avoid full-slicing slices

If we already have a slice, there is no need to get another full-range slice from that, just use the original.
clippy::redundant_slicing

compiler/rustc_builtin_macros/src/format_foreign.rs

@@ -312,7 +312,7 @@ pub mod printf {
                 return Some((Substitution::Escape, &s[start + 2..]));
             }
 
-            Cur::new_at(&s[..], start)
+            Cur::new_at(s, start)
         };
 
         // This is meant to be a translation of the following regex:
@@ -673,7 +673,7 @@ pub mod shell {
                 _ => { /* fall-through */ }
             }
 
-            Cur::new_at(&s[..], start)
+            Cur::new_at(s, start)
         };
 
         let at = at.at_next_cp()?;