Optimize Rc::<str>::default() implementation

This PR lets `impl Default for Rc<str>` re-use the implementation
for `Rc::<[u8]>::default()`. The previous version only calculted the
memory layout at runtime, even though it should be known at compile
time, resulting in an additional function call.

The same optimization is done for `Rc<CStr>`.

Generated byte code: <https://godbolt.org/z/dfq73jsoP>.

Resolves <rust-lang#135784>.

Author: Kijewski

Diff for: alloc/src/ffi/c_str.rs

@@ -965,8 +965,9 @@ impl Default for Rc<CStr> {
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
     fn default() -> Self {
-        let c_str: &CStr = Default::default();
-        Rc::from(c_str)
+        let rc = Rc::<[u8]>::from(*b"\0");
+        // `[u8]` has the same layout as `CStr`, and it is `NUL` terminated.
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const CStr) }
     }
 }
 

Diff for: alloc/src/rc.rs

@@ -2369,7 +2369,9 @@ impl Default for Rc<str> {
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
     fn default() -> Self {
-        Rc::from("")
+        let rc = Rc::<[u8]>::default();
+        // `[u8]` has the same layout as `str`.
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
     }
 }