Stop allocating vtable entries for non-object-safe methods

Author: nbdd0121

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -647,14 +647,14 @@ fn vtable_entries<'tcx>(
                     .filter(|item| item.kind == ty::AssocKind::Fn);
                 // Now list each method's DefId and InternalSubsts (for within its trait).
                 // If the method can never be called from this object, produce `Vacant`.
-                let own_entries = trait_methods.map(move |trait_method| {
+                let own_entries = trait_methods.filter_map(move |trait_method| {
                     debug!("vtable_entries: trait_method={:?}", trait_method);
                     let def_id = trait_method.def_id;
 
                     // Some methods cannot be called on an object; skip those.
                     if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
                         debug!("vtable_entries: not vtable safe");
-                        return VtblEntry::Vacant;
+                        return None;
                     }
 
                     // The method may have some early-bound lifetimes; add regions for those.
@@ -681,7 +681,7 @@ fn vtable_entries<'tcx>(
                     let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
                     if impossible_predicates(tcx, predicates.predicates) {
                         debug!("vtable_entries: predicates do not hold");
-                        return VtblEntry::Vacant;
+                        return Some(VtblEntry::Vacant);
                     }
 
                     let instance = ty::Instance::resolve_for_vtable(
@@ -691,7 +691,7 @@ fn vtable_entries<'tcx>(
                         substs,
                     )
                     .expect("resolution failed during building vtable representation");
-                    VtblEntry::Method(instance)
+                    Some(VtblEntry::Method(instance))
                 });
 
                 entries.extend(own_entries);

compiler/rustc_trait_selection/src/traits/util.rs

@@ -289,7 +289,9 @@ pub fn count_own_vtable_entries(tcx: TyCtxt<'tcx>, trait_ref: ty::PolyTraitRef<'
     // Count number of methods and add them to the total offset.
     // Skip over associated types and constants.
     for trait_item in tcx.associated_items(trait_ref.def_id()).in_definition_order() {
-        if trait_item.kind == ty::AssocKind::Fn {
+        let is_vtable_safe_method = trait_item.kind == ty::AssocKind::Fn
+            && super::is_vtable_safe_method(tcx, trait_ref.def_id(), trait_item);
+        if is_vtable_safe_method {
             entries += 1;
         }
     }
@@ -308,13 +310,16 @@ pub fn get_vtable_index_of_object_method<N>(
     // add them to the total offset.
     // Skip over associated types and constants, as those aren't stored in the vtable.
     let mut entries = object.vtable_base;
-    for trait_item in tcx.associated_items(object.upcast_trait_ref.def_id()).in_definition_order() {
+    let trait_def_id = object.upcast_trait_ref.def_id();
+    for trait_item in tcx.associated_items(trait_def_id).in_definition_order() {
+        let is_vtable_safe_method = trait_item.kind == ty::AssocKind::Fn
+            && super::is_vtable_safe_method(tcx, trait_def_id, trait_item);
         if trait_item.def_id == method_def_id {
             // The item with the ID we were given really ought to be a method.
-            assert_eq!(trait_item.kind, ty::AssocKind::Fn);
+            assert!(is_vtable_safe_method);
             return entries;
         }
-        if trait_item.kind == ty::AssocKind::Fn {
+        if is_vtable_safe_method {
             entries += 1;
         }
     }

src/test/ui/traits/vtable/vtable-non-object-safe.rs

@@ -0,0 +1,18 @@
+// build-fail
+#![feature(rustc_attrs)]
+
+// Ensure that non-object-safe methods in Iterator does not generate
+// vtable entries.
+
+#[rustc_dump_vtable]
+trait A: Iterator {}
+//~^ error Vtable
+
+impl<T> A for T where T: Iterator {}
+
+fn foo(_a: &mut dyn A<Item=u8>) {
+}
+
+fn main() {
+    foo(&mut vec![0, 1, 2, 3].into_iter());
+}

src/test/ui/traits/vtable/vtable-non-object-safe.stderr

@@ -0,0 +1,16 @@
+error: Vtable entries for `<std::vec::IntoIter<u8> as A>`: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(<std::vec::IntoIter<u8> as Iterator>::next),
+    Method(<std::vec::IntoIter<u8> as Iterator>::size_hint),
+    Method(<std::vec::IntoIter<u8> as Iterator>::advance_by),
+    Method(<std::vec::IntoIter<u8> as Iterator>::nth),
+]
+  --> $DIR/vtable-non-object-safe.rs:8:1
+   |
+LL | trait A: Iterator {}
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/traits/vtable/vtable-vacant.rs

@@ -1,22 +1,25 @@
 // build-fail
 #![feature(rustc_attrs)]
+#![feature(negative_impls)]
+#![allow(where_clauses_object_safety)]
 
 // B --> A
 
 #[rustc_dump_vtable]
 trait A {
     fn foo_a1(&self) {}
-    fn foo_a2(&self) where Self: Sized {}
+    fn foo_a2(&self) where Self: Send {}
 }
 
 #[rustc_dump_vtable]
 trait B: A {
     //~^ error Vtable
     fn foo_b1(&self) {}
-    fn foo_b2() where Self: Sized {}
+    fn foo_b2(&self) where Self: Send {}
 }
 
 struct S;
+impl !Send for S {}
 
 impl A for S {}
 impl B for S {}

src/test/ui/traits/vtable/vtable-vacant.stderr

@@ -7,12 +7,12 @@ error: Vtable entries for `<S as B>`: [
     Method(<S as B>::foo_b1),
     Vacant,
 ]
-  --> $DIR/vtable-vacant.rs:13:1
+  --> $DIR/vtable-vacant.rs:15:1
    |
 LL | / trait B: A {
 LL | |
 LL | |     fn foo_b1(&self) {}
-LL | |     fn foo_b2() where Self: Sized {}
+LL | |     fn foo_b2(&self) where Self: Send {}
 LL | | }
    | |_^