Implement refinement lint for RPITIT

26 files changed, 483 insertions, 21 deletions

diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 55c4ec66cd9..31547f19be0 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -44,7 +44,7 @@ use rustc_fluent_macro::fluent_messages;
 pub use rustc_lint_defs::{pluralize, Applicability};
 use rustc_span::source_map::SourceMap;
 pub use rustc_span::ErrorGuaranteed;
-use rustc_span::{Loc, Span};
+use rustc_span::{Loc, Span, DUMMY_SP};
 
 use std::borrow::Cow;
 use std::error::Report;
@@ -1754,7 +1754,7 @@ impl DelayedDiagnostic {
             BacktraceStatus::Captured => {
                 let inner = &self.inner;
                 self.inner.subdiagnostic(DelayedAtWithNewline {
-                    span: inner.span.primary_span().unwrap(),
+                    span: inner.span.primary_span().unwrap_or(DUMMY_SP),
                     emitted_at: inner.emitted_at.clone(),
                     note: self.note,
                 });
@@ -1764,7 +1764,7 @@ impl DelayedDiagnostic {
             _ => {
                 let inner = &self.inner;
                 self.inner.subdiagnostic(DelayedAtWithoutNewline {
-                    span: inner.span.primary_span().unwrap(),
+                    span: inner.span.primary_span().unwrap_or(DUMMY_SP),
                     emitted_at: inner.emitted_at.clone(),
                     note: self.note,
                 });
diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index 597cae6ff33..6a08fc528d7 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -222,6 +222,11 @@ hir_analysis_return_type_notation_on_non_rpitit =
     .note = function returns `{$ty}`, which is not compatible with associated type return bounds
     .label = this function must be `async` or return `impl Trait`
 
+hir_analysis_rpitit_refined = impl trait in impl method signature does not match trait method signature
+    .suggestion = replace the return type so that it matches the trait
+    .label = return type from trait method defined here
+    .unmatched_bound_label = this bound is stronger than that defined on the trait
+
 hir_analysis_self_in_impl_self =
     `Self` is not valid in the self type of an impl block
     .note = replace `Self` with a different type
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index bd0ab6463f0..92cc9759304 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -28,6 +28,8 @@ use rustc_trait_selection::traits::{
 use std::borrow::Cow;
 use std::iter;
 
+mod refine;
+
 /// Checks that a method from an impl conforms to the signature of
 /// the same method as declared in the trait.
 ///
@@ -53,6 +55,12 @@ pub(super) fn compare_impl_method<'tcx>(
             impl_trait_ref,
             CheckImpliedWfMode::Check,
         )?;
+        refine::check_refining_return_position_impl_trait_in_trait(
+            tcx,
+            impl_m,
+            trait_m,
+            impl_trait_ref,
+        );
     };
 }
 
