Improve const traits diagnostics for new desugaring

24 files changed, 224 insertions, 168 deletions

diff --git a/compiler/rustc_middle/src/macros.rs b/compiler/rustc_middle/src/macros.rs
index d385be007d3..39816c17b98 100644
--- a/compiler/rustc_middle/src/macros.rs
+++ b/compiler/rustc_middle/src/macros.rs
@@ -67,7 +67,7 @@ macro_rules! TrivialLiftImpls {
     };
 }
 
-/// Used for types that are `Copy` and which **do not care arena
+/// Used for types that are `Copy` and which **do not care about arena
 /// allocated data** (i.e., don't need to be folded).
 #[macro_export]
 macro_rules! TrivialTypeTraversalImpls {
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 8fd4c30457f..7ada5fd93ba 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1951,19 +1951,18 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
 
     fn pretty_print_bound_constness(
         &mut self,
-        trait_ref: ty::TraitRef<'tcx>,
+        constness: ty::BoundConstness,
     ) -> Result<(), PrintError> {
         define_scoped_cx!(self);
 
-        let Some(idx) = self.tcx().generics_of(trait_ref.def_id).host_effect_index else {
-            return Ok(());
-        };
-        let arg = trait_ref.args.const_at(idx);
-
-        if arg == self.tcx().consts.false_ {
-            p!("const ");
-        } else if arg != self.tcx().consts.true_ && !arg.has_infer() {
-            p!("~const ");
+        match constness {
+            ty::BoundConstness::NotConst => {}
+            ty::BoundConstness::Const => {
+                p!("const ");
+            }
+            ty::BoundConstness::ConstIfConst => {
+                p!("~const ");
+            }
         }
         Ok(())
     }
@@ -2948,6 +2947,15 @@ impl<'tcx> ty::TraitPredicate<'tcx> {
     }
 }
 
+#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Lift)]
+pub struct TraitPredPrintWithBoundConstness<'tcx>(ty::TraitPredicate<'tcx>, ty::BoundConstness);
+
+impl<'tcx> fmt::Debug for TraitPredPrintWithBoundConstness<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
 #[extension(pub trait PrintPolyTraitPredicateExt<'tcx>)]
 impl<'tcx> ty::PolyTraitPredicate<'tcx> {
     fn print_modifiers_and_trait_path(
@@ -2955,6 +2963,13 @@ impl<'tcx> ty::PolyTraitPredicate<'tcx> {
     ) -> ty::Binder<'tcx, TraitPredPrintModifiersAndPath<'tcx>> {
         self.map_bound(TraitPredPrintModifiersAndPath)
     }
+
+    fn print_with_bound_constness(
+        self,
+        constness: ty::BoundConstness,
+    ) -> ty::Binder<'tcx, TraitPredPrintWithBoundConstness<'tcx>> {
+        self.map_bound(|trait_pred| TraitPredPrintWithBoundConstness(trait_pred, constness))
+    }
 }
 
 #[derive(Debug, Copy, Clone, Lift)]
@@ -3052,7 +3067,6 @@ define_print! {
 
     ty::TraitPredicate<'tcx> {
         p!(print(self.trait_ref.self_ty()), ": ");
-        p!(pretty_print_bound_constness(self.trait_ref));
         if let ty::PredicatePolarity::Negative = self.polarity {
             p!("!");
         }
@@ -3184,13 +3198,21 @@ define_print_and_forward_display! {
     }
 
     TraitPredPrintModifiersAndPath<'tcx> {
-        p!(pretty_print_bound_constness(self.0.trait_ref));
         if let ty::PredicatePolarity::Negative = self.0.polarity {
             p!("!")
         }
         p!(print(self.0.trait_ref.print_trait_sugared()));
     }
 
+    TraitPredPrintWithBoundConstness<'tcx> {
+        p!(print(self.0.trait_ref.self_ty()), ": ");
+        p!(pretty_print_bound_constness(self.1));
+        if let ty::PredicatePolarity::Negative = self.0.polarity {
+            p!("!");
+        }
+        p!(print(self.0.trait_ref.print_trait_sugared()))
+    }
+
     PrintClosureAsImpl<'tcx> {
         p!(pretty_closure_as_impl(self.closure))
     }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index 34a0f182ab4..5af117a3f48 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -19,8 +19,8 @@ use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::print::{
-    FmtPrinter, Print, PrintTraitPredicateExt as _, PrintTraitRefExt as _,
-    with_forced_trimmed_paths,
+    FmtPrinter, Print, PrintPolyTraitPredicateExt, PrintTraitPredicateExt as _,
+    PrintTraitRefExt as _, with_forced_trimmed_paths,
 };
 use rustc_middle::ty::{
     self, ToPolyTraitRef, TraitRef, Ty, TyCtxt, TypeFoldable, TypeVisitableExt, Upcast,
@@ -154,6 +154,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         } else {
                             (leaf_trait_predicate, &obligation)
                         };
+
+                        let (main_trait_predicate, leaf_trait_predicate, predicate_constness) = self.get_effects_trait_pred_override(main_trait_predicate, leaf_trait_predicate, span);
+
                         let main_trait_ref = main_trait_predicate.to_poly_trait_ref();
                         let leaf_trait_ref = leaf_trait_predicate.to_poly_trait_ref();
 
