diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index 8678f388298..13cc5cbed44 100644
--- a/compiler/rustc_middle/src/ty/diagnostics.rs
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -98,6 +98,7 @@ impl<'tcx, T> IsSuggestable<'tcx> for T
 where
     T: TypeVisitable<TyCtxt<'tcx>> + TypeFoldable<TyCtxt<'tcx>>,
 {
+    #[tracing::instrument(level = "debug", skip(tcx))]
     fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool {
         self.visit_with(&mut IsSuggestableVisitor { tcx, infer_suggestable }).is_continue()
     }
@@ -533,6 +534,9 @@ fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         match c.kind() {
             ConstKind::Infer(InferConst::Var(_)) if self.infer_suggestable => {}
 
+            // effect variables are always suggestable, because they are not visible
+            ConstKind::Infer(InferConst::EffectVar(_)) => {}
+
             ConstKind::Infer(..)
             | ConstKind::Bound(..)
             | ConstKind::Placeholder(..)
diff --git a/library/core/src/ops/arith.rs b/library/core/src/ops/arith.rs
index bb3cdde66d1..fd50f804748 100644
--- a/library/core/src/ops/arith.rs
+++ b/library/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]
diff --git a/tests/ui/suggestions/invalid-bin-op.stderr b/tests/ui/suggestions/invalid-bin-op.stderr
index 2bd745c645a..018250c8c1b 100644
--- a/tests/ui/suggestions/invalid-bin-op.stderr
+++ b/tests/ui/suggestions/invalid-bin-op.stderr
@@ -16,6 +16,10 @@ help: consider annotating `S<T>` with `#[derive(PartialEq)]`
 LL + #[derive(PartialEq)]
 LL | struct S<T>(T);
    |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | pub fn foo<T>(s: S<T>, t: S<T>) where S<T>: PartialEq {
+   |                                 +++++++++++++++++++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr b/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
index e90784a54ae..c1281ce6a42 100644
--- a/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
+++ b/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
@@ -11,6 +11,19 @@ LL |     <i32 as Add<u32>>::add(1, 2);
              <&'a i32 as Add<i32>>
              <&i32 as Add<&i32>>
 
+error[E0277]: cannot add `u32` to `i32`
+  --> $DIR/ufcs-qpath-self-mismatch.rs:4:5
+   |
+LL |     <i32 as Add<u32>>::add(1, 2);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `i32 + u32`
+   |
+   = help: the trait `Add<u32>` is not implemented for `i32`
+   = help: the following other types implement trait `Add<Rhs>`:
+             <i32 as Add>
+             <i32 as Add<&i32>>
+             <&'a i32 as Add<i32>>
+             <&i32 as Add<&i32>>
+
 error[E0308]: mismatched types
   --> $DIR/ufcs-qpath-self-mismatch.rs:7:28
    |
@@ -55,19 +68,6 @@ help: change the type of the numeric literal from `u32` to `i32`
 LL |     <i32 as Add<i32>>::add(1, 2i32);
    |                                ~~~
 
-error[E0277]: cannot add `u32` to `i32`
-  --> $DIR/ufcs-qpath-self-mismatch.rs:4:5
-   |
-LL |     <i32 as Add<u32>>::add(1, 2);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `i32 + u32`
-   |
-   = help: the trait `Add<u32>` is not implemented for `i32`
-   = help: the following other types implement trait `Add<Rhs>`:
-             <i32 as Add>
-             <i32 as Add<&i32>>
-             <&'a i32 as Add<i32>>
-             <&i32 as Add<&i32>>
-
 error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0277, E0308.