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs
new file mode 100644
index 00000000000..bc80cd62057
--- /dev/null
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs
@@ -0,0 +1,271 @@
+use rustc_data_structures::fx::FxIndexSet;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::{outlives::env::OutlivesEnvironment, TyCtxtInferExt};
+use rustc_lint_defs::builtin::REFINING_IMPL_TRAIT;
+use rustc_middle::traits::{ObligationCause, Reveal};
+use rustc_middle::ty::{
+    self, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitor,
+};
+use rustc_span::{Span, DUMMY_SP};
+use rustc_trait_selection::traits::{
+    elaborate, normalize_param_env_or_error, outlives_bounds::InferCtxtExt, ObligationCtxt,
+};
+use std::ops::ControlFlow;
+
+/// Check that an implementation does not refine an RPITIT from a trait method signature.
+pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    impl_m: ty::AssocItem,
+    trait_m: ty::AssocItem,
+    impl_trait_ref: ty::TraitRef<'tcx>,
+) {
+    if !tcx.impl_method_has_trait_impl_trait_tys(impl_m.def_id) {
+        return;
+    }
+
+    let impl_def_id = impl_m.container_id(tcx);
+    let impl_m_args = ty::GenericArgs::identity_for_item(tcx, impl_m.def_id);
+    let trait_m_to_impl_m_args = impl_m_args.rebase_onto(tcx, impl_def_id, impl_trait_ref.args);
+    let bound_trait_m_sig = tcx.fn_sig(trait_m.def_id).instantiate(tcx, trait_m_to_impl_m_args);
+    let trait_m_sig = tcx.liberate_late_bound_regions(impl_m.def_id, bound_trait_m_sig);
+
+    let Ok(hidden_tys) = tcx.collect_return_position_impl_trait_in_trait_tys(impl_m.def_id) else {
+        // Error already emitted, no need to delay another.
+        return;
+    };
+
+    let mut collector = ImplTraitInTraitCollector { tcx, types: FxIndexSet::default() };
+    trait_m_sig.visit_with(&mut collector);
+
+    // Bound that we find on RPITITs in the trait signature.
+    let mut trait_bounds = vec![];
+    // Bounds that we find on the RPITITs in the impl signature.
+    let mut impl_bounds = vec![];
+
+    for trait_projection in collector.types.into_iter().rev() {
+        let impl_opaque_args = trait_projection.args.rebase_onto(tcx, trait_m.def_id, impl_m_args);
+        let hidden_ty = hidden_tys[&trait_projection.def_id].instantiate(tcx, impl_opaque_args);
+
+        // If the hidden type is not an opaque, then we have "refined" the trait signature.
+        let ty::Alias(ty::Opaque, impl_opaque) = *hidden_ty.kind() else {
+            report_mismatched_rpitit_signature(
+                tcx,
+                trait_m_sig,
+                trait_m.def_id,
+                impl_m.def_id,
+                None,
+            );
+            return;
+        };
+
+        // This opaque also needs to be from the impl method -- otherwise,
+        // it's a refinement to a TAIT.
+        if !tcx.hir().get_if_local(impl_opaque.def_id).map_or(false, |node| {
+            matches!(
+                node.expect_item().expect_opaque_ty().origin,
+                hir::OpaqueTyOrigin::AsyncFn(def_id)  | hir::OpaqueTyOrigin::FnReturn(def_id)
+                    if def_id == impl_m.def_id.expect_local()
+            )
+        }) {
+            report_mismatched_rpitit_signature(
+                tcx,
+                trait_m_sig,
+                trait_m.def_id,
+                impl_m.def_id,
+                None,
+            );
+            return;
+        }
+
+        trait_bounds.extend(
+            tcx.item_bounds(trait_projection.def_id).iter_instantiated(tcx, trait_projection.args),
+        );
+        impl_bounds.extend(elaborate(
+            tcx,
+            tcx.explicit_item_bounds(impl_opaque.def_id)
+                .iter_instantiated_copied(tcx, impl_opaque.args),
+        ));
+    }
+
+    let hybrid_preds = tcx
+        .predicates_of(impl_def_id)
+        .instantiate_identity(tcx)
+        .into_iter()
+        .chain(tcx.predicates_of(trait_m.def_id).instantiate_own(tcx, trait_m_to_impl_m_args))
+        .map(|(clause, _)| clause);
+    let param_env = ty::ParamEnv::new(tcx.mk_clauses_from_iter(hybrid_preds), Reveal::UserFacing);
+    let param_env = normalize_param_env_or_error(tcx, param_env, ObligationCause::dummy());
+
+    let ref infcx = tcx.infer_ctxt().build();
+    let ocx = ObligationCtxt::new(infcx);
+
+    // Normalize the bounds. This has two purposes:
+    //
+    // 1. Project the RPITIT projections from the trait to the opaques on the impl,
+    //    which means that they don't need to be mapped manually.
+    //
+    // 2. Project any other projections that show up in the bound. That makes sure that
+    //    we don't consider `tests/ui/async-await/in-trait/async-associated-types.rs`
+    //    to be refining.
+    let (trait_bounds, impl_bounds) =
+        ocx.normalize(&ObligationCause::dummy(), param_env, (trait_bounds, impl_bounds));
+
+    // Since we've normalized things, we need to resolve regions, since we'll
+    // possibly have introduced region vars during projection. We don't expect
+    // this resolution to have incurred any region errors -- but if we do, then
+    // just delay a bug.
+    let mut implied_wf_types = FxIndexSet::default();
+    implied_wf_types.extend(trait_m_sig.inputs_and_output);
+    implied_wf_types.extend(ocx.normalize(
+        &ObligationCause::dummy(),
+        param_env,
+        trait_m_sig.inputs_and_output,
+    ));
+    if !ocx.select_all_or_error().is_empty() {
+        tcx.sess.delay_span_bug(
+            DUMMY_SP,
+            "encountered errors when checking RPITIT refinement (selection)",
+        );
+        return;
+    }
+    let outlives_env = OutlivesEnvironment::with_bounds(
+        param_env,
+        infcx.implied_bounds_tys(param_env, impl_m.def_id.expect_local(), implied_wf_types),
+    );
+    let errors = infcx.resolve_regions(&outlives_env);
+    if !errors.is_empty() {
+        tcx.sess.delay_span_bug(
+            DUMMY_SP,
+            "encountered errors when checking RPITIT refinement (regions)",
+        );
+        return;
+    }