@@ -164,9 +167,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             return guar;
                         }
 
-                        // FIXME(effects)
-                        let predicate_is_const = false;
-
                         if let Err(guar) = leaf_trait_predicate.error_reported()
                         {
                             return guar;
@@ -227,7 +227,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         let err_msg = self.get_standard_error_message(
                             main_trait_predicate,
                             message,
-                            predicate_is_const,
+                            predicate_constness,
                             append_const_msg,
                             post_message,
                         );
@@ -286,7 +286,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         }
 
                         if tcx.is_lang_item(leaf_trait_ref.def_id(), LangItem::Drop)
-                            && predicate_is_const
+                            && matches!(predicate_constness, ty::BoundConstness::ConstIfConst | ty::BoundConstness::Const)
                         {
                             err.note("`~const Drop` was renamed to `~const Destruct`");
                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
@@ -2187,29 +2187,34 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         &self,
         trait_predicate: ty::PolyTraitPredicate<'tcx>,
         message: Option<String>,
-        predicate_is_const: bool,
+        predicate_constness: ty::BoundConstness,
         append_const_msg: Option<AppendConstMessage>,
         post_message: String,
     ) -> String {
         message
             .and_then(|cannot_do_this| {
-                match (predicate_is_const, append_const_msg) {
+                match (predicate_constness, append_const_msg) {
                     // do nothing if predicate is not const
-                    (false, _) => Some(cannot_do_this),
+                    (ty::BoundConstness::NotConst, _) => Some(cannot_do_this),
                     // suggested using default post message
-                    (true, Some(AppendConstMessage::Default)) => {
-                        Some(format!("{cannot_do_this} in const contexts"))
-                    }
+                    (
+                        ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst,
+                        Some(AppendConstMessage::Default),
+                    ) => Some(format!("{cannot_do_this} in const contexts")),
                     // overridden post message
-                    (true, Some(AppendConstMessage::Custom(custom_msg, _))) => {
-                        Some(format!("{cannot_do_this}{custom_msg}"))
-                    }
+                    (
+                        ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst,
+                        Some(AppendConstMessage::Custom(custom_msg, _)),
+                    ) => Some(format!("{cannot_do_this}{custom_msg}")),
                     // fallback to generic message
-                    (true, None) => None,
+                    (ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst, None) => None,
                 }
             })
             .unwrap_or_else(|| {
-                format!("the trait bound `{trait_predicate}` is not satisfied{post_message}")
+                format!(
+                    "the trait bound `{}` is not satisfied{post_message}",
+                    trait_predicate.print_with_bound_constness(predicate_constness)
+                )
             })
     }
 
