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Also convert a number of `static mut` to just a plain old `static` and remove
some unsafe blocks.

src/libstd/dynamic_lib.rs

@@ -230,11 +230,11 @@ pub mod dl {
 
     pub fn check_for_errors_in<T>(f: || -> T) -> Result<T, String> {
         use rt::mutex::{StaticNativeMutex, NATIVE_MUTEX_INIT};
-        static mut lock: StaticNativeMutex = NATIVE_MUTEX_INIT;
+        static LOCK: StaticNativeMutex = NATIVE_MUTEX_INIT;
         unsafe {
             // dlerror isn't thread safe, so we need to lock around this entire
             // sequence
-            let _guard = lock.lock();
+            let _guard = LOCK.lock();
             let _old_error = dlerror();
 
             let result = f();