+    // Resolve any lifetime variables that may have been introduced during normalization.
+    let Ok((trait_bounds, impl_bounds)) = infcx.fully_resolve((trait_bounds, impl_bounds)) else {
+        tcx.sess.delay_span_bug(
+            DUMMY_SP,
+            "encountered errors when checking RPITIT refinement (resolution)",
+        );
+        return;
+    };
+
+    // For quicker lookup, use an `IndexSet`
+    // (we don't use one earlier because it's not foldable..)
+    let trait_bounds = FxIndexSet::from_iter(trait_bounds);
+
+    // Find any clauses that are present in the impl's RPITITs that are not
+    // present in the trait's RPITITs. This will trigger on trivial predicates,
+    // too, since we *do not* use the trait solver to prove that the RPITIT's
+    // bounds are not stronger -- we're doing a simple, syntactic compatibility
+    // check between bounds. This is strictly forwards compatible, though.
+    for (clause, span) in impl_bounds {
+        if !trait_bounds.contains(&clause) {
+            report_mismatched_rpitit_signature(
+                tcx,
+                trait_m_sig,
+                trait_m.def_id,
+                impl_m.def_id,
+                Some(span),
+            );
+            return;
+        }
+    }
+}
+
+struct ImplTraitInTraitCollector<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    types: FxIndexSet<ty::AliasTy<'tcx>>,
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInTraitCollector<'tcx> {
+    type BreakTy = !;
+
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> std::ops::ControlFlow<Self::BreakTy> {
+        if let ty::Alias(ty::Projection, proj) = *ty.kind()
+            && self.tcx.is_impl_trait_in_trait(proj.def_id)
+        {
+            if self.types.insert(proj) {
+                for (pred, _) in self
+                    .tcx
+                    .explicit_item_bounds(proj.def_id)
+                    .iter_instantiated_copied(self.tcx, proj.args)
+                {
+                    pred.visit_with(self)?;
+                }
+            }
+            ControlFlow::Continue(())
+        } else {
+            ty.super_visit_with(self)
+        }
+    }
+}
+
+fn report_mismatched_rpitit_signature<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    trait_m_sig: ty::FnSig<'tcx>,
+    trait_m_def_id: DefId,
+    impl_m_def_id: DefId,
+    unmatched_bound: Option<Span>,
+) {
+    let mapping = std::iter::zip(
+        tcx.fn_sig(trait_m_def_id).skip_binder().bound_vars(),
+        tcx.fn_sig(impl_m_def_id).skip_binder().bound_vars(),
+    )
+    .filter_map(|(impl_bv, trait_bv)| {
+        if let ty::BoundVariableKind::Region(impl_bv) = impl_bv
+            && let ty::BoundVariableKind::Region(trait_bv) = trait_bv
+        {
+            Some((impl_bv, trait_bv))
+        } else {
+            None
+        }
+    })
+    .collect();
+
+    let mut return_ty =
+        trait_m_sig.output().fold_with(&mut super::RemapLateBound { tcx, mapping: &mapping });
+
+    if tcx.asyncness(impl_m_def_id).is_async() && tcx.asyncness(trait_m_def_id).is_async() {
+        let ty::Alias(ty::Projection, future_ty) = return_ty.kind() else {
+            bug!();
+        };
+        let Some(future_output_ty) = tcx
+            .explicit_item_bounds(future_ty.def_id)
+            .iter_instantiated_copied(tcx, future_ty.args)
+            .find_map(|(clause, _)| match clause.kind().no_bound_vars()? {
+                ty::ClauseKind::Projection(proj) => proj.term.ty(),
+                _ => None,
+            })
+        else {
+            bug!()
+        };
+        return_ty = future_output_ty;
+    }
+
+    let (span, impl_return_span, pre, post) =
+        match tcx.hir().get_by_def_id(impl_m_def_id.expect_local()).fn_decl().unwrap().output {
+            hir::FnRetTy::DefaultReturn(span) => (tcx.def_span(impl_m_def_id), span, "-> ", " "),
+            hir::FnRetTy::Return(ty) => (ty.span, ty.span, "", ""),
+        };
+    let trait_return_span =
+        tcx.hir().get_if_local(trait_m_def_id).map(|node| match node.fn_decl().unwrap().output {
+            hir::FnRetTy::DefaultReturn(_) => tcx.def_span(trait_m_def_id),
+            hir::FnRetTy::Return(ty) => ty.span,
+        });
+
+    let span = unmatched_bound.unwrap_or(span);
+    tcx.emit_spanned_lint(
+        REFINING_IMPL_TRAIT,
+        tcx.local_def_id_to_hir_id(impl_m_def_id.expect_local()),
+        span,
+        crate::errors::ReturnPositionImplTraitInTraitRefined {
+            impl_return_span,
+            trait_return_span,
+            pre,
+            post,
+            return_ty,
+            unmatched_bound,
+        },
+    );
+}
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index 9471ad9ca90..a1b306edd64 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -919,6 +919,21 @@ pub struct UnusedAssociatedTypeBounds {
     pub span: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(hir_analysis_rpitit_refined)]
+pub(crate) struct ReturnPositionImplTraitInTraitRefined<'tcx> {
+    #[suggestion(applicability = "maybe-incorrect", code = "{pre}{return_ty}{post}")]
+    pub impl_return_span: Span,
+    #[label]
+    pub trait_return_span: Option<Span>,
+    #[label(hir_analysis_unmatched_bound_label)]
+    pub unmatched_bound: Option<Span>,
+
+    pub pre: &'static str,
+    pub post: &'static str,
+    pub return_ty: Ty<'tcx>,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_assoc_bound_on_const)]
 #[note]
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 394b6aa1a06..df0151dc499 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -3381,6 +3381,7 @@ declare_lint_pass! {
         PROC_MACRO_BACK_COMPAT,
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
+        REFINING_IMPL_TRAIT,
         RENAMED_AND_REMOVED_LINTS,
         REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
         RUST_2021_INCOMPATIBLE_CLOSURE_CAPTURES,
@@ -4448,7 +4449,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail
-    ///
     /// #![deny(ambiguous_glob_imports)]
     /// pub fn foo() -> u32 {
     ///     use sub::*;
@@ -4485,6 +4485,50 @@ declare_lint! {
 }
 
