Rework maybe_suggest_add_generic_impl_trait

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -123,6 +123,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
     }
 
+    /// For a struct or enum with an invalid bare trait object field, suggest turning
+    /// it into a generic type bound.
     fn maybe_suggest_add_generic_impl_trait(
         &self,
         self_ty: &hir::Ty<'_>,
@@ -132,21 +134,38 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let msg = "you might be missing a type parameter";
         let mut sugg = vec![];
 
-        let parent_id = tcx.hir_get_parent_item(self_ty.hir_id).def_id;
-        let parent_item = tcx.hir_node_by_def_id(parent_id).expect_item();
-        match parent_item.kind {
-            hir::ItemKind::Struct(_, generics) | hir::ItemKind::Enum(_, generics) => {
-                sugg.push((
-                    generics.where_clause_span,
-                    format!(
-                        "<T: {}>",
-                        self.tcx().sess.source_map().span_to_snippet(self_ty.span).unwrap()
-                    ),
-                ));
-                sugg.push((self_ty.span, "T".to_string()));
+        let parent_hir_id = tcx.parent_hir_id(self_ty.hir_id);
+        let parent_item = tcx.hir_get_parent_item(self_ty.hir_id).def_id;
+
+        let generics = match tcx.hir_node_by_def_id(parent_item) {
+            hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Struct(variant, generics), ..
+            }) => {
+                if !variant.fields().iter().any(|field| field.hir_id == parent_hir_id) {
+                    return false;
+                }
+                generics
             }
-            _ => {}
-        }
+            hir::Node::Item(hir::Item { kind: hir::ItemKind::Enum(def, generics), .. }) => {
+                if !def
+                    .variants
+                    .iter()
+                    .flat_map(|variant| variant.data.fields().iter())
+                    .any(|field| field.hir_id == parent_hir_id)
+                {
+                    return false;
+                }
+                generics
+            }
+            _ => return false,
+        };
+
+        // FIXME: `T` may already be taken.
+        sugg.push((
+            generics.where_clause_span,
+            format!("<T: {}>", self.tcx().sess.source_map().span_to_snippet(self_ty.span).unwrap()),
+        ));
+        sugg.push((self_ty.span, "T".to_string()));
         diag.multipart_suggestion_verbose(msg, sugg, Applicability::MachineApplicable);
         true
     }
@@ -198,6 +217,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
     }
 
+    /// Try our best to approximate when adding `dyn` would be helpful for a bare
+    /// trait object.
+    ///
+    /// Right now, this is if the type is either directly nested in another ty,
+    /// or if it's in the tail field within a struct. This approximates what the
+    /// user would've gotten on edition 2015, except for the case where we have
+    /// an *obvious* knock-on `Sized` error.
     fn maybe_suggest_dyn_trait(
         &self,
         self_ty: &hir::Ty<'_>,
@@ -206,19 +232,40 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         diag: &mut Diag<'_>,
     ) -> bool {
         let tcx = self.tcx();
-        let parent_id = tcx.hir_get_parent_item(self_ty.hir_id).def_id;
-        let parent_item = tcx.hir_node_by_def_id(parent_id).expect_item();
 
-        // If the parent item is an enum, don't suggest the dyn trait.
-        if let hir::ItemKind::Enum(..) = parent_item.kind {
-            return false;
-        }
+        // Look at the direct HIR parent, since we care about the relationship between
+        // the type and the thing that directly encloses it.
+        match tcx.parent_hir_node(self_ty.hir_id) {
+            // These are all generally ok. Namely, when a trait object is nested
+            // into another expression or ty, it's either very certain that they
+            // missed the ty (e.g. `&Trait`) or it's not really possible to tell
+            // what their intention is, so let's not give confusing suggestions and
+            // just mention `dyn`. The user can make up their mind what to do here.
+            hir::Node::Ty(_)
+            | hir::Node::Expr(_)
+            | hir::Node::PatExpr(_)
+            | hir::Node::PathSegment(_)
+            | hir::Node::AssocItemConstraint(_)
+            | hir::Node::TraitRef(_)
+            | hir::Node::Item(_)
+            | hir::Node::WherePredicate(_) => {}
 
-        // If the parent item is a struct, check if self_ty is the last field.
-        if let hir::ItemKind::Struct(variant_data, _) = parent_item.kind {
-            if variant_data.fields().last().unwrap().ty.span != self_ty.span {
-                return false;
+            hir::Node::Field(field) => {
+                // Enums can't have unsized fields, fields can only have an unsized tail field.
+                if let hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Struct(variant, _), ..
+                }) = tcx.parent_hir_node(field.hir_id)
+                    && variant
+                        .fields()
+                        .last()
+                        .is_some_and(|tail_field| tail_field.hir_id == field.hir_id)
+                {
+                    // Ok
+                } else {
+                    return false;
+                }
             }
+            _ => return false,
         }
 
         // FIXME: Only emit this suggestion if the trait is dyn-compatible.