@@ -2333,6 +2338,51 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
     }
 
+    /// For effects predicates such as `<u32 as Add>::Effects: Compat<host>`, pretend that the
+    /// predicate that failed was `u32: Add`. Return the constness of such predicate to later
+    /// print as `u32: ~const Add`.
+    fn get_effects_trait_pred_override(
+        &self,
+        p: ty::PolyTraitPredicate<'tcx>,
+        leaf: ty::PolyTraitPredicate<'tcx>,
+        span: Span,
+    ) -> (ty::PolyTraitPredicate<'tcx>, ty::PolyTraitPredicate<'tcx>, ty::BoundConstness) {
+        let trait_ref = p.to_poly_trait_ref();
+        if !self.tcx.is_lang_item(trait_ref.def_id(), LangItem::EffectsCompat) {
+            return (p, leaf, ty::BoundConstness::NotConst);
+        }
+
+        let Some(ty::Alias(ty::AliasTyKind::Projection, projection)) =
+            trait_ref.self_ty().no_bound_vars().map(Ty::kind)
+        else {
+            return (p, leaf, ty::BoundConstness::NotConst);
+        };
+
+        let constness = trait_ref.skip_binder().args.const_at(1);
+
+        let constness = if constness == self.tcx.consts.true_ || constness.is_ct_infer() {
+            ty::BoundConstness::NotConst
+        } else if constness == self.tcx.consts.false_ {
+            ty::BoundConstness::Const
+        } else if matches!(constness.kind(), ty::ConstKind::Param(_)) {
+            ty::BoundConstness::ConstIfConst
+        } else {
+            self.dcx().span_bug(span, format!("Unknown constness argument: {constness:?}"));
+        };
+
+        let new_pred = p.map_bound(|mut trait_pred| {
+            trait_pred.trait_ref = projection.trait_ref(self.tcx);
+            trait_pred
+        });
+
+        let new_leaf = leaf.map_bound(|mut trait_pred| {
+            trait_pred.trait_ref = projection.trait_ref(self.tcx);
+            trait_pred
+        });
+
+        (new_pred, new_leaf, constness)
+    }
+
     fn add_tuple_trait_message(
         &self,
         obligation_cause_code: &ObligationCauseCode<'tcx>,
diff --git a/compiler/rustc_type_ir/src/predicate.rs b/compiler/rustc_type_ir/src/predicate.rs
index 76065c10d19..8146181df6c 100644
--- a/compiler/rustc_type_ir/src/predicate.rs
+++ b/compiler/rustc_type_ir/src/predicate.rs
@@ -755,3 +755,10 @@ impl fmt::Display for BoundConstness {
         }
     }
 }
+
+impl<I> Lift<I> for BoundConstness {
+    type Lifted = BoundConstness;
+    fn lift_to_interner(self, _: I) -> Option<Self::Lifted> {
+        Some(self)
+    }
+}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.stderr
index fb491453b37..8288c660ce7 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-0.stderr
@@ -3,11 +3,11 @@ error: using `#![feature(effects)]` without enabling next trait solver globally
    = note: the next trait solver must be enabled globally for the effects feature to work correctly
    = help: use `-Znext-solver` to enable
 
-error[E0277]: the trait bound `Trait::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `<T as Trait>::Assoc: Trait` is not satisfied
   --> $DIR/assoc-type-const-bound-usage-0.rs:13:5
    |
 LL |     T::Assoc::func()
-   |     ^^^^^^^^ the trait `Compat` is not implemented for `Trait::{synthetic#0}`
+   |     ^^^^^^^^ the trait `Trait` is not implemented for `<T as Trait>::Assoc`
    |
 note: required by a bound in `Trait::func`
   --> $DIR/assoc-type-const-bound-usage-0.rs:6:1
@@ -17,12 +17,16 @@ LL | #[const_trait]
 ...
 LL |     fn func() -> i32;
    |        ---- required by a bound in this associated function
+help: consider further restricting the associated type
+   |
+LL | const fn unqualified<T: ~const Trait>() -> i32 where <T as Trait>::Assoc: Trait {
+   |                                                ++++++++++++++++++++++++++++++++
 
-error[E0277]: the trait bound `Trait::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `<T as Trait>::Assoc: Trait` is not satisfied
   --> $DIR/assoc-type-const-bound-usage-0.rs:17:5
    |
 LL |     <T as Trait>::Assoc::func()
-   |     ^^^^^^^^^^^^^^^^^^^ the trait `Compat` is not implemented for `Trait::{synthetic#0}`
+   |     ^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `<T as Trait>::Assoc`
    |
 note: required by a bound in `Trait::func`
   --> $DIR/assoc-type-const-bound-usage-0.rs:6:1
@@ -32,6 +36,10 @@ LL | #[const_trait]
 ...
 LL |     fn func() -> i32;
    |        ---- required by a bound in this associated function
+help: consider further restricting the associated type
+   |
+LL | const fn qualified<T: ~const Trait>() -> i32 where <T as Trait>::Assoc: Trait {
+   |                                              ++++++++++++++++++++++++++++++++
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.stderr
index 392b310a4c9..0792d090321 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage-1.stderr
@@ -3,11 +3,11 @@ error: using `#![feature(effects)]` without enabling next trait solver globally
    = note: the next trait solver must be enabled globally for the effects feature to work correctly
    = help: use `-Znext-solver` to enable
 
