diff --git a/compiler/rustc_hir_analysis/src/impl_wf_check.rs b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
index 5cc1ec71757be..0056ceff6f529 100644
--- a/compiler/rustc_hir_analysis/src/impl_wf_check.rs
+++ b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
@@ -11,7 +11,7 @@
 use crate::constrained_generic_params as cgp;
 use min_specialization::check_min_specialization;
 
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::{codes::*, struct_span_code_err};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
@@ -88,14 +88,36 @@ fn enforce_impl_params_are_constrained(
 
     impl_trait_ref.error_reported()?;
 
-    let mut input_parameters = cgp::parameters_for_impl(tcx, impl_self_ty, impl_trait_ref);
+    let mut constrained_parameters = cgp::parameters_for_impl(tcx, impl_self_ty, impl_trait_ref);
     cgp::identify_constrained_generic_params(
         tcx,
         impl_predicates,
         impl_trait_ref,
-        &mut input_parameters,
+        &mut constrained_parameters,
     );
 
+    // Reasons each generic is unconstrained, or Other if not present
+    let mut unconstrained_reasons = FxHashMap::default();
+    for (clause, _) in impl_predicates.predicates {
+        if let Some(projection) = clause.as_projection_clause() {
+            let mentioned_params = cgp::parameters_for(tcx, projection.term().skip_binder(), true);
+            let is_clause_circular =
+                Some(projection.required_poly_trait_ref(tcx).skip_binder()) == impl_trait_ref;
+
+            for param in mentioned_params {
+                if is_clause_circular {
+                    // Potentially override BoundToUnconstrainedProjection
+                    unconstrained_reasons.insert(param, UnconstrainedReason::BoundCircularly);
+                } else {
+                    // Don't override BoundCircularly
+                    unconstrained_reasons
+                        .entry(param)
+                        .or_insert(UnconstrainedReason::BoundToUnconstrainedProjection);
+                }
+            }
+        }
+    }
+
     // Disallow unconstrained lifetimes, but only if they appear in assoc types.
     let lifetimes_in_associated_types: FxHashSet<_> = tcx
         .associated_item_def_ids(impl_def_id)
@@ -115,42 +137,56 @@ fn enforce_impl_params_are_constrained(
         })
         .collect();
 
+    let impl_kind = match impl_trait_ref {
+        Some(_) => ImplKind::ImplTrait,
+        None => ImplKind::InherentImpl,
+    };
+
     let mut res = Ok(());
     for param in &impl_generics.own_params {
+        let cgp_param = cgp::Parameter(param.index);
+        let unconstrained_reason =
+            unconstrained_reasons.get(&cgp_param).copied().unwrap_or_default();
+
         match param.kind {
             // Disallow ANY unconstrained type parameters.
             ty::GenericParamDefKind::Type { .. } => {
                 let param_ty = ty::ParamTy::for_def(param);
-                if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
+                if !constrained_parameters.contains(&cgp_param) {
                     res = Err(report_unused_parameter(
                         tcx,
                         tcx.def_span(param.def_id),
                         "type",
                         param_ty.name,
+                        impl_kind,
+                        unconstrained_reason,
                     ));
                 }
             }
             ty::GenericParamDefKind::Lifetime => {
-                let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
-                if lifetimes_in_associated_types.contains(&param_lt) && // (*)
-                    !input_parameters.contains(&param_lt)
+                if lifetimes_in_associated_types.contains(&cgp_param) && // (*)
+                    !constrained_parameters.contains(&cgp_param)
                 {
                     res = Err(report_unused_parameter(
                         tcx,
                         tcx.def_span(param.def_id),
                         "lifetime",
                         param.name,
+                        impl_kind,
+                        unconstrained_reason,
                     ));
                 }
             }
             ty::GenericParamDefKind::Const { .. } => {
                 let param_ct = ty::ParamConst::for_def(param);
-                if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
+                if !constrained_parameters.contains(&cgp_param) {
                     res = Err(report_unused_parameter(
                         tcx,
                         tcx.def_span(param.def_id),
                         "const",
                         param_ct.name,
+                        impl_kind,
+                        unconstrained_reason,
                     ));
                 }
             }
@@ -178,11 +214,30 @@ fn enforce_impl_params_are_constrained(
     // used elsewhere are not projected back out.
 }
 
