replace as usize casts with usize::from in slice sort

Author: WaffleLapkin

library/core/src/slice/sort.rs

@@ -365,12 +365,12 @@ where
         if count > 0 {
             macro_rules! left {
                 () => {
-                    l.add(*start_l as usize)
+                    l.add(usize::from(*start_l))
                 };
             }
             macro_rules! right {
                 () => {
-                    r.sub((*start_r as usize) + 1)
+                    r.sub(usize::from(*start_r) + 1)
                 };
             }
 
@@ -458,7 +458,7 @@ where
             //    the last block, so the `l.offset` calls are valid.
             unsafe {
                 end_l = end_l.sub(1);
-                ptr::swap(l.add(*end_l as usize), r.sub(1));
+                ptr::swap(l.add(usize::from(*end_l)), r.sub(1));
                 r = r.sub(1);
             }
         }
@@ -471,7 +471,7 @@ where
             // SAFETY: See the reasoning in [remaining-elements-safety].
             unsafe {
                 end_r = end_r.sub(1);
-                ptr::swap(l, r.sub((*end_r as usize) + 1));
+                ptr::swap(l, r.sub(usize::from(*end_r) + 1));
                 l = l.add(1);
             }
         }