Add the unstability tag

Author: SciMind2460

library/core/src/ops/arith.rs

@@ -208,6 +208,7 @@ pub trait Sub<Rhs = Self> {
 macro_rules! sub_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[rustc_const_unstable(feature = "const_ops", issue = "90080")]
         impl const Sub for $t {
             type Output = $t;
 
@@ -340,6 +341,7 @@ pub trait Mul<Rhs = Self> {
 macro_rules! mul_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[rustc_const_unstable(feature = "const_ops", issue = "90080")]
         impl const Mul for $t {
             type Output = $t;
 
@@ -458,6 +460,7 @@ mul_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f16 f32 f64 f128
 pub trait Div<Rhs = Self> {
     /// The resulting type after applying the `/` operator.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_ops", issue = "90080")]
     type Output;
 
     /// Performs the `/` operation.