Don't require a C++ compiler for -musl targets.

Also, don't build/run `bn_tests` unless RSA signing is enabled, since
the functionality that `bn_tests` tests is RSA-signing-specific.

Cargo.toml

@@ -287,10 +287,11 @@ rayon = "0.6"
 default = ["use_heap", "dev_urandom_fallback"]
 dev_urandom_fallback = []
 internal_benches = []
-rsa_signing = []
+rsa_signing = ["bn_tests"]
 slow_tests = []
 test_logging = []
 use_heap = []
+bn_tests = []
 
 # XXX: debug = false because of https://github.com/rust-lang/rust/issues/34122
 

build.rs

@@ -482,9 +482,13 @@ fn build_c_code(target: &Target, out_dir: &Path) {
 
     let test_srcs = RING_TEST_SRCS.iter()
         .map(PathBuf::from)
-        // Avoid the libstdc++ vs libc++ issue for macOS/iOS by just avoiding
-        // all the C++-based tests.
-        .filter(|p| !(target.os() == "macos" || target.os == "ios") ||
+        // Allow the C++-based tests to be disabled
+        .filter(|p| cfg!(feature = "bn_tests") ||
+            p.extension().unwrap().to_str().unwrap() != "cc")
+        // Avoid the libstdc++ vs libc++ issue for macOS/iOS, and g++ issue on
+        // musl by just avoiding all the C++-based tests.
+        .filter(|p| !(target.os() == "macos" || target.os == "ios" ||
+                      target.env() == "musl") ||
                     p.extension().unwrap().to_str().unwrap() != "cc")
         .collect::<Vec<_>>();
 
@@ -502,6 +506,8 @@ fn build_c_code(target: &Target, out_dir: &Path) {
                           includes_modified));
 
     if target.env() != "msvc" &&
+        target.env() != "musl" &&
+        cfg!(feature = "bn_tests") &&
        !(target.os() == "macos" || target.os() == "ios") {
         let libcxx = if use_libcxx(target) {
             "c++"

src/lib.rs

@@ -189,7 +189,8 @@ mod private {
 #[cfg(test)]
 mod tests {
     #[cfg(all(feature = "use_heap",
-              not(any(target_os = "macos", target_os = "ios"))))]
+              feature = "bn_tests",
+              not(any(target_os = "macos", target_os = "ios", target_env = "musl"))))]
     bssl_test!(test_bn, bssl_bn_test_main);
 
     bssl_test!(test_constant_time, bssl_constant_time_test_main);