-error[E0277]: the trait bound `Trait::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `<T as Trait>::Assoc: Trait` is not satisfied
   --> $DIR/assoc-type-const-bound-usage-1.rs:15:44
    |
 LL | fn unqualified<T: const Trait>() -> Type<{ T::Assoc::func() }> {
-   |                                            ^^^^^^^^ the trait `Compat` is not implemented for `Trait::{synthetic#0}`
+   |                                            ^^^^^^^^ the trait `Trait` is not implemented for `<T as Trait>::Assoc`
    |
 note: required by a bound in `Trait::func`
   --> $DIR/assoc-type-const-bound-usage-1.rs:7:1
@@ -17,12 +17,16 @@ LL | #[const_trait]
 ...
 LL |     fn func() -> i32;
    |        ---- required by a bound in this associated function
+help: consider further restricting the associated type
+   |
+LL | fn unqualified<T: const Trait>() -> Type<{ T::Assoc::func() }> where <T as Trait>::Assoc: Trait {
+   |                                                                ++++++++++++++++++++++++++++++++
 
-error[E0277]: the trait bound `Trait::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `<T as Trait>::Assoc: Trait` is not satisfied
   --> $DIR/assoc-type-const-bound-usage-1.rs:19:42
    |
 LL | fn qualified<T: const Trait>() -> Type<{ <T as Trait>::Assoc::func() }> {
-   |                                          ^^^^^^^^^^^^^^^^^^^ the trait `Compat` is not implemented for `Trait::{synthetic#0}`
+   |                                          ^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `<T as Trait>::Assoc`
    |
 note: required by a bound in `Trait::func`
   --> $DIR/assoc-type-const-bound-usage-1.rs:7:1
@@ -32,6 +36,10 @@ LL | #[const_trait]
 ...
 LL |     fn func() -> i32;
    |        ---- required by a bound in this associated function
+help: consider further restricting the associated type
+   |
+LL | fn qualified<T: const Trait>() -> Type<{ <T as Trait>::Assoc::func() }> where <T as Trait>::Assoc: Trait {
+   |                                                                         ++++++++++++++++++++++++++++++++
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs
index bb9e9045f8f..878f9a713a0 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs
@@ -1,4 +1,6 @@
-#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
+//@ compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
 
 #[const_trait]
 pub trait Plus {
@@ -23,7 +25,7 @@ pub const fn add_i32(a: i32, b: i32) -> i32 {
 
 pub const fn add_u32(a: u32, b: u32) -> u32 {
     a.plus(b)
-    //~^ ERROR the trait bound
+    //~^ ERROR the trait bound `u32: ~const Plus`
 }
 
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr
index 73ea1422bf9..5d2333d94fe 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr
@@ -1,33 +1,22 @@
-warning: the feature `effects` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/call-const-trait-method-fail.rs:1:30
-   |
-LL | #![feature(const_trait_impl, effects)]
-   |                              ^^^^^^^
-   |
-   = note: see issue #102090 <https://github.com/rust-lang/rust/issues/102090> for more information
-   = note: `#[warn(incomplete_features)]` on by default
-
-error: using `#![feature(effects)]` without enabling next trait solver globally
-   |
-   = note: the next trait solver must be enabled globally for the effects feature to work correctly
-   = help: use `-Znext-solver` to enable
-
-error[E0277]: the trait bound `Runtime: ~const Compat` is not satisfied
-  --> $DIR/call-const-trait-method-fail.rs:25:5
+error[E0277]: the trait bound `u32: ~const Plus` is not satisfied
+  --> $DIR/call-const-trait-method-fail.rs:27:5
    |
 LL |     a.plus(b)
-   |     ^ the trait `~const Compat` is not implemented for `Runtime`
+   |     ^ the trait `Plus` is not implemented for `u32`
    |
-   = help: the trait `Compat` is implemented for `Runtime`
 note: required by a bound in `Plus::plus`
-  --> $DIR/call-const-trait-method-fail.rs:3:1
+  --> $DIR/call-const-trait-method-fail.rs:5:1
    |
 LL | #[const_trait]
    | ^^^^^^^^^^^^^^ required by this bound in `Plus::plus`
 LL | pub trait Plus {
 LL |     fn plus(self, rhs: Self) -> Self;
    |        ---- required by a bound in this associated function
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | pub const fn add_u32(a: u32, b: u32) -> u32 where u32: Plus {
+   |                                             +++++++++++++++
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs
index 74e33ca72ff..f9e79d41752 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs
@@ -1,4 +1,6 @@
-#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
+//@ compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
 
 struct S;
 
@@ -21,7 +23,6 @@ const fn equals_self<T: ~const Foo>(t: &T) -> bool {
 // it not using the impl.
 
