Suggest relaxing implicit type Assoc: Sized; bound

Fix #85378.

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
                         {

tests/ui/object-safety/assoc_type_bounds_implicit_sized.fixed

@@ -0,0 +1,10 @@
+// run-rustfix
+trait TraitWithAType {
+    type Item: ?Sized;
+}
+trait Trait {}
+struct A {}
+impl TraitWithAType for A {
+    type Item = dyn Trait; //~ ERROR E0277
+}
+fn main() {}

tests/ui/object-safety/assoc_type_bounds_implicit_sized.rs

@@ -0,0 +1,10 @@
+// run-rustfix
+trait TraitWithAType {
+    type Item;
+}
+trait Trait {}
+struct A {}
+impl TraitWithAType for A {
+    type Item = dyn Trait; //~ ERROR E0277
+}
+fn main() {}

tests/ui/object-safety/assoc_type_bounds_implicit_sized.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
+  --> $DIR/assoc_type_bounds_implicit_sized.rs:8:17
+   |
+LL |     type Item = dyn Trait;
+   |                 ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Trait + 'static)`
+note: required by a bound in `TraitWithAType::Item`
+  --> $DIR/assoc_type_bounds_implicit_sized.rs:3:5
+   |
+LL |     type Item;
+   |     ^^^^^^^^^^ required by this bound in `TraitWithAType::Item`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Item: ?Sized;
+   |              ++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/object-safety/assoc_type_bounds_sized_used.stderr

@@ -33,6 +33,10 @@ help: consider removing the `?Sized` bound to make the type parameter `Sized`
 LL - fn bop<T: Bop + ?Sized>() {
 LL + fn bop<T: Bop>() {
    |
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Bar: Default + ?Sized
+   |                       ++++++++
 
 error: aborting due to 2 previous errors