+#[derive(Copy, Clone)]
+enum ImplKind {
+    ImplTrait,
+    InherentImpl,
+}
+
+#[derive(Copy, Clone, Default)]
+enum UnconstrainedReason {
+    /// Not used in the impl trait, self type, nor bound to any projection
+    #[default]
+    Other,
+    /// Bound to a projection, but the LHS was not constrained
+    BoundToUnconstrainedProjection,
+    /// Bound to a projection, but the LHS is the trait being implemented
+    BoundCircularly,
+}
+
 fn report_unused_parameter(
     tcx: TyCtxt<'_>,
     span: Span,
     kind: &str,
     name: Symbol,
+    impl_kind: ImplKind,
+    unconstrained_reason: UnconstrainedReason,
 ) -> ErrorGuaranteed {
     let mut err = struct_span_code_err!(
         tcx.dcx(),
@@ -194,6 +249,38 @@ fn report_unused_parameter(
         name
     );
     err.span_label(span, format!("unconstrained {kind} parameter"));
+
+    match impl_kind {
+        ImplKind::ImplTrait => {
+            err.note(format!(
+                "to constrain `{name}`, use it in the implemented trait, in the self type, \
+                or in an equality with an associated type"
+            ));
+        }
+        ImplKind::InherentImpl => {
+            err.note(format!(
+                "to constrain `{name}`, use it in the self type, \
+                or in an equality with an associated type"
+            ));
+        }
+    }
+
+    match unconstrained_reason {
+        UnconstrainedReason::BoundToUnconstrainedProjection => {
+            err.note(format!(
+                "`{name}` is bound to an associated type, \
+                but the reference to the associated type itself uses unconstrained generic parameters"
+            ));
+        }
+        UnconstrainedReason::BoundCircularly => {
+            err.note(format!(
+                "`{name}` is bound to an associated type, \
+                but the associated type is circularly defined in this impl"
+            ));
+        }
+        UnconstrainedReason::Other => {}
+    }
+
     if kind == "const" {
         err.note(
             "expressions using a const parameter must map each value to a distinct output value",
@@ -202,5 +289,6 @@ fn report_unused_parameter(
             "proving the result of expressions other than the parameter are unique is not supported",
         );
     }
+
     err.emit()
 }
diff --git a/tests/rustdoc-ui/not-wf-ambiguous-normalization.stderr b/tests/rustdoc-ui/not-wf-ambiguous-normalization.stderr
index 55ab144b92419..677c8f4e05a92 100644
--- a/tests/rustdoc-ui/not-wf-ambiguous-normalization.stderr
+++ b/tests/rustdoc-ui/not-wf-ambiguous-normalization.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> Allocator for DefaultAllocator {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/rustdoc-ui/synthetic-auto-trait-impls/unconstrained-param-in-impl-ambiguity.stderr b/tests/rustdoc-ui/synthetic-auto-trait-impls/unconstrained-param-in-impl-ambiguity.stderr
index 38d1a537fe458..91e34fbf1bc49 100644
--- a/tests/rustdoc-ui/synthetic-auto-trait-impls/unconstrained-param-in-impl-ambiguity.stderr
+++ b/tests/rustdoc-ui/synthetic-auto-trait-impls/unconstrained-param-in-impl-ambiguity.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `Q` is not constrained by the impl trait, self
    |
 LL | unsafe impl<Q: Trait> Send for Inner {}
    |             ^ unconstrained type parameter
+   |
+   = note: to constrain `Q`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/associated-types/issue-26262.stderr b/tests/ui/associated-types/issue-26262.stderr
index 90e2d0d930164..639f0f6667117 100644
--- a/tests/ui/associated-types/issue-26262.stderr
+++ b/tests/ui/associated-types/issue-26262.stderr
@@ -3,12 +3,16 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: Tr> S<T::Assoc> {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
 
