depend/secp256k1/src")
         .flag_if_supported("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
-        .define("SECP256K1_BUILD", Some("1"))
+        .define("SECP256K1_BUILD", Some(""))
         .define("ENABLE_MODULE_SURJECTIONPROOF", Some("1"))
         .define("ENABLE_MODULE_GENERATOR", Some("1"))
         .define("ENABLE_MODULE_RANGEPROOF", Some("1"))

src/zkp/ecdsa_adaptor.rs

@@ -14,9 +14,9 @@ use rand::thread_rng;
 #[cfg(any(test, feature = "rand"))]
 use rand::{CryptoRng, Rng};
 use {constants, PublicKey, Secp256k1, SecretKey};
+use {ecdsa::Signature, Verification};
 use {from_hex, Error};
 use {Message, Signing};
-use {ecdsa::Signature, Verification};
 
 /// Represents an adaptor signature and dleq proof.
 #[derive(Debug, PartialEq, Clone, Copy, Eq)]
@@ -287,9 +287,11 @@ impl EcdsaAdaptorSignature {
 mod tests {
     use super::Message;
     use super::*;
+    #[cfg(not(rust_secp_fuzz))]
     use rand::{rngs::ThreadRng, thread_rng, RngCore};
     use SECP256K1;
 
+    #[cfg(not(rust_secp_fuzz))]
     fn test_ecdsa_adaptor_signature_helper(
         encrypt: fn(&Message, &SecretKey, &PublicKey, &mut ThreadRng) -> EcdsaAdaptorSignature,
     ) {

src/zkp/generator.rs

@@ -177,6 +177,7 @@ impl Generator {
     ///
     /// This is `pub(crate)` because generators have a different serialization from regular public keys.
     /// As such, certain invariants need to be upheld which is easier if we don't allow users to access the internal representation of generators.
+    #[cfg(feature = "std")] // for un-used warnings
     pub(crate) fn as_inner(&self) -> &ffi::PublicKey {
         &self.0
     }

src/zkp/tag.rs

@@ -12,7 +12,7 @@ impl Tag {
         self.0
     }
 
-    #[cfg(feature = "rand")]
+    #[cfg(all(feature = "rand", feature = "std"))]
     pub(crate) fn as_inner(&self) -> &ffi::Tag {
         &self.0
     }