replace cargo external-symbols feature with a rustc --cfg flag

This feature was not useful for Cargo users, since Cargo does not give you
the kind of fine-grained control over C library linkage that you need. So
it was just unnecessarily confusing and would cause the build to break if
you enabled it accidentally, say, with --all-features.

Author: apoelstra

Cargo.toml

@@ -26,10 +26,6 @@ endomorphism = ["secp256k1-sys/endomorphism"]
 lowmemory = ["secp256k1-sys/lowmemory"]
 global-context = ["std", "rand-std"]
 
-# Use this feature to not compile the bundled libsecp256k1 C symbols,
-# but use external ones. Use this only if you know what you are doing!
-external-symbols = ["secp256k1-sys/external-symbols"]
-
 # Do not use this feature! HAZMAT. (meant for Fuzzing only. this is *BROKEN CRYPTOGRAPHY*)
 fuzztarget = ["secp256k1-sys/fuzztarget"]
 

secp256k1-sys/Cargo.toml

@@ -31,9 +31,5 @@ endomorphism = []
 lowmemory = []
 std = []
 
-# Use this feature to not compile the bundled libsecp256k1 C symbols,
-# but use external ones. Use this only if you know what you are doing!
-external-symbols = []
-
 # Do not use this feature! HAZMAT. (meant for Fuzzing only. this is *BROKEN CRYPTOGRAPHY*)
 fuzztarget = []

secp256k1-sys/README.md

@@ -29,6 +29,7 @@ $ ./vendor-libsecp.sh depend <version-code> <rev>
 
 ## Linking to external symbols
 
-For the more exotic use cases, this crate can be used with existing libsecp256k1
-symbols by using the `external-symbols` feature. How to setup rustc to link
-against those existing symbols is left as an exercise to the reader.
+If you want to compile this library without using the bundled symbols (which may
+be required for integration into other build systems), you can do so by adding
+`--cfg=rust_secp_no_symbol_renaming'` to your `RUSTFLAGS` variable.
+

secp256k1-sys/build.rs

@@ -26,11 +26,6 @@ extern crate cc;
 use std::env;
 
 fn main() {
-    if cfg!(feature = "external-symbols") {
-        println!("cargo:rustc-link-lib=static=secp256k1");
-        return;
-    }
-
     // Actual build
     let mut base_config = cc::Build::new();
     base_config.include("depend/secp256k1/")

secp256k1-sys/src/lib.rs

@@ -38,21 +38,21 @@ impl Default for RecoverableSignature {
 
 #[cfg(not(feature = "fuzztarget"))]
 extern "C" {
-    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_parse_compact")]
+    #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_parse_compact")]
     pub fn secp256k1_ecdsa_recoverable_signature_parse_compact(cx: *const Context, sig: *mut RecoverableSignature,
                                                                input64: *const c_uchar, recid: c_int)
                                                                -> c_int;
 
-    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_serialize_compact")]
+    #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_serialize_compact")]
     pub fn secp256k1_ecdsa_recoverable_signature_serialize_compact(cx: *const Context, output64: *mut c_uchar,
                                                                    recid: *mut c_int, sig: *const RecoverableSignature)
                                                                    -> c_int;
 
-    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_convert")]
+    #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_3_1_ecdsa_recoverable_signature_convert")]
     pub fn secp256k1_ecdsa_recoverable_signature_convert(cx: *const Context, sig: *mut Signature,
                                                          input: *const RecoverableSignature)
                                                          -> c_int;
-    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_3_1_ecdsa_sign_recoverable")]
+    #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_3_1_ecdsa_sign_recoverable")]
     pub fn secp256k1_ecdsa_sign_recoverable(cx: *const Context,
                                             sig: *mut RecoverableSignature,
                                             msg32: *const c_uchar,
@@ -61,7 +61,7 @@ extern "C" {
                                             noncedata: *const c_void)
                                             -> c_int;
 
-    #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_3_1_ecdsa_recover")]
+    #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_3_1_ecdsa_recover")]
     pub fn secp256k1_ecdsa_recover(cx: *const Context,
                                    pk: *mut PublicKey,
                                    sig: *const RecoverableSignature,

secp256k1-sys/src/recovery.rs

@@ -40,7 +40,7 @@ impl AlignedType {
     }
 }
 
-#[cfg(all(feature = "std", not(feature = "external-symbols")))]
+#[cfg(all(feature = "std", not(rust_secp_no_symbol_renaming)))]
 pub(crate) const ALIGN_TO: usize = mem::align_of::<AlignedType>();