diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index 8678f388298c2..13cc5cbed443f 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 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IsSuggestableVisitor<'tcx> {
         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 bb3cdde66d173..fd50f80474833 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 2bd745c645a1c..018250c8c1b60 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.rs b/tests/ui/ufcs/ufcs-qpath-self-mismatch.rs
index ec86213f8629d..1a07dc46c5e8a 100644
--- a/tests/ui/ufcs/ufcs-qpath-self-mismatch.rs
+++ b/tests/ui/ufcs/ufcs-qpath-self-mismatch.rs
@@ -4,6 +4,7 @@ fn main() {
     <i32 as Add<u32>>::add(1, 2);
     //~^ ERROR cannot add `u32` to `i32`
     //~| ERROR cannot add `u32` to `i32`
+    //~| ERROR cannot add `u32` to `i32`
     <i32 as Add<i32>>::add(1u32, 2);
     //~^ ERROR mismatched types
     <i32 as Add<i32>>::add(1, 2u32);
diff --git a/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr b/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
index e90784a54ae0d..0e60779dac5e1 100644
--- a/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
+++ b/tests/ui/ufcs/ufcs-qpath-self-mismatch.stderr
@@ -11,8 +11,21 @@ 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
+  --> $DIR/ufcs-qpath-self-mismatch.rs:8:28
    |
 LL |     <i32 as Add<i32>>::add(1u32, 2);
    |     ---------------------- ^^^^ expected `i32`, found `u32`
@@ -20,7 +33,7 @@ LL |     <i32 as Add<i32>>::add(1u32, 2);
    |     arguments to this function are incorrect
    |
 help: the return type of this call is `u32` due to the type of the argument passed
-  --> $DIR/ufcs-qpath-self-mismatch.rs:7:5
+  --> $DIR/ufcs-qpath-self-mismatch.rs:8:5
    |
 LL |     <i32 as Add<i32>>::add(1u32, 2);
    |     ^^^^^^^^^^^^^^^^^^^^^^^----^^^^
@@ -34,7 +47,7 @@ LL |     <i32 as Add<i32>>::add(1i32, 2);
    |                             ~~~
 
 error[E0308]: mismatched types
-  --> $DIR/ufcs-qpath-self-mismatch.rs:9:31
+  --> $DIR/ufcs-qpath-self-mismatch.rs:10:31
    |
 LL |     <i32 as Add<i32>>::add(1, 2u32);
    |     ----------------------    ^^^^ expected `i32`, found `u32`
@@ -42,7 +55,7 @@ LL |     <i32 as Add<i32>>::add(1, 2u32);
    |     arguments to this function are incorrect
    |
 help: the return type of this call is `u32` due to the type of the argument passed
-  --> $DIR/ufcs-qpath-self-mismatch.rs:9:5
+  --> $DIR/ufcs-qpath-self-mismatch.rs:10:5
    |
 LL |     <i32 as Add<i32>>::add(1, 2u32);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^----^
@@ -68,7 +81,7 @@ LL |     <i32 as Add<u32>>::add(1, 2);
              <&'a i32 as Add<i32>>
              <&i32 as Add<&i32>>
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.