Rollup merge of rust-lang#133237 - fee1-dead-contrib:constadd, r=compiler-errors

Minimally constify `Add`

* This PR removes the requirement for `impl const` to have a const stability attribute. cc ``@RalfJung`` I believe you mentioned that it would make much more sense to require `const_trait`s to have const stability instead. I agree with that sentiment but I don't think that is _required_ for a small scale experimentation like this PR. rust-lang/project-const-traits#16 should definitely be prioritized in the future, but removing the impl check should be good for now as all callers need `const_trait_impl` enabled for any const impl to work.
* This PR is intentionally minimal as constifying other traits can become more complicated (`PartialEq`, for example, would run into requiring implementing it for `str` as that is used in matches, which runs into the implementation for slice equality which uses specialization)

Per the reasons above, anyone who is interested in making traits `const` in the standard library are **strongly encouraged** to reach out to us on the [Zulip channel](https://rust-lang.zulipchat.com/#narrow/channel/419616-t-compiler.2Fproject-const-traits) before proceeding with the work.

cc ``@rust-lang/project-const-traits``

I believe there is prior approval from libs that we can experiment, so

r? project-const-traits

Author: jieyouxu

core/src/lib.rs

@@ -174,6 +174,7 @@
 #![feature(const_is_char_boundary)]
 #![feature(const_precise_live_drops)]
 #![feature(const_str_split_at)]
+#![feature(const_trait_impl)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
 #![feature(doc_cfg)]

core/src/ops/arith.rs

@@ -73,6 +73,7 @@
     append_const_msg
 )]
 #[doc(alias = "+")]
+#[cfg_attr(not(bootstrap), const_trait)]
 pub trait Add<Rhs = Self> {
     /// The resulting type after applying the `+` operator.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -94,6 +95,7 @@ pub trait Add<Rhs = Self> {
 macro_rules! add_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg(bootstrap)]
         impl Add for $t {
             type Output = $t;
 
@@ -103,6 +105,17 @@ macro_rules! add_impl {
             fn add(self, other: $t) -> $t { self + other }
         }
 
+        #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg(not(bootstrap))]
+        impl const Add for $t {
+            type Output = $t;
+
+            #[inline]
+            #[track_caller]
+            #[rustc_inherit_overflow_checks]
+            fn add(self, other: $t) -> $t { self + other }
+        }
+
         forward_ref_binop! { impl Add, add for $t, $t }
     )*)
 }