 error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
   --> $DIR/issue-26262.rs:17:6
    |
 LL | impl<'a,T: Trait2<'a>> Trait1<<T as Trait2<'a>>::Foo> for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/async-await/in-trait/unconstrained-impl-region.stderr b/tests/ui/async-await/in-trait/unconstrained-impl-region.stderr
index 66819d1fcf7db..f6ba561a3792c 100644
--- a/tests/ui/async-await/in-trait/unconstrained-impl-region.stderr
+++ b/tests/ui/async-await/in-trait/unconstrained-impl-region.stderr
@@ -19,6 +19,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Actor for () {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/async-await/issues/issue-78654.full.stderr b/tests/ui/async-await/issues/issue-78654.full.stderr
index 0d12a948c68bd..a7c3f8e79ad89 100644
--- a/tests/ui/async-await/issues/issue-78654.full.stderr
+++ b/tests/ui/async-await/issues/issue-78654.full.stderr
@@ -10,6 +10,7 @@ error[E0207]: the const parameter `H` is not constrained by the impl trait, self
 LL | impl<const H: feature> Foo {
    |      ^^^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `H`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/async-await/issues/issue-78654.min.stderr b/tests/ui/async-await/issues/issue-78654.min.stderr
index 0d12a948c68bd..a7c3f8e79ad89 100644
--- a/tests/ui/async-await/issues/issue-78654.min.stderr
+++ b/tests/ui/async-await/issues/issue-78654.min.stderr
@@ -10,6 +10,7 @@ error[E0207]: the const parameter `H` is not constrained by the impl trait, self
 LL | impl<const H: feature> Foo {
    |      ^^^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `H`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr b/tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr
index afad3388145c7..8366f956546d3 100644
--- a/tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr
+++ b/tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr
@@ -52,6 +52,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}> {
    |             ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `N`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/const-generics/issues/issue-68366.full.stderr b/tests/ui/const-generics/issues/issue-68366.full.stderr
index 3363a895e4725..c65f0361bea08 100644
--- a/tests/ui/const-generics/issues/issue-68366.full.stderr
+++ b/tests/ui/const-generics/issues/issue-68366.full.stderr
@@ -16,6 +16,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl <const N: usize> Collatz<{Some(N)}> {}
    |       ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `N`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -25,6 +26,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl<const N: usize> Foo {}
    |      ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `N`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/const-generics/issues/issue-68366.min.stderr b/tests/ui/const-generics/issues/issue-68366.min.stderr
index 276f91e76dd42..4a3f5106a2bad 100644
--- a/tests/ui/const-generics/issues/issue-68366.min.stderr
+++ b/tests/ui/const-generics/issues/issue-68366.min.stderr
@@ -25,6 +25,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl <const N: usize> Collatz<{Some(N)}> {}
    |       ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `N`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -34,6 +35,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl<const N: usize> Foo {}
    |      ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: to constrain `N`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/consts/rustc-impl-const-stability.stderr b/tests/ui/consts/rustc-impl-const-stability.stderr
index ba8e6c1555ce2..ce5e1f977a7bb 100644
--- a/tests/ui/consts/rustc-impl-const-stability.stderr
+++ b/tests/ui/consts/rustc-impl-const-stability.stderr
@@ -13,6 +13,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const Default for Data {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/error-codes/E0207.stderr b/tests/ui/error-codes/E0207.stderr
index 01d7c41854412..493b7695e7672 100644
--- a/tests/ui/error-codes/E0207.stderr
+++ b/tests/ui/error-codes/E0207.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: Default> Foo {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/generic-associated-types/bugs/issue-87735.stderr b/tests/ui/generic-associated-types/bugs/issue-87735.stderr
index d80050652389d..4619109793c60 100644
--- a/tests/ui/generic-associated-types/bugs/issue-87735.stderr
+++ b/tests/ui/generic-associated-types/bugs/issue-87735.stderr
@@ -3,6 +3,9 @@ error[E0207]: the type parameter `U` is not constrained by the impl trait, self
    |
 LL | impl<'b, T, U> AsRef2 for Foo<T>
    |             ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `U` is bound to an associated type, but the reference to the associated type itself uses unconstrained generic parameters
 
 error[E0309]: the parameter type `U` may not live long enough
   --> $DIR/issue-87735.rs:34:21
diff --git a/tests/ui/generic-associated-types/bugs/issue-88526.stderr b/tests/ui/generic-associated-types/bugs/issue-88526.stderr
index 5da3e3ff64ab9..01ed5928243b6 100644
--- a/tests/ui/generic-associated-types/bugs/issue-88526.stderr
+++ b/tests/ui/generic-associated-types/bugs/issue-88526.stderr
@@ -3,6 +3,9 @@ error[E0207]: the type parameter `I` is not constrained by the impl trait, self
    |
 LL | impl<'q, Q, I, F> A for TestB<Q, F>
    |             ^ unconstrained type parameter
+   |
+   = note: to constrain `I`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `I` is bound to an associated type, but the reference to the associated type itself uses unconstrained generic parameters
 
