Auto merge of rust-lang#120381 - fee1-dead-contrib:reconstify-add, r=compiler-errors

Reconstify `Add`

r? project-const-traits

I'm not happy with the ui test changes (or failures because I did not bless them and include the diffs in this PR). There is at least some bugs I need to look and try fix:

1. A third duplicated diagnostic when a consumer crate that does not have `effects` enabled has a trait selection error for an upstream const_trait trait. See tests/ui/ufcs/ufcs-qpath-self-mismatch.rs.
2. For some reason, making `Add` a const trait would stop us from suggesting `T: Add` when we try to add two `T`s without that bound. See tests/ui/suggestions/issue-97677.rs

Author: bors

Diff for: core/src/ops/arith.rs

@@ -73,6 +73,7 @@
     append_const_msg
 )]
 #[doc(alias = "+")]
+#[const_trait]
 pub trait Add<Rhs = Self> {
     /// The resulting type after applying the `+` operator.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -94,7 +95,8 @@ pub trait Add<Rhs = Self> {
 macro_rules! add_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl Add for $t {
+        #[rustc_const_unstable(feature = "const_ops", issue = "90080")]
+        impl const Add for $t {
             type Output = $t;
 
             #[inline]