 pub const EQ: bool = equals_self(&S);
-//~^ ERROR: the trait bound `Runtime: const Compat` is not satisfied
-// FIXME(effects) diagnostic
+//~^ ERROR: the trait bound `S: const Foo` is not satisfied
 
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.stderr
index b2a98041c1c..68c9fc40010 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/call-generic-method-nonconst.stderr
@@ -1,32 +1,21 @@
-warning: the feature `effects` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/call-generic-method-nonconst.rs:1:30
-   |
-LL | #![feature(const_trait_impl, effects)]
-   |                              ^^^^^^^
-   |
-   = note: see issue #102090 <https://github.com/rust-lang/rust/issues/102090> for more information
-   = note: `#[warn(incomplete_features)]` on by default
-
-error: using `#![feature(effects)]` without enabling next trait solver globally
-   |
-   = note: the next trait solver must be enabled globally for the effects feature to work correctly
-   = help: use `-Znext-solver` to enable
-
-error[E0277]: the trait bound `Runtime: const Compat` is not satisfied
-  --> $DIR/call-generic-method-nonconst.rs:23:34
+error[E0277]: the trait bound `S: const Foo` is not satisfied
+  --> $DIR/call-generic-method-nonconst.rs:25:34
    |
 LL | pub const EQ: bool = equals_self(&S);
-   |                      ----------- ^^ the trait `const Compat` is not implemented for `Runtime`
+   |                      ----------- ^^ the trait `Foo` is not implemented for `S`
    |                      |
    |                      required by a bound introduced by this call
    |
-   = help: the trait `Compat` is implemented for `Runtime`
 note: required by a bound in `equals_self`
-  --> $DIR/call-generic-method-nonconst.rs:16:25
+  --> $DIR/call-generic-method-nonconst.rs:18:25
    |
 LL | const fn equals_self<T: ~const Foo>(t: &T) -> bool {
    |                         ^^^^^^^^^^ required by this bound in `equals_self`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | pub const EQ: bool where S: Foo = equals_self(&S);
+   |                    ++++++++++++
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.rs
index 2fd58b05178..a0333153f85 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.rs
@@ -1,4 +1,6 @@
-#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
+//@ compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
 
 #[const_trait]
 trait ConstDefaultFn: Sized {
@@ -22,7 +24,7 @@ impl const ConstDefaultFn for ConstImpl {
 
 const fn test() {
     NonConstImpl.a();
-    //~^ ERROR the trait bound
+    //~^ ERROR the trait bound `NonConstImpl: ~const ConstDefaultFn` is not satisfied
     ConstImpl.a();
 }
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.stderr
index 02f9dffba32..0809d9c1e1d 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-default-method-bodies.stderr
@@ -1,33 +1,22 @@
-warning: the feature `effects` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/const-default-method-bodies.rs:1:30
-   |
-LL | #![feature(const_trait_impl, effects)]
-   |                              ^^^^^^^
-   |
-   = note: see issue #102090 <https://github.com/rust-lang/rust/issues/102090> for more information
-   = note: `#[warn(incomplete_features)]` on by default
-
-error: using `#![feature(effects)]` without enabling next trait solver globally
-   |
-   = note: the next trait solver must be enabled globally for the effects feature to work correctly
-   = help: use `-Znext-solver` to enable
-
-error[E0277]: the trait bound `Runtime: ~const Compat` is not satisfied
-  --> $DIR/const-default-method-bodies.rs:24:18
+error[E0277]: the trait bound `NonConstImpl: ~const ConstDefaultFn` is not satisfied
+  --> $DIR/const-default-method-bodies.rs:26:18
    |
 LL |     NonConstImpl.a();
-   |                  ^ the trait `~const Compat` is not implemented for `Runtime`
+   |                  ^ the trait `ConstDefaultFn` is not implemented for `NonConstImpl`
    |
-   = help: the trait `Compat` is implemented for `Runtime`
 note: required by a bound in `ConstDefaultFn::a`
-  --> $DIR/const-default-method-bodies.rs:3:1
+  --> $DIR/const-default-method-bodies.rs:5:1
    |
 LL | #[const_trait]
    | ^^^^^^^^^^^^^^ required by this bound in `ConstDefaultFn::a`
 ...
 LL |     fn a(self) {
    |        - required by a bound in this associated function
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | const fn test() where NonConstImpl: ConstDefaultFn {
+   |                 ++++++++++++++++++++++++++++++++++
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-fns-are-early-bound.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-fns-are-early-bound.stderr
index 7aa3aa8c6bb..9eda9d98ec5 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-fns-are-early-bound.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-fns-are-early-bound.stderr
@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `FnOnce<()>::{synthetic#0}: const Compat` is not satisfied
+error[E0277]: the trait bound `fn() {foo}: const FnOnce()` is not satisfied
   --> $DIR/const-fns-are-early-bound.rs:31:17
    |
 LL |     is_const_fn(foo);
-   |     ----------- ^^^ the trait `const Compat` is not implemented for `FnOnce<()>::{synthetic#0}`
+   |     ----------- ^^^ the trait `FnOnce()` is not implemented for fn item `fn() {foo}`
    |     |
    |     required by a bound introduced by this call
    |
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/cross-crate.gatednc.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/cross-crate.gatednc.stderr
index b7209827c22..a34bae843c8 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/cross-crate.gatednc.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/cross-crate.gatednc.stderr
@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `cross_crate::MyTrait::{synthetic#0}: ~const Compat` is not satisfied
+error[E0277]: the trait bound `cross_crate::NonConst: ~const cross_crate::MyTrait` is not satisfied
   --> $DIR/cross-crate.rs:19:14
    |
 LL |     NonConst.func();
-   |              ^^^^ the trait `~const Compat` is not implemented for `cross_crate::MyTrait::{synthetic#0}`
+   |              ^^^^ the trait `cross_crate::MyTrait` is not implemented for `cross_crate::NonConst`
    |
 note: required by a bound in `func`
   --> $DIR/auxiliary/cross-crate.rs:5:1
@@ -12,6 +12,10 @@ LL | #[const_trait]
 ...
 LL |     fn func(self);
    |        ---- required by a bound in this associated function
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | const fn const_context() where cross_crate::NonConst: cross_crate::MyTrait {
+   |                          +++++++++++++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.rs
index 64f23824b39..0c2d93775a4 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.rs
@@ -1,4 +1,6 @@
-#![feature(const_trait_impl, effects)] //~ WARN the feature `effects` is incomplete
+//@ compile-flags: -Znext-solver
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
 
