adt_const_params: check referred-to types when checking impls of ConstParamTy on refs

compiler/rustc_hir_analysis/messages.ftl

@@ -78,9 +78,16 @@ hir_analysis_const_impl_for_non_const_trait =
     .note = marking a trait with `#[const_trait]` ensures all default method bodies are `const`
     .adding = adding a non-const method body in the future would be a breaking change
 
+hir_analysis_const_param_ty_impl_on_infringing_inner_ty =
+    the trait `ConstParamTy` cannot be implemented for this type
+    .label = this type does not implement `ConstParamTy`
+
+hir_analysis_const_param_ty_impl_on_infringing_inner_ty_sugg =
+    consider annotating with `#[derive(ConstParamTy)]`
+
 hir_analysis_const_param_ty_impl_on_non_adt =
     the trait `ConstParamTy` may not be implemented for this type
-    .label = type is not a structure or enumeration
+    .label = the type `{$ty}` is not a structure or enumeration
 
 hir_analysis_const_specialize = cannot specialize on const impl with non-const impl
 

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -939,6 +939,22 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
                     Ok(())
                 })
             } else {
+                fn ty_is_local(ty: Ty<'_>) -> bool {
+                    match ty.kind() {
+                        ty::Adt(adt_def, ..) => adt_def.did().is_local(),
+                        // Arrays and slices use the inner type's `ConstParamTy`.
+                        ty::Array(ty, ..) => ty_is_local(*ty),
+                        ty::Slice(ty) => ty_is_local(*ty),
+                        // `&` references use the inner type's `ConstParamTy`.
+                        // `&mut` are not supported.
+                        ty::Ref(_, ty, ast::Mutability::Not) => ty_is_local(*ty),
+                        // Say that a tuple is local if any of its components are local.
+                        // This is not strictly correct, but it's likely that the user can fix the local component.
+                        ty::Tuple(tys) => tys.iter().any(|ty| ty_is_local(ty)),
+                        _ => false,
+                    }
+                }
+
                 let mut diag = match ty.kind() {
                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => return Ok(()),
                     ty::FnPtr(_) => tcx.dcx().struct_span_err(
@@ -962,31 +978,16 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
                     tcx,
                     tcx.param_env(param.def_id),
                     ty,
-                    cause,
+                    &cause,
                 ) {
                     // Can never implement `ConstParamTy`, don't suggest anything.
-                    Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed) => false,
+                    Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed(..)) => false,
                     // May be able to implement `ConstParamTy`. Only emit the feature help
                     // if the type is local, since the user may be able to fix the local type.
-                    Err(ConstParamTyImplementationError::InfrigingFields(..)) => {
-                        fn ty_is_local(ty: Ty<'_>) -> bool {
-                            match ty.kind() {
-                                ty::Adt(adt_def, ..) => adt_def.did().is_local(),
-                                // Arrays and slices use the inner type's `ConstParamTy`.
-                                ty::Array(ty, ..) => ty_is_local(*ty),
-                                ty::Slice(ty) => ty_is_local(*ty),
-                                // `&` references use the inner type's `ConstParamTy`.
-                                // `&mut` are not supported.
-                                ty::Ref(_, ty, ast::Mutability::Not) => ty_is_local(*ty),
-                                // Say that a tuple is local if any of its components are local.
-                                // This is not strictly correct, but it's likely that the user can fix the local component.
-                                ty::Tuple(tys) => tys.iter().any(|ty| ty_is_local(ty)),
-                                _ => false,
-                            }
-                        }
-
-                        ty_is_local(ty)
-                    }
+                    Err(
+                        ConstParamTyImplementationError::InfrigingFields(..)
+                        | ConstParamTyImplementationError::InfringingInnerTy(..),
+                    ) => ty_is_local(ty),
                     // Implments `ConstParamTy`, suggest adding the feature to enable.
                     Ok(..) => true,
                 };

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -127,13 +127,31 @@ fn visit_implementation_of_const_param_ty(
     };
 
     let cause = traits::ObligationCause::misc(span, impl_did);
-    match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, cause) {
+    match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, &cause) {
         Ok(()) => Ok(()),
         Err(ConstParamTyImplementationError::InfrigingFields(fields)) => {
             Err(infringing_fields_error(tcx, fields, LangItem::ConstParamTy, impl_did, span))
         }
-        Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed) => {
-            Err(tcx.dcx().emit_err(errors::ConstParamTyImplOnNonAdt { span }))
+        Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed(ty)) => {
+            Err(tcx.dcx().emit_err(errors::ConstParamTyImplOnNonAdt { span, ty }))
+        }
+        Err(ConstParamTyImplementationError::InfringingInnerTy(def_ids)) => {
+            let mut def_spans = vec![];
+            let mut suggs = vec![];
+            for def_id in def_ids {
+                let def_span = tcx.def_span(def_id);
+                def_spans.push(def_span);
+                if def_id.is_local() {
+                    suggs.push(errors::ConstParamTyImplOnInfringingInnerTySugg {
+                        span: def_span.shrink_to_lo(),
+                    });
+                }
+            }
+            Err(tcx.dcx().emit_err(errors::ConstParamTyImplOnInfringingInnerTy {
+                span,
+                def_spans,
+                suggs,
+            }))
         }
     }
 }

compiler/rustc_hir_analysis/src/errors.rs

@@ -221,10 +221,34 @@ pub struct CopyImplOnNonAdt {
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_const_param_ty_impl_on_non_adt)]
-pub struct ConstParamTyImplOnNonAdt {
+pub struct ConstParamTyImplOnNonAdt<'tcx> {
     #[primary_span]
     #[label]
     pub span: Span,
+    pub ty: Ty<'tcx>,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_const_param_ty_impl_on_infringing_inner_ty)]
+pub struct ConstParamTyImplOnInfringingInnerTy {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub def_spans: Vec<Span>,
+    #[subdiagnostic]
+    pub suggs: Vec<ConstParamTyImplOnInfringingInnerTySugg>,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    hir_analysis_const_param_ty_impl_on_infringing_inner_ty_sugg,
+    applicability = "maybe-incorrect",
+    style = "verbose",
+    code = "#[derive(ConstParamTy)]\n"
+)]
+pub struct ConstParamTyImplOnInfringingInnerTySugg {
+    #[primary_span]
+    pub span: Span,
 }
 
 #[derive(Diagnostic)]