 declare_lint! {
+    /// The `refining_impl_trait` lint detects usages of return-position impl
+    /// traits in trait signatures which are refined by implementations.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(return_position_impl_trait_in_trait)]
+    /// #![deny(refining_impl_trait)]
+    ///
+    /// use std::fmt::Display;
+    ///
+    /// trait AsDisplay {
+    ///     fn as_display(&self) -> impl Display;
+    /// }
+    ///
+    /// impl<'s> AsDisplay for &'s str {
+    ///     fn as_display(&self) -> Self {
+    ///         *self
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // users can observe that the return type of
+    ///     // `<&str as AsDisplay>::as_display()` is `&str`.
+    ///     let x: &str = "".as_display();
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Return-position impl trait in traits (RPITITs) desugar to associated types,
+    /// and callers of methods for types where the implementation is known are
+    /// able to observe the types written in the impl signature. This may be
+    /// intended behavior, but may also pose a semver hazard for authors of libraries
+    /// who do not wish to make stronger guarantees about the types than what is
+    /// written in the trait signature.
+    pub REFINING_IMPL_TRAIT,
+    Warn,
+    "impl trait in impl method signature does not match trait method signature",
+}
+
+declare_lint! {
     /// The `elided_lifetimes_in_associated_constant` lint detects elided lifetimes
     /// that were erroneously allowed in associated constants.
     ///
diff --git a/compiler/rustc_middle/src/ty/generic_args.rs b/compiler/rustc_middle/src/ty/generic_args.rs
index 97dab5cb47e..53a3d5fc6be 100644
--- a/compiler/rustc_middle/src/ty/generic_args.rs
+++ b/compiler/rustc_middle/src/ty/generic_args.rs
@@ -440,7 +440,7 @@ impl<'tcx> GenericArgs<'tcx> {
         target_args: GenericArgsRef<'tcx>,
     ) -> GenericArgsRef<'tcx> {
         let defs = tcx.generics_of(source_ancestor);
-        tcx.mk_args_from_iter(target_args.iter().chain(self.iter().skip(defs.params.len())))
+        tcx.mk_args_from_iter(target_args.iter().chain(self.iter().skip(defs.count())))
     }
 
     pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> GenericArgsRef<'tcx> {
diff --git a/tests/ui/async-await/in-trait/async-example-desugared-extra.rs b/tests/ui/async-await/in-trait/async-example-desugared-extra.rs
index 81e1e59a362..1bbf4836a2a 100644
--- a/tests/ui/async-await/in-trait/async-example-desugared-extra.rs
+++ b/tests/ui/async-await/in-trait/async-example-desugared-extra.rs
@@ -27,8 +27,7 @@ impl Future for MyFuture {
 }
 
 impl MyTrait for i32 {
-    // FIXME: this should eventually require `#[refine]` to compile, because it also provides
-    // `Clone`.
+    #[allow(refining_impl_trait)]
     fn foo(&self) -> impl Future<Output = i32> + Clone {
         MyFuture(*self)
     }
diff --git a/tests/ui/async-await/in-trait/async-example-desugared.rs b/tests/ui/async-await/in-trait/async-example-desugared.rs
index fb92ec78674..0a5023176fe 100644
--- a/tests/ui/async-await/in-trait/async-example-desugared.rs
+++ b/tests/ui/async-await/in-trait/async-example-desugared.rs
@@ -12,7 +12,7 @@ trait MyTrait {
 }
 
 impl MyTrait for i32 {
-    fn foo(&self) -> impl Future<Output = i32> + '_ {
+    fn foo(&self) -> impl Future<Output = i32> {
         async { *self }
     }
 }
diff --git a/tests/ui/impl-trait/in-trait/auxiliary/rpitit.rs b/tests/ui/impl-trait/in-trait/auxiliary/rpitit.rs
index cfc2193f633..5bd5c48ca69 100644
--- a/tests/ui/impl-trait/in-trait/auxiliary/rpitit.rs
+++ b/tests/ui/impl-trait/in-trait/auxiliary/rpitit.rs
@@ -8,6 +8,7 @@ pub trait Foo {
 
 pub struct Foreign;
 impl Foo for Foreign {
+    #[allow(refining_impl_trait)]
     fn bar(self) -> &'static () {
         &()
     }
diff --git a/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.rs b/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.rs
index ff7ad4bf389..063adb7c378 100644
--- a/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.rs
+++ b/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.rs
@@ -17,6 +17,7 @@ impl<'a, I: 'a + Iterable> Iterable for &'a I {
     //~^ ERROR impl has stricter requirements than trait
 
     fn iter(&self) -> impl 'a + Iterator<Item = I::Item<'a>> {
+        //~^ WARN impl trait in impl method signature does not match trait method signature
         (*self).iter()
     }
 }
diff --git a/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr b/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr
index 106b8a7c804..8d08d5e521e 100644
--- a/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr
+++ b/tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr
@@ -12,6 +12,21 @@ help: copy the `where` clause predicates from the trait
 LL |     where Self: 'b;
    |     ~~~~~~~~~~~~~~
 
-error: aborting due to previous error
+warning: impl trait in impl method signature does not match trait method signature
+  --> $DIR/bad-item-bound-within-rpitit.rs:19:28
+   |
+LL |     fn iter(&self) -> impl '_ + Iterator<Item = Self::Item<'_>>;
+   |                       ----------------------------------------- return type from trait method defined here
+...
+LL |     fn iter(&self) -> impl 'a + Iterator<Item = I::Item<'a>> {
+   |                            ^^ this bound is stronger than that defined on the trait
+   |
+   = note: `#[warn(refining_impl_trait)]` on by default
+help: replace the return type so that it matches the trait
+   |
+LL |     fn iter(&self) -> impl Iterator<Item = <&'a I as Iterable>::Item<'_>> + '_ {
+   |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0276`.
diff --git a/tests/ui/impl-trait/in-trait/deep-match-works.rs b/tests/ui/impl-trait/in-trait/deep-match-works.rs
index 78cff97c616..ffd10d9b1ff 100644
--- a/tests/ui/impl-trait/in-trait/deep-match-works.rs
+++ b/tests/ui/impl-trait/in-trait/deep-match-works.rs
@@ -10,6 +10,7 @@ trait Foo {
 }
 
