allow using the system-wide llvm-libunwind as the unwinder

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

std/Cargo.toml

@@ -61,6 +61,7 @@ profiler = ["profiler_builtins"]
 compiler-builtins-c = ["alloc/compiler-builtins-c"]
 compiler-builtins-mem = ["alloc/compiler-builtins-mem"]
 llvm-libunwind = ["unwind/llvm-libunwind"]
+system-llvm-libunwind = ["unwind/system-llvm-libunwind"]
 
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = ["core/panic_immediate_abort"]

test/Cargo.toml

@@ -27,6 +27,7 @@ backtrace = ["std/backtrace"]
 compiler-builtins-c = ["std/compiler-builtins-c"]
 compiler-builtins-mem = ["std/compiler-builtins-mem"]
 llvm-libunwind = ["std/llvm-libunwind"]
+system-llvm-libunwind = ["std/system-llvm-libunwind"]
 panic-unwind = ["std/panic_unwind"]
 panic_immediate_abort = ["std/panic_immediate_abort"]
 profiler = ["std/profiler"]

unwind/Cargo.toml

@@ -23,3 +23,4 @@ cc = { version = "1.0.1" }
 
 [features]
 llvm-libunwind = []
+system-llvm-libunwind = []

unwind/src/lib.rs

@@ -45,10 +45,24 @@ extern "C" {}
 // When building with crt-static, we get `gcc_eh` from the `libc` crate, since
 // glibc needs it, and needs it listed later on the linker command line. We
 // don't want to duplicate it here.
-#[cfg(all(target_os = "linux", target_env = "gnu", not(feature = "llvm-libunwind")))]
+#[cfg(all(
+    target_os = "linux",
+    target_env = "gnu",
+    not(feature = "llvm-libunwind"),
+    not(feature = "system-llvm-libunwind")
+))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
 extern "C" {}
 
+#[cfg(all(
+    target_os = "linux",
+    target_env = "gnu",
+    not(feature = "llvm-libunwind"),
+    feature = "system-llvm-libunwind"
+))]
+#[link(name = "unwind", cfg(not(target_feature = "crt-static")))]
+extern "C" {}
+
 #[cfg(target_os = "redox")]
 #[link(name = "gcc_eh", kind = "static-nobundle", cfg(target_feature = "crt-static"))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]