 #[const_trait]
 pub trait Tr {
@@ -6,7 +8,7 @@ pub trait Tr {
 
     fn b(&self) {
         ().a()
-        //~^ ERROR the trait bound
+        //~^ ERROR the trait bound `(): ~const Tr` is not satisfied
     }
 }
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.stderr
index 1b5aa9c9191..d0f22c0b9b6 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.stderr
@@ -1,33 +1,22 @@
-warning: the feature `effects` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/default-method-body-is-const-same-trait-ck.rs:1:30
-   |
-LL | #![feature(const_trait_impl, effects)]
-   |                              ^^^^^^^
-   |
-   = note: see issue #102090 <https://github.com/rust-lang/rust/issues/102090> for more information
-   = note: `#[warn(incomplete_features)]` on by default
-
-error: using `#![feature(effects)]` without enabling next trait solver globally
-   |
-   = note: the next trait solver must be enabled globally for the effects feature to work correctly
-   = help: use `-Znext-solver` to enable
-
-error[E0277]: the trait bound `Runtime: ~const Compat` is not satisfied
-  --> $DIR/default-method-body-is-const-same-trait-ck.rs:8:12
+error[E0277]: the trait bound `(): ~const Tr` is not satisfied
+  --> $DIR/default-method-body-is-const-same-trait-ck.rs:10:12
    |
 LL |         ().a()
-   |            ^ the trait `~const Compat` is not implemented for `Runtime`
+   |            ^ the trait `Tr` is not implemented for `()`
    |
-   = help: the trait `Compat` is implemented for `Runtime`
 note: required by a bound in `Tr::a`
-  --> $DIR/default-method-body-is-const-same-trait-ck.rs:3:1
+  --> $DIR/default-method-body-is-const-same-trait-ck.rs:5:1
    |
 LL | #[const_trait]
    | ^^^^^^^^^^^^^^ required by this bound in `Tr::a`
 LL | pub trait Tr {
 LL |     fn a(&self) {}
    |        - required by a bound in this associated function
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | pub trait Tr where (): Tr {
+   |              ++++++++++++
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.rs
index 0d659744e70..93a6f385e47 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.rs
@@ -17,8 +17,7 @@ trait Bar: ~const Foo {}
 
 const fn foo<T: Bar>(x: &T) {
     x.a();
-    //[yy,yn]~^ ERROR the trait bound
-    // FIXME(effects) diagnostic
+    //[yy,yn]~^ ERROR the trait bound `T: ~const Foo`
 }
 