 impl Foo for () {
+    #[allow(refining_impl_trait)]
     fn bar() -> Wrapper<i32> {
         Wrapper(0)
     }
diff --git a/tests/ui/impl-trait/in-trait/foreign.rs b/tests/ui/impl-trait/in-trait/foreign.rs
index b0c93a02935..035d91c1876 100644
--- a/tests/ui/impl-trait/in-trait/foreign.rs
+++ b/tests/ui/impl-trait/in-trait/foreign.rs
@@ -9,6 +9,7 @@ use std::sync::Arc;
 // Implement an RPITIT from another crate.
 struct Local;
 impl Foo for Local {
+    #[allow(refining_impl_trait)]
     fn bar(self) -> Arc<String> {
         Arc::new(String::new())
     }
diff --git a/tests/ui/impl-trait/in-trait/issue-102571.rs b/tests/ui/impl-trait/in-trait/issue-102571.rs
index 61c91e64417..ccb53031c44 100644
--- a/tests/ui/impl-trait/in-trait/issue-102571.rs
+++ b/tests/ui/impl-trait/in-trait/issue-102571.rs
@@ -8,14 +8,6 @@ trait Foo {
     fn bar(self) -> impl Deref<Target = impl Display + ?Sized>;
 }
 
-struct A;
-
-impl Foo for A {
-    fn bar(self) -> &'static str {
-        "Hello, world"
-    }
-}
-
 fn foo<T: Foo>(t: T) {
     let () = t.bar();
     //~^ ERROR mismatched types
diff --git a/tests/ui/impl-trait/in-trait/issue-102571.stderr b/tests/ui/impl-trait/in-trait/issue-102571.stderr
index 87219941d91..594b9ae9cd6 100644
--- a/tests/ui/impl-trait/in-trait/issue-102571.stderr
+++ b/tests/ui/impl-trait/in-trait/issue-102571.stderr
@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-102571.rs:20:9
+  --> $DIR/issue-102571.rs:12:9
    |
 LL |     let () = t.bar();
    |         ^^   ------- this expression has type `impl Deref<Target = impl std::fmt::Display + ?Sized>`
diff --git a/tests/ui/impl-trait/in-trait/nested-rpitit.rs b/tests/ui/impl-trait/in-trait/nested-rpitit.rs
index 65285e3a3cc..cb943eb5af9 100644
--- a/tests/ui/impl-trait/in-trait/nested-rpitit.rs
+++ b/tests/ui/impl-trait/in-trait/nested-rpitit.rs
@@ -13,6 +13,7 @@ trait Foo {
 struct A;
 
 impl Foo for A {
+    #[allow(refining_impl_trait)]
     fn bar(self) -> &'static str {
         "Hello, world"
     }
@@ -21,6 +22,7 @@ impl Foo for A {
 struct B;
 
 impl Foo for B {
+    #[allow(refining_impl_trait)]
     fn bar(self) -> Box<i32> {
         Box::new(42)
     }
diff --git a/tests/ui/impl-trait/in-trait/object-safety.rs b/tests/ui/impl-trait/in-trait/object-safety.rs
index 9a231e59b09..d1c9fba4e99 100644
--- a/tests/ui/impl-trait/in-trait/object-safety.rs
+++ b/tests/ui/impl-trait/in-trait/object-safety.rs
@@ -8,7 +8,7 @@ trait Foo {
 }
 
