Auto merge of #47004 - nvzqz:rc-conversions, r=bluss,kennytm

Remove transmute in From<&str> impls for Arc/Rc

Performs conversion via raw pointer casts.

Author: bors

src/liballoc/arc.rs

@@ -1377,7 +1377,8 @@ impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
 impl<'a> From<&'a str> for Arc<str> {
     #[inline]
     fn from(v: &str) -> Arc<str> {
-        unsafe { mem::transmute(<Arc<[u8]>>::from(v.as_bytes())) }
+        let arc = Arc::<[u8]>::from(v.as_bytes());
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
     }
 }
 

src/liballoc/rc.rs

@@ -1099,7 +1099,8 @@ impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
 impl<'a> From<&'a str> for Rc<str> {
     #[inline]
     fn from(v: &str) -> Rc<str> {
-        unsafe { mem::transmute(<Rc<[u8]>>::from(v.as_bytes())) }
+        let rc = Rc::<[u8]>::from(v.as_bytes());
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
     }
 }