 error[E0309]: the parameter type `F` may not live long enough
   --> $DIR/issue-88526.rs:16:18
diff --git a/tests/ui/generic-associated-types/gat-trait-path-generic-type-arg.stderr b/tests/ui/generic-associated-types/gat-trait-path-generic-type-arg.stderr
index cb2b9f32bfe72..f274fc34eac42 100644
--- a/tests/ui/generic-associated-types/gat-trait-path-generic-type-arg.stderr
+++ b/tests/ui/generic-associated-types/gat-trait-path-generic-type-arg.stderr
@@ -18,6 +18,8 @@ error[E0207]: the type parameter `T1` is not constrained by the impl trait, self
    |
 LL | impl <T, T1> Foo for T {
    |          ^^ unconstrained type parameter
+   |
+   = note: to constrain `T1`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/impl-trait/in-trait/unconstrained-lt.stderr b/tests/ui/impl-trait/in-trait/unconstrained-lt.stderr
index 4c5a42c0b4b47..24d00663a2090 100644
--- a/tests/ui/impl-trait/in-trait/unconstrained-lt.stderr
+++ b/tests/ui/impl-trait/in-trait/unconstrained-lt.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, T> Foo for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/impl-trait/issues/issue-87340.stderr b/tests/ui/impl-trait/issues/issue-87340.stderr
index 1be4087be4242..aad8cd8509a0b 100644
--- a/tests/ui/impl-trait/issues/issue-87340.stderr
+++ b/tests/ui/impl-trait/issues/issue-87340.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> X for () {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0282]: type annotations needed
   --> $DIR/issue-87340.rs:11:23
diff --git a/tests/ui/impl-unused-rps-in-assoc-type.stderr b/tests/ui/impl-unused-rps-in-assoc-type.stderr
index ef61fa4be4830..beb077dbdec4f 100644
--- a/tests/ui/impl-unused-rps-in-assoc-type.stderr
+++ b/tests/ui/impl-unused-rps-in-assoc-type.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Fun for Holder {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/impl-unused-tps-inherent.stderr b/tests/ui/impl-unused-tps-inherent.stderr
index 43f63cf968cf5..d24d4ca1a054e 100644
--- a/tests/ui/impl-unused-tps-inherent.stderr
+++ b/tests/ui/impl-unused-tps-inherent.stderr
@@ -3,12 +3,16 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> MyType {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps-inherent.rs:17:8
    |
 LL | impl<T,U> MyType1<T> {
    |        ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the self type, or in an equality with an associated type
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/impl-unused-tps.stderr b/tests/ui/impl-unused-tps.stderr
index af427cb5f3e3c..0c9140c6caa4e 100644
--- a/tests/ui/impl-unused-tps.stderr
+++ b/tests/ui/impl-unused-tps.stderr
@@ -25,30 +25,42 @@ error[E0207]: the type parameter `U` is not constrained by the impl trait, self
    |
 LL | impl<T,U> Foo<T> for [isize;1] {
    |        ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:31:8
    |
 LL | impl<T,U> Bar for T {
    |        ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:39:8
    |
 LL | impl<T,U> Bar for T
    |        ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `U` is bound to an associated type, but the associated type is circularly defined in this impl
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:47:8
    |
 LL | impl<T,U,V> Foo<T> for T
    |        ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0207]: the type parameter `V` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:47:10
    |
 LL | impl<T,U,V> Foo<T> for T
    |          ^ unconstrained type parameter
+   |
+   = note: to constrain `V`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `V` is bound to an associated type, but the reference to the associated type itself uses unconstrained generic parameters
 
