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

@@ -771,25 +771,14 @@ where
                         });
                     }
 
-                    let mk_dyn_vtable = || {
+                    let mk_dyn_vtable = |principal: Option<ty::PolyExistentialTraitRef<'tcx>>| {
+                        let min_count = ty::vtable_min_entries(tcx, principal);
                         Ty::new_imm_ref(
                             tcx,
                             tcx.lifetimes.re_static,
-                            Ty::new_array(tcx, tcx.types.usize, 3),
+                            // FIXME: properly type (e.g. usize and fn pointers) the fields.
+                            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
-                        vtable by counting the methods on the trait + methods on
-                        all parent traits does not work, because some methods can
-                        be not object safe and thus excluded from the vtable.
-                        Increase this counter if you tried to implement this but
-                        failed to do it without duplicating a lot of code from
-                        other places in the compiler: 2
-                        Ty::new_tup(tcx,&[
-                            Ty::new_array(tcx,tcx.types.usize, 3),
-                            Ty::new_array(tcx,Option<fn()>),
-                        ])
-                        */
                     };
 
                     let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type()
@@ -808,16 +797,16 @@ 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),
                         }
                     };

compiler/rustc_middle/src/ty/vtable.rs

@@ -3,6 +3,8 @@ use std::fmt;
 use crate::mir::interpret::{alloc_range, AllocId, Allocation, Pointer, Scalar};
 use crate::ty::{self, Instance, PolyTraitRef, Ty, TyCtxt};
 use rustc_ast::Mutability;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_hir::def_id::DefId;
 
 #[derive(Clone, Copy, PartialEq, HashStable)]
 pub enum VtblEntry<'tcx> {
@@ -45,6 +47,65 @@ pub const COMMON_VTABLE_ENTRIES_DROPINPLACE: usize = 0;
 pub const COMMON_VTABLE_ENTRIES_SIZE: usize = 1;
 pub const COMMON_VTABLE_ENTRIES_ALIGN: usize = 2;
 
+// FIXME: This is duplicating equivalent code in compiler/rustc_trait_selection/src/traits/util.rs
+// But that is a downstream crate, and this code is pretty simple. Probably OK for now.
+struct SupertraitDefIds<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    stack: Vec<DefId>,
+    visited: FxHashSet<DefId>,
+}
+
+fn supertrait_def_ids(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SupertraitDefIds<'_> {
+    SupertraitDefIds {
+        tcx,
+        stack: vec![trait_def_id],
+        visited: Some(trait_def_id).into_iter().collect(),
+    }
+}
+
+impl Iterator for SupertraitDefIds<'_> {
+    type Item = DefId;
+
+    fn next(&mut self) -> Option<DefId> {
+        let def_id = self.stack.pop()?;
+        let predicates = self.tcx.super_predicates_of(def_id);
+        let visited = &mut self.visited;
+        self.stack.extend(
+            predicates
+                .predicates
+                .iter()
+                .filter_map(|(pred, _)| pred.as_trait_clause())
+                .map(|trait_ref| trait_ref.def_id())
+                .filter(|&super_def_id| visited.insert(super_def_id)),
+        );
+        Some(def_id)
+    }
+}
+
+// Note that we don't have access to a self type here, this has to be purely based on the trait (and
+// supertrait) definitions. That means we can't call into the same vtable_entries code since that
+// returns a specific instantiation (e.g., with Vacant slots when bounds aren't satisfied). The goal
+// here is to do a best-effort approximation without duplicating a lot of code.
+//
+// This function is used in layout computation for e.g. &dyn Trait, so it's critical that this
+// function is an accurate approximation. We verify this when actually computing the vtable below.
+pub(crate) fn vtable_min_entries<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
+) -> usize {
+    let mut count = TyCtxt::COMMON_VTABLE_ENTRIES.len();
+    let Some(trait_ref) = trait_ref else {
+        return count;
+    };
+
+    // This includes self in supertraits.
+    for def_id in supertrait_def_ids(tcx, trait_ref.def_id()) {
+        count += tcx.own_existential_vtable_entries(def_id).len();
+    }
+
+    count
+}
+
 /// Retrieves an allocation that represents the contents of a vtable.
 /// Since this is a query, allocations are cached and not duplicated.
 pub(super) fn vtable_allocation_provider<'tcx>(
@@ -62,6 +123,9 @@ pub(super) fn vtable_allocation_provider<'tcx>(
         TyCtxt::COMMON_VTABLE_ENTRIES
     };
 
+    // This confirms that the layout computation for &dyn Trait has an accurate sizing.
+    assert!(vtable_entries.len() >= vtable_min_entries(tcx, poly_trait_ref));
+
     let layout = tcx
         .layout_of(ty::ParamEnv::reveal_all().and(ty))
         .expect("failed to build vtable representation");

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -497,59 +497,12 @@ fn virtual_call_violations_for_method<'tcx>(
             };
             errors.push(MethodViolationCode::UndispatchableReceiver(span));
         } else {
-            // Do sanity check to make sure the receiver actually has the layout of a pointer.
-
-            use rustc_target::abi::Abi;
-
-            let param_env = tcx.param_env(method.def_id);
-
-            let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
-                match tcx.layout_of(param_env.and(ty)) {
-                    Ok(layout) => Some(layout.abi),
-                    Err(err) => {
-                        // #78372
-                        tcx.dcx().span_delayed_bug(
-                            tcx.def_span(method.def_id),
-                            format!("error: {err}\n while computing layout for type {ty:?}"),
-                        );
-                        None
-                    }
-                }
-            };
-
-            // e.g., `Rc<()>`
-            let unit_receiver_ty =
-                receiver_for_self_ty(tcx, receiver_ty, Ty::new_unit(tcx), method.def_id);
-
-            match abi_of_ty(unit_receiver_ty) {
-                Some(Abi::Scalar(..)) => (),
-                abi => {
-                    tcx.dcx().span_delayed_bug(
-                        tcx.def_span(method.def_id),
-                        format!(
-                            "receiver when `Self = ()` should have a Scalar ABI; found {abi:?}"
-                        ),
-                    );
-                }
-            }
-
-            let trait_object_ty = object_ty_for_trait(tcx, trait_def_id, tcx.lifetimes.re_static);
-
-            // e.g., `Rc<dyn Trait>`
-            let trait_object_receiver =
-                receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
-
-            match abi_of_ty(trait_object_receiver) {
-                Some(Abi::ScalarPair(..)) => (),
-                abi => {
-                    tcx.dcx().span_delayed_bug(
-                        tcx.def_span(method.def_id),
-                        format!(
-                            "receiver when `Self = {trait_object_ty}` should have a ScalarPair ABI; found {abi:?}"
-                        ),
-                    );
-                }
-            }
+            // We used to have a sanity check here for whether the ABI of the receiver with `()` and `dyn Trait`
+            // self types was correct. For now that code has been deleted since
+            // computing the layout_of such types requires knowing the number of methods in the
+            // virtual table, which in turn requires this code. So we skip these checks. Both
+            // of these are just sanity checking (i.e. this code is not responsible for ABI) so this
+            // should be fine.
         }
     }