Reduce duplication

Extract code for checking range validity into helper functions, and use
macros to implement SliceIndex for the new range types from
`std::range`.

Author: Kmeakin

library/core/src/range.rs

@@ -639,3 +639,20 @@ impl<T> IntoBounds<T> for RangeToInclusive<T> {
         (Unbounded, Included(self.last))
     }
 }
+
+#[unstable(feature = "new_range_api", issue = "125687")]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<RangeToInclusive<T>> for legacy::RangeToInclusive<T> {
+    #[inline]
+    fn from(value: RangeToInclusive<T>) -> Self {
+        Self { end: value.last }
+    }
+}
+#[unstable(feature = "new_range_api", issue = "125687")]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<legacy::RangeToInclusive<T>> for RangeToInclusive<T> {
+    #[inline]
+    fn from(value: legacy::RangeToInclusive<T>) -> Self {
+        Self { last: value.end }
+    }
+}