 error: aborting due to 7 previous errors
 
diff --git a/tests/ui/issues/issue-16562.stderr b/tests/ui/issues/issue-16562.stderr
index efbd7f712a45a..07c1f58e6be2f 100644
--- a/tests/ui/issues/issue-16562.stderr
+++ b/tests/ui/issues/issue-16562.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T, M: MatrixShape> Collection for Col<M, usize> {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/issues/issue-22886.stderr b/tests/ui/issues/issue-22886.stderr
index a04fa677f9ec5..de6f0db1c7c83 100644
--- a/tests/ui/issues/issue-22886.stderr
+++ b/tests/ui/issues/issue-22886.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Iterator for Newtype {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/issues/issue-29861.stderr b/tests/ui/issues/issue-29861.stderr
index a25cbf0515d84..1011caedf752f 100644
--- a/tests/ui/issues/issue-29861.stderr
+++ b/tests/ui/issues/issue-29861.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, T: 'a> MakeRef2 for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/issue-29861.rs:16:43
diff --git a/tests/ui/issues/issue-35139.stderr b/tests/ui/issues/issue-35139.stderr
index 875af70483224..54e2fb2ff28b5 100644
--- a/tests/ui/issues/issue-35139.stderr
+++ b/tests/ui/issues/issue-35139.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> MethodType for MTFn {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/lazy-type-alias/unconstrained-params-in-impl-due-to-overflow.stderr b/tests/ui/lazy-type-alias/unconstrained-params-in-impl-due-to-overflow.stderr
index bcffa02ddd424..45ec6d29096c4 100644
--- a/tests/ui/lazy-type-alias/unconstrained-params-in-impl-due-to-overflow.stderr
+++ b/tests/ui/lazy-type-alias/unconstrained-params-in-impl-due-to-overflow.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> Loop<T> {}
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
 
 error[E0275]: overflow normalizing the type alias `Loop<T>`
   --> $DIR/unconstrained-params-in-impl-due-to-overflow.rs:6:16
diff --git a/tests/ui/lazy-type-alias/unconstrained-params-in-impl.stderr b/tests/ui/lazy-type-alias/unconstrained-params-in-impl.stderr
index 2419c78cba8d9..d67d950224d9e 100644
--- a/tests/ui/lazy-type-alias/unconstrained-params-in-impl.stderr
+++ b/tests/ui/lazy-type-alias/unconstrained-params-in-impl.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> NotInjective<T> {}
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-impl-requires-const-trait.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-impl-requires-const-trait.stderr
index d20404e63b3fb..234c0ce26a8b1 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-impl-requires-const-trait.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-impl-requires-const-trait.stderr
@@ -16,6 +16,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const A for () {}
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-non-const-type.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-non-const-type.stderr
index 4b009446dbc00..61124f4047987 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-non-const-type.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-non-const-type.stderr
@@ -10,6 +10,7 @@ LL | #[derive_const(Default)]
 
 error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-use.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-use.stderr
index e45c1a1f46fb6..0780760079cae 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-use.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-use.stderr
@@ -19,6 +19,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const Default for A {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -34,6 +35,7 @@ LL | #[derive_const(Default, PartialEq)]
 
