use Self alias in self types rather than manually substituting it

library/alloc/src/str.rs

@@ -234,7 +234,7 @@ impl str {
     #[stable(feature = "str_box_extras", since = "1.20.0")]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
-    pub fn into_boxed_bytes(self: Box<str>) -> Box<[u8]> {
+    pub fn into_boxed_bytes(self: Box<Self>) -> Box<[u8]> {
         self.into()
     }
 
@@ -501,7 +501,7 @@ impl str {
     #[rustc_allow_incoherent_impl]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
-    pub fn into_string(self: Box<str>) -> String {
+    pub fn into_string(self: Box<Self>) -> String {
         let slice = Box::<[u8]>::from(self);
         unsafe { String::from_utf8_unchecked(slice.into_vec()) }
     }