Add example of c calling rust (#74)

* Add example of c calling rust staticlib.

* Remove trailing newlines.

* Set visibility to private , run formatter.

examples/ffi/c_calling_rust/BUILD

@@ -0,0 +1,21 @@
+package(default_visibility = ["//visibility:private"])
+
+load("@//rust:rust.bzl", "rust_library", "rust_test", "rust_binary")
+
+rust_library(
+    name = "rusty",
+    srcs = ["lib.rs"],
+    crate_type = "staticlib",
+)
+
+# cc_* rules expect cc_* deps, so we need to wrap our rust staticlib.
+cc_library(
+    name = "wrapper",
+    srcs = [":rusty"],
+)
+
+cc_test(
+    name = "main",
+    srcs = ["main.c"],
+    deps = [":wrapper"],
+)

examples/ffi/c_calling_rust/lib.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+pub extern fn my_favorite_number() -> i32 {
+    4
+}

examples/ffi/c_calling_rust/main.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+#include <stdint.h>
+
+extern int32_t my_favorite_number();
+
+int main(int argc, char** argv) {
+  assert(my_favorite_number() == 4);
+  return 0;
+}