 error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/span-bug-issue-121418.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/span-bug-issue-121418.stderr
index 3ff1efb59883d..1dab189391736 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/span-bug-issue-121418.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/span-bug-issue-121418.stderr
@@ -14,6 +14,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const dyn T {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.stderr
index d18063f8d3d88..5408153e9c6ea 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.stderr
@@ -34,6 +34,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl<T> const Foo for T {}
    |         ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -43,6 +44,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl<T> const Foo for T where T: const Specialize {}
    |         ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-default-body-stability.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-default-body-stability.stderr
index 62c4bc3b7ae60..acd5eb066f286 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-default-body-stability.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/trait-default-body-stability.stderr
@@ -22,6 +22,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const Try for T {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -31,6 +32,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const FromResidual for T {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: to constrain `host`, use it in the implemented trait, in the self type, or in an equality with an associated type
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
diff --git a/tests/ui/specialization/min_specialization/ice-const-not-fully-resolved-113045.stderr b/tests/ui/specialization/min_specialization/ice-const-not-fully-resolved-113045.stderr
index acbdb9b0a308e..1c451178b3305 100644
--- a/tests/ui/specialization/min_specialization/ice-const-not-fully-resolved-113045.stderr
+++ b/tests/ui/specialization/min_specialization/ice-const-not-fully-resolved-113045.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `Unconstrained` is not constrained by the impl
    |
 LL | impl<'a, Unconstrained> X for [(); 0] {}
    |          ^^^^^^^^^^^^^ unconstrained type parameter
+   |
+   = note: to constrain `Unconstrained`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: specialization impl does not specialize any associated items
   --> $DIR/ice-const-not-fully-resolved-113045.rs:11:1
diff --git a/tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.stderr b/tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.stderr
index 06e2fa5d4d1fd..05ab8fbb6b348 100644
--- a/tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.stderr
+++ b/tests/ui/traits/associated_type_bound/116464-invalid-assoc-type-suggestion-in-trait-impl.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `S` is not constrained by the impl trait, self
    |
 LL | impl<T, S> Trait<T> for i32 {
    |         ^ unconstrained type parameter
+   |
+   = note: to constrain `S`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0107]: trait takes 1 generic argument but 2 generic arguments were supplied
   --> $DIR/116464-invalid-assoc-type-suggestion-in-trait-impl.rs:15:12
diff --git a/tests/ui/type-alias-impl-trait/assoc-type-lifetime-unconstrained.stderr b/tests/ui/type-alias-impl-trait/assoc-type-lifetime-unconstrained.stderr
index e6b94c525ff23..a7b75a5ac9452 100644
--- a/tests/ui/type-alias-impl-trait/assoc-type-lifetime-unconstrained.stderr
+++ b/tests/ui/type-alias-impl-trait/assoc-type-lifetime-unconstrained.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, I> UnwrapItemsExt for I {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0792]: expected generic lifetime parameter, found `'_`
   --> $DIR/assoc-type-lifetime-unconstrained.rs:22:9
diff --git a/tests/ui/type-alias-impl-trait/ice-failed-to-resolve-instance-for-110696.stderr b/tests/ui/type-alias-impl-trait/ice-failed-to-resolve-instance-for-110696.stderr
index a39f77ce17f61..de8dbcd0e5000 100644
--- a/tests/ui/type-alias-impl-trait/ice-failed-to-resolve-instance-for-110696.stderr