 impl Foo for u32 {
-    fn baz(&self) -> u32 {
+    fn baz(&self) -> impl Debug {
         32
     }
 }
diff --git a/tests/ui/impl-trait/in-trait/refine.rs b/tests/ui/impl-trait/in-trait/refine.rs
new file mode 100644
index 00000000000..e9b5e669fde
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/refine.rs
@@ -0,0 +1,36 @@
+#![feature(return_position_impl_trait_in_trait, async_fn_in_trait)]
+#![deny(refining_impl_trait)]
+
+trait Foo {
+    fn bar() -> impl Sized;
+}
+
+struct A;
+impl Foo for A {
+    fn bar() -> impl Copy {}
+    //~^ ERROR impl method signature does not match trait method signature
+}
+
+struct B;
+impl Foo for B {
+    fn bar() {}
+    //~^ ERROR impl method signature does not match trait method signature
+}
+
+struct C;
+impl Foo for C {
+    fn bar() -> () {}
+    //~^ ERROR impl method signature does not match trait method signature
+}
+
+trait Late {
+    fn bar<'a>(&'a self) -> impl Sized + 'a;
+}
+
+struct D;
+impl Late for D {
+    fn bar(&self) -> impl Copy + '_ {}
+    //~^ ERROR impl method signature does not match trait method signature
+}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/in-trait/refine.stderr b/tests/ui/impl-trait/in-trait/refine.stderr
new file mode 100644
index 00000000000..28d53270da3
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/refine.stderr
@@ -0,0 +1,63 @@
+error: impl trait in impl method signature does not match trait method signature
+  --> $DIR/refine.rs:10:22
+   |
+LL |     fn bar() -> impl Sized;
+   |                 ---------- return type from trait method defined here
+...
+LL |     fn bar() -> impl Copy {}
+   |                      ^^^^ this bound is stronger than that defined on the trait
+   |
+note: the lint level is defined here
+  --> $DIR/refine.rs:2:9
+   |
+LL | #![deny(refining_impl_trait)]
+   |         ^^^^^^^^^^^^^^^^^^^
+help: replace the return type so that it matches the trait
+   |
+LL |     fn bar() -> impl Sized {}
+   |                 ~~~~~~~~~~
+
+error: impl trait in impl method signature does not match trait method signature
+  --> $DIR/refine.rs:16:5
+   |
+LL |     fn bar() -> impl Sized;
+   |                 ---------- return type from trait method defined here
+...
+LL |     fn bar() {}
+   |     ^^^^^^^^
+   |
+help: replace the return type so that it matches the trait
+   |
+LL |     fn bar() -> impl Sized {}
+   |              +++++++++++++
+
+error: impl trait in impl method signature does not match trait method signature
+  --> $DIR/refine.rs:22:17
+   |
+LL |     fn bar() -> impl Sized;
+   |                 ---------- return type from trait method defined here
+...
+LL |     fn bar() -> () {}
+   |                 ^^
+   |
+help: replace the return type so that it matches the trait
+   |
+LL |     fn bar() -> impl Sized {}
+   |                 ~~~~~~~~~~
+
+error: impl trait in impl method signature does not match trait method signature
+  --> $DIR/refine.rs:32:27
+   |
+LL |     fn bar<'a>(&'a self) -> impl Sized + 'a;
+   |                             --------------- return type from trait method defined here
+...
+LL |     fn bar(&self) -> impl Copy + '_ {}
+   |                           ^^^^ this bound is stronger than that defined on the trait
+   |
+help: replace the return type so that it matches the trait
+   |
+LL |     fn bar(&self) -> impl Sized + '_ {}
+   |                      ~~~~~~~~~~~~~~~
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/impl-trait/in-trait/reveal.rs b/tests/ui/impl-trait/in-trait/reveal.rs
index d6ede1cc495..65425ba1a84 100644
--- a/tests/ui/impl-trait/in-trait/reveal.rs
+++ b/tests/ui/impl-trait/in-trait/reveal.rs
@@ -8,6 +8,7 @@ trait Foo {
 }
 
 impl Foo for () {
+    #[allow(refining_impl_trait)]
     fn f() -> Box<String> {
         Box::new(String::new())
     }
diff --git a/tests/ui/impl-trait/in-trait/rpitit-shadowed-by-missing-adt.rs b/tests/ui/impl-trait/in-trait/rpitit-shadowed-by-missing-adt.rs
index 7682884f879..747f7ed9e72 100644
--- a/tests/ui/impl-trait/in-trait/rpitit-shadowed-by-missing-adt.rs
+++ b/tests/ui/impl-trait/in-trait/rpitit-shadowed-by-missing-adt.rs
@@ -10,6 +10,7 @@ pub trait Tr {
 }
 
