Increase vtable layout size

This improves LLVM's codegen by allowing vtable loads to be hoisted out
of loops (as just one example).

Author: Mark-Simulacrum

compiler/rustc_middle/src/ty/layout.rs

@@ -771,11 +771,25 @@ where
                         });
                     }
 
-                    let mk_dyn_vtable = || {
+                    let mk_dyn_vtable = |principal: Option<ty::PolyExistentialTraitRef<'tcx>>| {
+                        let min_count = TyCtxt::COMMON_VTABLE_ENTRIES.len()
+                            // As the comment below notes, it's hard to get access to supertraits
+                            // here. Supertraits currently are determined by resolving predicates.
+                            // It's not clear that it's easy to do that here. So, for now, we're
+                            // just increasing the vtable size by the local, direct vtable entries.
+                            // We know that *at least* that many pointers will be needed,
+                            // which is already a win for some cases. As an example, this lets LLVM
+                            // better hoist vtable loads out of loops for calls to `&mut dyn FnMut`
+                            // function arguments.
+                            + principal
+                                .map(|principal| {
+                                    tcx.own_existential_vtable_entries(principal.def_id()).len()
+                                })
+                                .unwrap_or(0);
                         Ty::new_imm_ref(
                             tcx,
                             tcx.lifetimes.re_static,
-                            Ty::new_array(tcx, tcx.types.usize, 3),
+                            Ty::new_array(tcx, tcx.types.usize, min_count.try_into().unwrap()),
                         )
                         /* FIXME: use actual fn pointers
                         Warning: naively computing the number of entries in the
@@ -808,16 +822,16 @@ where
                         // `std::mem::uninitialized::<&dyn Trait>()`, for example.
                         if let ty::Adt(def, args) = metadata.kind()
                             && Some(def.did()) == tcx.lang_items().dyn_metadata()
-                            && args.type_at(0).is_trait()
+                            && let ty::Dynamic(data, _, ty::Dyn) = args.type_at(0).kind()
                         {
-                            mk_dyn_vtable()
+                            mk_dyn_vtable(data.principal())
                         } else {
                             metadata
                         }
                     } else {
                         match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).kind() {
                             ty::Slice(_) | ty::Str => tcx.types.usize,
-                            ty::Dynamic(_, _, ty::Dyn) => mk_dyn_vtable(),
+                            ty::Dynamic(data, _, ty::Dyn) => mk_dyn_vtable(data.principal()),
                             _ => bug!("TyAndLayout::field({:?}): not applicable", this),
                         }
                     };