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yn.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yn.stderr
index d4064e01ef1..873c57ec71f 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yn.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yn.stderr
@@ -10,11 +10,11 @@ note: this trait is not a `#[const_trait]`, so it cannot have `~const` trait bou
 LL | trait Bar: ~const Foo {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `Foo::{synthetic#0}: ~const Compat` is not satisfied
+error[E0277]: the trait bound `T: ~const Foo` is not satisfied
   --> $DIR/super-traits-fail-2.rs:19:7
    |
 LL |     x.a();
-   |       ^ the trait `~const Compat` is not implemented for `Foo::{synthetic#0}`
+   |       ^ the trait `Foo` is not implemented for `T`
    |
 note: required by a bound in `Foo::a`
   --> $DIR/super-traits-fail-2.rs:6:25
@@ -24,10 +24,10 @@ LL | #[cfg_attr(any(yy, yn), const_trait)]
 LL | trait Foo {
 LL |     fn a(&self);
    |        - required by a bound in this associated function
-help: consider further restricting the associated type
+help: consider further restricting this bound
    |
-LL | const fn foo<T: Bar>(x: &T) where Foo::{synthetic#0}: ~const Compat {
-   |                             +++++++++++++++++++++++++++++++++++++++
+LL | const fn foo<T: Bar + Foo>(x: &T) {
+   |                     +++++
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yy.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yy.stderr
index 9f9f96c6b48..bea3aea2f3a 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yy.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-2.yy.stderr
@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `Foo::{synthetic#0}: ~const Compat` is not satisfied
+error[E0277]: the trait bound `T: ~const Foo` is not satisfied
   --> $DIR/super-traits-fail-2.rs:19:7
    |
 LL |     x.a();
-   |       ^ the trait `~const Compat` is not implemented for `Foo::{synthetic#0}`
+   |       ^ the trait `Foo` is not implemented for `T`
    |
 note: required by a bound in `Foo::a`
   --> $DIR/super-traits-fail-2.rs:6:25
@@ -12,10 +12,10 @@ LL | #[cfg_attr(any(yy, yn), const_trait)]
 LL | trait Foo {
 LL |     fn a(&self);
    |        - required by a bound in this associated function
-help: consider further restricting the associated type
+help: consider further restricting this bound
    |
-LL | const fn foo<T: Bar>(x: &T) where Foo::{synthetic#0}: ~const Compat {
-   |                             +++++++++++++++++++++++++++++++++++++++
+LL | const fn foo<T: Bar + Foo>(x: &T) {
+   |                     +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.rs
index c7e224dcce0..b5643b11700 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.rs
@@ -20,7 +20,7 @@ trait Bar: ~const Foo {}
 const fn foo<T: ~const Bar>(x: &T) {
     //[yn,nn]~^ ERROR: `~const` can only be applied to `#[const_trait]`
     x.a();
-    //[yn]~^ ERROR: the trait bound
+    //[yn]~^ ERROR: the trait bound `T: ~const Foo` is not satisfied
 }
 
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.yn.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.yn.stderr
index 0b48633a10e..bbc95948a59 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.yn.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.yn.stderr
@@ -16,11 +16,11 @@ error: `~const` can only be applied to `#[const_trait]` traits
 LL | const fn foo<T: ~const Bar>(x: &T) {
    |                        ^^^
 
-error[E0277]: the trait bound `Foo::{synthetic#0}: ~const Compat` is not satisfied
+error[E0277]: the trait bound `T: ~const Foo` is not satisfied
   --> $DIR/super-traits-fail-3.rs:22:7
    |
 LL |     x.a();
-   |       ^ the trait `~const Compat` is not implemented for `Foo::{synthetic#0}`
+   |       ^ the trait `Foo` is not implemented for `T`
    |
 note: required by a bound in `Foo::a`
   --> $DIR/super-traits-fail-3.rs:8:25
@@ -30,10 +30,10 @@ LL | #[cfg_attr(any(yy, yn), const_trait)]
 LL | trait Foo {
 LL |     fn a(&self);
    |        - required by a bound in this associated function
-help: consider further restricting the associated type
+help: consider further restricting this bound
    |
-LL | const fn foo<T: ~const Bar>(x: &T) where Foo::{synthetic#0}: ~const Compat {
-   |                                    +++++++++++++++++++++++++++++++++++++++
+LL | const fn foo<T: ~const Bar + Foo>(x: &T) {
+   |                            +++++
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-where-clause-const.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-where-clause-const.stderr
index 979f1e798e0..eaa981ec744 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-where-clause-const.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-where-clause-const.stderr
@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `Foo::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `T: Foo` is not satisfied
   --> $DIR/trait-where-clause-const.rs:22:5
    |
 LL |     T::b();
-   |     ^ the trait `Compat` is not implemented for `Foo::{synthetic#0}`
+   |     ^ the trait `Foo` is not implemented for `T`
    |
 note: required by a bound in `Foo::b`
   --> $DIR/trait-where-clause-const.rs:13:1
@@ -12,10 +12,6 @@ LL | #[const_trait]
 ...
 LL |     fn b() where Self: ~const Bar;
    |        - required by a bound in this associated function
-help: consider further restricting the associated type
-   |
-LL | const fn test1<T: ~const Foo + Bar>() where Foo::{synthetic#0}: Compat {
-   |                                       ++++++++++++++++++++++++++++++++
 