 impl Tr for () {
+    #[allow(refining_impl_trait)]
     fn w() -> &'static () {
         &()
     }
diff --git a/tests/ui/impl-trait/in-trait/signature-mismatch.failure.stderr b/tests/ui/impl-trait/in-trait/signature-mismatch.failure.stderr
index 186580f5756..468cf12f1bc 100644
--- a/tests/ui/impl-trait/in-trait/signature-mismatch.failure.stderr
+++ b/tests/ui/impl-trait/in-trait/signature-mismatch.failure.stderr
@@ -1,5 +1,5 @@
 error[E0623]: lifetime mismatch
-  --> $DIR/signature-mismatch.rs:77:10
+  --> $DIR/signature-mismatch.rs:79:10
    |
 LL |         &'a self,
    |         -------- this parameter and the return type are declared with different lifetimes...
diff --git a/tests/ui/impl-trait/in-trait/signature-mismatch.rs b/tests/ui/impl-trait/in-trait/signature-mismatch.rs
index c84a3b8f46b..135467d57ee 100644
--- a/tests/ui/impl-trait/in-trait/signature-mismatch.rs
+++ b/tests/ui/impl-trait/in-trait/signature-mismatch.rs
@@ -3,7 +3,6 @@
 //[success] check-pass
 
 #![feature(return_position_impl_trait_in_trait)]
-#![allow(incomplete_features)]
 
 use std::future::Future;
 
@@ -45,6 +44,7 @@ impl AsyncTrait for Struct {
     // Does not capture more lifetimes that trait def'n, since trait def'n
     // implicitly captures all in-scope lifetimes.
     #[cfg(success)]
+    #[allow(refining_impl_trait)]
     fn async_fn<'a>(&self, buff: &'a [u8]) -> impl Future<Output = Vec<u8>> + 'a {
         async move { buff.to_vec() }
     }
@@ -52,6 +52,7 @@ impl AsyncTrait for Struct {
     // Does not capture more lifetimes that trait def'n, since trait def'n
     // implicitly captures all in-scope lifetimes.
     #[cfg(success)]
+    #[allow(refining_impl_trait)]
     fn async_fn_early<'a: 'a>(&self, buff: &'a [u8]) -> impl Future<Output = Vec<u8>> + 'a {
         async move { buff.to_vec() }
     }
@@ -59,6 +60,7 @@ impl AsyncTrait for Struct {
     // Does not capture more lifetimes that trait def'n, since trait def'n
     // implicitly captures all in-scope lifetimes.
     #[cfg(success)]
+    #[allow(refining_impl_trait)]
     fn async_fn_multiple<'a, 'b>(
         &'a self,
         buff: &'b [u8],
diff --git a/tests/ui/impl-trait/in-trait/specialization-substs-remap.rs b/tests/ui/impl-trait/in-trait/specialization-substs-remap.rs
index c9ee877db8e..83812d6cd8a 100644
--- a/tests/ui/impl-trait/in-trait/specialization-substs-remap.rs
+++ b/tests/ui/impl-trait/in-trait/specialization-substs-remap.rs
@@ -12,6 +12,7 @@ impl<U> Foo for U
 where
     U: Copy,
 {
+    #[allow(refining_impl_trait)]
     fn bar(&self) -> U {
         *self
     }
diff --git a/tests/ui/impl-trait/in-trait/success.rs b/tests/ui/impl-trait/in-trait/success.rs
index 4cbe682b46f..f583604a021 100644
--- a/tests/ui/impl-trait/in-trait/success.rs
+++ b/tests/ui/impl-trait/in-trait/success.rs
@@ -10,12 +10,14 @@ trait Foo {
 }
 
 impl Foo for i32 {
+    #[allow(refining_impl_trait)]
     fn bar(&self) -> i32 {
         *self
     }
 }
 
 impl Foo for &'static str {
+    #[allow(refining_impl_trait)]
     fn bar(&self) -> &'static str {
         *self
     }
@@ -24,6 +26,7 @@ impl Foo for &'static str {
 struct Yay;
 
 impl Foo for Yay {
+    #[allow(refining_impl_trait)]
     fn bar(&self) -> String {
         String::from(":^)")
     }