Rollup merge of #58442 - cuviper:unix-weak, r=alexcrichton

Simplify the unix `Weak` functionality

- We can avoid allocation by adding a NUL to the function name.
- We can get `Option<F>` directly, rather than aliasing the inner `AtomicUsize`.

src/libstd/sys/unix/weak.rs

@@ -18,7 +18,7 @@
 
 use libc;
 
-use ffi::CString;
+use ffi::CStr;
 use marker;
 use mem;
 use sync::atomic::{AtomicUsize, Ordering};
@@ -26,7 +26,7 @@
 macro_rules! weak {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
         static $name: ::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
-            ::sys::weak::Weak::new(stringify!($name));
+            ::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
     )
 }
 
@@ -45,23 +45,22 @@ pub const fn new(name: &'static str) -> Weak<F> {
         }
     }
 
-    pub fn get(&self) -> Option<&F> {
+    pub fn get(&self) -> Option<F> {
         assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
         unsafe {
             if self.addr.load(Ordering::SeqCst) == 1 {
                 self.addr.store(fetch(self.name), Ordering::SeqCst);
             }
-            if self.addr.load(Ordering::SeqCst) == 0 {
-                None
-            } else {
-                mem::transmute::<&AtomicUsize, Option<&F>>(&self.addr)
+            match self.addr.load(Ordering::SeqCst) {
+                0 => None,
+                addr => Some(mem::transmute_copy::<usize, F>(&addr)),
             }
         }
     }
 }
 
 unsafe fn fetch(name: &str) -> usize {
-    let name = match CString::new(name) {
+    let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
         Ok(cstr) => cstr,
         Err(..) => return 0,
     };