 error[E0308]: mismatched types
   --> $DIR/trait-where-clause-const.rs:22:5
@@ -26,11 +22,11 @@ LL |     T::b();
    = note: expected constant `host`
               found constant `true`
 
-error[E0277]: the trait bound `Foo::{synthetic#0}: Compat` is not satisfied
+error[E0277]: the trait bound `T: Foo` is not satisfied
   --> $DIR/trait-where-clause-const.rs:25:5
    |
 LL |     T::c::<T>();
-   |     ^ the trait `Compat` is not implemented for `Foo::{synthetic#0}`
+   |     ^ the trait `Foo` is not implemented for `T`
    |
 note: required by a bound in `Foo::c`
   --> $DIR/trait-where-clause-const.rs:13:1
@@ -40,10 +36,6 @@ LL | #[const_trait]
 ...
 LL |     fn c<T: ~const Bar>();
    |        - required by a bound in this associated function
-help: consider further restricting the associated type
-   |
-LL | const fn test1<T: ~const Foo + Bar>() where Foo::{synthetic#0}: Compat {
-   |                                       ++++++++++++++++++++++++++++++++
 
 error[E0308]: mismatched types
   --> $DIR/trait-where-clause-const.rs:25:5
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.rs
index 5fffe54fc1a..d336719f52e 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.rs
@@ -19,7 +19,6 @@ impl Trait for Ty {
 }
 
 fn main() {
-    // FIXME(effects): improve diagnostics on this
     require::<Ty>();
 }
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.stderr
index 0806ffa4b5d..848aa68689b 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/unsatisfied-const-trait-bound.stderr
@@ -7,7 +7,7 @@ LL | #![feature(const_trait_impl, effects, generic_const_exprs)]
    = help: remove one of these features
 
 error[E0308]: mismatched types
-  --> $DIR/unsatisfied-const-trait-bound.rs:30:37
+  --> $DIR/unsatisfied-const-trait-bound.rs:29:37
    |
 LL | fn accept0<T: Trait>(_: Container<{ T::make() }>) {}
    |                                     ^^^^^^^^^ expected `false`, found `true`
@@ -16,7 +16,7 @@ LL | fn accept0<T: Trait>(_: Container<{ T::make() }>) {}
               found constant `true`
 
 error[E0308]: mismatched types
-  --> $DIR/unsatisfied-const-trait-bound.rs:34:50
+  --> $DIR/unsatisfied-const-trait-bound.rs:33:50
    |
 LL | const fn accept1<T: ~const Trait>(_: Container<{ T::make() }>) {}
    |                                                  ^^^^^^^^^ expected `false`, found `host`
@@ -24,17 +24,21 @@ LL | const fn accept1<T: ~const Trait>(_: Container<{ T::make() }>) {}
    = note: expected constant `false`
               found constant `host`
 
-error[E0277]: the trait bound `Trait::{synthetic#0}: const Compat` is not satisfied
-  --> $DIR/unsatisfied-const-trait-bound.rs:23:15
+error[E0277]: the trait bound `Ty: const Trait` is not satisfied
+  --> $DIR/unsatisfied-const-trait-bound.rs:22:15
    |
 LL |     require::<Ty>();
-   |               ^^ the trait `const Compat` is not implemented for `Trait::{synthetic#0}`
+   |               ^^ the trait `Trait` is not implemented for `Ty`
    |
 note: required by a bound in `require`
   --> $DIR/unsatisfied-const-trait-bound.rs:8:15
    |
 LL | fn require<T: const Trait>() {}
    |               ^^^^^^^^^^^ required by this bound in `require`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() where Ty: Trait {
+   |           +++++++++++++++
 
 error: aborting due to 4 previous errors