Suggest relaxing implicit `type Assoc: Sized;` bound

Fix #85378.

1 files changed, 23 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 78f7f915f72..8f1ebc24b94 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -2652,6 +2652,29 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             // Check for foreign traits being reachable.
                             self.tcx.visible_parent_map(()).get(&def_id).is_some()
                         };
+                        if Some(def_id) == self.tcx.lang_items().sized_trait()
+                            && let Some(hir::Node::TraitItem(hir::TraitItem {
+                                ident,
+                                kind: hir::TraitItemKind::Type(bounds, None),
+                                ..
+                            })) = tcx.hir().get_if_local(item_def_id)
+                            // Do not suggest relaxing if there is an explicit `Sized` obligation.
+                            && !bounds.iter()
+                                .filter_map(|bound| bound.trait_ref())
+                                .any(|tr| tr.trait_def_id() == self.tcx.lang_items().sized_trait())
+                        {
+                            let (span, separator) = if let [.., last] = bounds {
+                                (last.span().shrink_to_hi(), " +")
+                            } else {
+                                (ident.span.shrink_to_hi(), ":")
+                            };
+                            err.span_suggestion_verbose(
+                                span,
+                                "consider relaxing the implicit `Sized` restriction",
+                                format!("{separator} ?Sized"),
+                                Applicability::MachineApplicable,
+                            );
+                        }
                         if let DefKind::Trait = tcx.def_kind(item_def_id)
                             && !visible_item
                         {