+++ b/tests/ui/type-alias-impl-trait/ice-failed-to-resolve-instance-for-110696.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: MyFrom<Phantom2<DummyT<U>>>, U> MyIndex<DummyT<T>> for Scope<U> {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/type-alias-impl-trait/impl-with-unconstrained-param.stderr b/tests/ui/type-alias-impl-trait/impl-with-unconstrained-param.stderr
index bb0e11d314c33..ac4bda0061b89 100644
--- a/tests/ui/type-alias-impl-trait/impl-with-unconstrained-param.stderr
+++ b/tests/ui/type-alias-impl-trait/impl-with-unconstrained-param.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> X for () {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0282]: type annotations needed
   --> $DIR/impl-with-unconstrained-param.rs:14:23
diff --git a/tests/ui/type-alias-impl-trait/issue-74244.stderr b/tests/ui/type-alias-impl-trait/issue-74244.stderr
index d2b50ffd86b59..def3b5bf77480 100644
--- a/tests/ui/type-alias-impl-trait/issue-74244.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-74244.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> Allocator for DefaultAllocator {
    |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0282]: type annotations needed
   --> $DIR/issue-74244.rs:16:13
diff --git a/tests/ui/type-alias-impl-trait/issue-74761-2.stderr b/tests/ui/type-alias-impl-trait/issue-74761-2.stderr
index 26babc29000c0..ef85b17751651 100644
--- a/tests/ui/type-alias-impl-trait/issue-74761-2.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-74761-2.stderr
@@ -3,12 +3,16 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, 'b> A for () {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0207]: the lifetime parameter `'b` is not constrained by the impl trait, self type, or predicates
   --> $DIR/issue-74761-2.rs:7:10
    |
 LL | impl<'a, 'b> A for () {
    |          ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'b`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0792]: expected generic lifetime parameter, found `'_`
   --> $DIR/issue-74761-2.rs:12:28
diff --git a/tests/ui/type-alias-impl-trait/issue-74761.stderr b/tests/ui/type-alias-impl-trait/issue-74761.stderr
index a4826c293467e..a29443ed325d9 100644
--- a/tests/ui/type-alias-impl-trait/issue-74761.stderr
+++ b/tests/ui/type-alias-impl-trait/issue-74761.stderr
@@ -3,12 +3,16 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, 'b> A for () {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0207]: the lifetime parameter `'b` is not constrained by the impl trait, self type, or predicates
   --> $DIR/issue-74761.rs:7:10
    |
 LL | impl<'a, 'b> A for () {
    |          ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'b`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0792]: expected generic lifetime parameter, found `'_`
   --> $DIR/issue-74761.rs:12:28
diff --git a/tests/ui/type-alias-impl-trait/type-alias-impl-trait-unconstrained-lifetime.stderr b/tests/ui/type-alias-impl-trait/type-alias-impl-trait-unconstrained-lifetime.stderr
index 5f9c56f1ca9d0..d4c1c4ed91da1 100644
--- a/tests/ui/type-alias-impl-trait/type-alias-impl-trait-unconstrained-lifetime.stderr
+++ b/tests/ui/type-alias-impl-trait/type-alias-impl-trait-unconstrained-lifetime.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, I: Iterator<Item = i32>> Trait for (i32, I) {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0792]: expected generic lifetime parameter, found `'_`
   --> $DIR/type-alias-impl-trait-unconstrained-lifetime.rs:14:9
diff --git a/tests/ui/type-alias-impl-trait/unconstrained-impl-param.stderr b/tests/ui/type-alias-impl-trait/unconstrained-impl-param.stderr
index 6206f169c5b9e..c191fe97aa693 100644
--- a/tests/ui/type-alias-impl-trait/unconstrained-impl-param.stderr
+++ b/tests/ui/type-alias-impl-trait/unconstrained-impl-param.stderr
@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Trait for Opaque<&'a str> {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: to constrain `'a`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/typeck/issue-13853-5.stderr b/tests/ui/typeck/issue-13853-5.stderr
index 388d5ec746ce6..cab3ba9fe3741 100644
--- a/tests/ui/typeck/issue-13853-5.stderr
+++ b/tests/ui/typeck/issue-13853-5.stderr
@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<'a, T: Deserializable> Deserializable for &'a str {
    |          ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
 
 error[E0308]: mismatched types
   --> $DIR/issue-13853-5.rs:9:70
diff --git a/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.rs b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.rs
new file mode 100644
index 0000000000000..a8f2d02637703
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.rs
@@ -0,0 +1,17 @@
+trait Foo1 {}
+
+trait Foo2 {
+    type Bar;
+}
+
+impl<const N: usize> Foo1 for u32 where [u32; N]: Clone {}
+//~^ ERROR: the const parameter `N` is not constrained
+
+impl<T, const N: usize> Foo1 for String where T: Iterator<Item = [u32; N]> {}
+//~^ ERROR: the type parameter `T` is not constrained
+//~| ERROR: the const parameter `N` is not constrained
+
+impl<T, const N: usize> Foo2 for T where T: Foo2<Bar = [u32; N]> { type Bar = [u32; N]; }
+//~^ ERROR: the const parameter `N` is not constrained
+
+fn main() {}
diff --git a/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.stderr b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.stderr
new file mode 100644
index 0000000000000..a3545ba84e36e
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-const-generic.stderr
@@ -0,0 +1,43 @@
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-const-generic.rs:7:6
+   |
+LL | impl<const N: usize> Foo1 for u32 where [u32; N]: Clone {}
+   |      ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: to constrain `N`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-const-generic.rs:10:6
+   |
+LL | impl<T, const N: usize> Foo1 for String where T: Iterator<Item = [u32; N]> {}
+   |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
+
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-const-generic.rs:10:9
+   |
+LL | impl<T, const N: usize> Foo1 for String where T: Iterator<Item = [u32; N]> {}
+   |         ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: to constrain `N`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `N` is bound to an associated type, but the reference to the associated type itself uses unconstrained generic parameters
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-const-generic.rs:14:9
+   |
+LL | impl<T, const N: usize> Foo2 for T where T: Foo2<Bar = [u32; N]> { type Bar = [u32; N]; }
+   |         ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: to constrain `N`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `N` is bound to an associated type, but the associated type is circularly defined in this impl
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0207`.
diff --git a/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.rs b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.rs
new file mode 100644
index 0000000000000..3946a83f174b9
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.rs
@@ -0,0 +1,24 @@
+trait Foo1 {}
+
+trait Foo2 {
+    type Bar;
+}
+
+trait Foo3 {
+    type Bar;
+}
+
+impl<T> Foo1 for u32 where T: Clone {}
+//~^ ERROR: the type parameter `T` is not constrained
+
+impl<T, U> Foo1 for String where T: Iterator<Item = U> {}
+//~^ ERROR: the type parameter `T` is not constrained
+//~| ERROR: the type parameter `U` is not constrained
+
+impl<T, U> Foo2 for T where T: Foo2<Bar = U> { type Bar = U; }
+//~^ ERROR: the type parameter `U` is not constrained
+
+impl<T, U> Foo3 for T where T: Foo3<Bar = Option<U>> { type Bar = Option<U>; }
+//~^ ERROR: the type parameter `U` is not constrained
+
+fn main() {}
diff --git a/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.stderr b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.stderr
new file mode 100644
index 0000000000000..1467e479853b2
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/mentioned-but-unconstrained-type.stderr
@@ -0,0 +1,46 @@
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-type.rs:11:6
+   |
+LL | impl<T> Foo1 for u32 where T: Clone {}
+   |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
+
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-type.rs:14:6
+   |
+LL | impl<T, U> Foo1 for String where T: Iterator<Item = U> {}
+   |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
+
+error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-type.rs:14:9
+   |
+LL | impl<T, U> Foo1 for String where T: Iterator<Item = U> {}
+   |         ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `U` is bound to an associated type, but the reference to the associated type itself uses unconstrained generic parameters
+
+error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-type.rs:18:9
+   |
+LL | impl<T, U> Foo2 for T where T: Foo2<Bar = U> { type Bar = U; }
+   |         ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `U` is bound to an associated type, but the associated type is circularly defined in this impl
+
+error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/mentioned-but-unconstrained-type.rs:21:9
+   |
+LL | impl<T, U> Foo3 for T where T: Foo3<Bar = Option<U>> { type Bar = Option<U>; }
+   |         ^ unconstrained type parameter
+   |
+   = note: to constrain `U`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: `U` is bound to an associated type, but the associated type is circularly defined in this impl
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0207`.
diff --git a/tests/ui/unconstrained-generic-parameters/unused-const-generic.rs b/tests/ui/unconstrained-generic-parameters/unused-const-generic.rs
new file mode 100644
index 0000000000000..e6acb79999d1a
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/unused-const-generic.rs
@@ -0,0 +1,14 @@
+struct Foo;
+
+impl<const N: usize> Foo {}
+//~^ ERROR: the const parameter `N` is not constrained
+
+impl<const N: usize> Default for Foo {
+    //~^ ERROR: the const parameter `N` is not constrained
+    fn default() -> Self { Foo }
+}
+
+// This one isn't an error
+fn foo<const N: usize>() {}
+
+fn main() {}
diff --git a/tests/ui/unconstrained-generic-parameters/unused-const-generic.stderr b/tests/ui/unconstrained-generic-parameters/unused-const-generic.stderr
new file mode 100644
index 0000000000000..95dd7b7d566f9
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/unused-const-generic.stderr
@@ -0,0 +1,23 @@
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unused-const-generic.rs:3:6
+   |
+LL | impl<const N: usize> Foo {}
+   |      ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: to constrain `N`, use it in the self type, or in an equality with an associated type
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unused-const-generic.rs:6:6
+   |
+LL | impl<const N: usize> Default for Foo {
+   |      ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: to constrain `N`, use it in the implemented trait, in the self type, or in an equality with an associated type
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0207`.
diff --git a/tests/ui/unconstrained-generic-parameters/unused-type-generic.rs b/tests/ui/unconstrained-generic-parameters/unused-type-generic.rs
new file mode 100644
index 0000000000000..ba64d4e5b6ccf
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/unused-type-generic.rs
@@ -0,0 +1,14 @@
+struct Foo;
+
+impl<T> Foo {}
+//~^ ERROR: the type parameter `T` is not constrained
+
+impl<T> Default for Foo {
+    //~^ ERROR: the type parameter `T` is not constrained
+    fn default() -> Self { Foo }
+}
+
+// This one isn't an error
+fn foo<T>() {}
+
+fn main() {}
diff --git a/tests/ui/unconstrained-generic-parameters/unused-type-generic.stderr b/tests/ui/unconstrained-generic-parameters/unused-type-generic.stderr
new file mode 100644
index 0000000000000..9491e923e2a78
--- /dev/null
+++ b/tests/ui/unconstrained-generic-parameters/unused-type-generic.stderr
@@ -0,0 +1,19 @@
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unused-type-generic.rs:3:6
+   |
+LL | impl<T> Foo {}
+   |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the self type, or in an equality with an associated type
+
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unused-type-generic.rs:6:6
+   |
+LL | impl<T> Default for Foo {
+   |      ^ unconstrained type parameter
+   |
+   = note: to constrain `T`, use it in the implemented trait, in the self type, or in an equality with an associated type
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0207`.