Auto merge of rust-lang#123572 - Mark-Simulacrum:vtable-methods, r=oli-obk

Increase vtable layout size

This improves LLVM's codegen by allowing vtable loads to be hoisted out of loops (as just one example). The calculation here is an under-approximation but works for simple trait hierarchies (e.g., FnMut will be improved). We have a runtime assert that the approximation is accurate, so there's no risk of UB as a result of getting this wrong.

```rust
#[no_mangle]
pub fn foo(elements: &[u32], callback: &mut dyn Callback) {
    for element in elements.iter() {
        if *element != 0 {
            callback.call(*element);
        }
    }
}

pub trait Callback {
    fn call(&mut self, _: u32);
}
```

Simplifying a bit (e.g., numbering ends up different):

```diff
 ; Function Attrs: nonlazybind uwtable
-define void `@foo(ptr` noalias noundef nonnull readonly align 4 %elements.0, i64 noundef %elements.1, ptr noundef nonnull align 1 %callback.0, ptr noalias nocapture noundef readonly align 8 dereferenceable(24) %callback.1) unnamed_addr #0 {
+define void `@foo(ptr` noalias noundef nonnull readonly align 4 %elements.0, i64 noundef %elements.1, ptr noundef nonnull align 1 %callback.0, ptr noalias nocapture noundef readonly align 8 dereferenceable(32) %callback.1) unnamed_addr #0 {
 start:
   %_15 = getelementptr inbounds i32, ptr %elements.0, i64 %elements.1
`@@` -13,4 +13,5 `@@`
 bb4.lr.ph:                                        ; preds = %start
   %1 = getelementptr inbounds i8, ptr %callback.1, i64 24
+  %2 = load ptr, ptr %1, align 8, !nonnull !3
   br label %bb4

 bb6:                                              ; preds = %bb4
-  %4 = load ptr, ptr %1, align 8, !invariant.load !3, !nonnull !3
-  tail call void %4(ptr noundef nonnull align 1 %callback.0, i32 noundef %_9)
+  tail call void %2(ptr noundef nonnull align 1 %callback.0, i32 noundef %_9)
   br label %bb7
 }
```

Author: bors

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -12,7 +12,6 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_session::parse::feature_err;
 use rustc_span::{sym, ErrorGuaranteed};
-use rustc_trait_selection::traits;
 
 mod builtin;
 mod inherent_impls;
@@ -199,7 +198,7 @@ fn check_object_overlap<'tcx>(
                 // With the feature enabled, the trait is not implemented automatically,
                 // so this is valid.
             } else {
-                let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
+                let mut supertrait_def_ids = tcx.supertrait_def_ids(component_def_id);
                 if supertrait_def_ids.any(|d| d == trait_def_id) {
                     let span = tcx.def_span(impl_def_id);
                     return Err(struct_span_code_err!(

compiler/rustc_middle/src/ty/layout.rs

@@ -827,25 +827,14 @@ 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()
@@ -864,16 +853,16 @@ 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;
 use rustc_macros::HashStable;
 
 #[derive(Clone, Copy, PartialEq, HashStable)]
@@ -40,12 +42,69 @@ impl<'tcx> fmt::Debug for VtblEntry<'tcx> {
 impl<'tcx> TyCtxt<'tcx> {
     pub const COMMON_VTABLE_ENTRIES: &'tcx [VtblEntry<'tcx>] =
         &[VtblEntry::MetadataDropInPlace, VtblEntry::MetadataSize, VtblEntry::MetadataAlign];
+
+    pub fn supertrait_def_ids(self, trait_def_id: DefId) -> SupertraitDefIds<'tcx> {
+        SupertraitDefIds {
+            tcx: self,
+            stack: vec![trait_def_id],
+            visited: Some(trait_def_id).into_iter().collect(),
+        }
+    }
 }
 
 pub const COMMON_VTABLE_ENTRIES_DROPINPLACE: usize = 0;
 pub const COMMON_VTABLE_ENTRIES_SIZE: usize = 1;
 pub const COMMON_VTABLE_ENTRIES_ALIGN: usize = 2;
 
+pub struct SupertraitDefIds<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    stack: Vec<DefId>,
+    visited: FxHashSet<DefId>,
+}
+
+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 tcx.supertrait_def_ids(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>(
@@ -63,6 +122,9 @@ 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/solve/trait_goals.rs

@@ -1,7 +1,5 @@
 //! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
 
-use crate::traits::supertrait_def_ids;
-
 use super::assembly::structural_traits::AsyncCallableRelevantTypes;
 use super::assembly::{self, structural_traits, Candidate};
 use super::{EvalCtxt, GoalSource, SolverMode};
@@ -837,7 +835,8 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
         let a_auto_traits: FxIndexSet<DefId> = a_data
             .auto_traits()
             .chain(a_data.principal_def_id().into_iter().flat_map(|principal_def_id| {
-                supertrait_def_ids(self.interner(), principal_def_id)
+                self.interner()
+                    .supertrait_def_ids(principal_def_id)
                     .filter(|def_id| self.interner().trait_is_auto(*def_id))
             }))
             .collect();

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -65,10 +65,7 @@ pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::elaborate;
 pub use self::util::{expand_trait_aliases, TraitAliasExpander, TraitAliasExpansionInfo};
 pub use self::util::{get_vtable_index_of_object_method, impl_item_is_final, upcast_choices};
-pub use self::util::{
-    supertrait_def_ids, supertraits, transitive_bounds, transitive_bounds_that_define_assoc_item,
-    SupertraitDefIds,
-};
+pub use self::util::{supertraits, transitive_bounds, transitive_bounds_that_define_assoc_item};
 pub use self::util::{with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
 
 pub use rustc_infer::traits::*;

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -26,6 +26,7 @@ use rustc_middle::ty::{TypeVisitableExt, Upcast};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
+use rustc_target::abi::Abi;
 use smallvec::SmallVec;
 
 use std::iter;
@@ -44,7 +45,8 @@ pub fn hir_ty_lowering_object_safety_violations(
     trait_def_id: DefId,
 ) -> Vec<ObjectSafetyViolation> {
     debug_assert!(tcx.generics_of(trait_def_id).has_self);
-    let violations = traits::supertrait_def_ids(tcx, trait_def_id)
+    let violations = tcx
+        .supertrait_def_ids(trait_def_id)
         .map(|def_id| predicates_reference_self(tcx, def_id, true))
         .filter(|spans| !spans.is_empty())
         .map(ObjectSafetyViolation::SupertraitSelf)
@@ -58,7 +60,7 @@ fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [Object
     debug!("object_safety_violations: {:?}", trait_def_id);
 
     tcx.arena.alloc_from_iter(
-        traits::supertrait_def_ids(tcx, trait_def_id)
+        tcx.supertrait_def_ids(trait_def_id)
             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
     )
 }
@@ -145,6 +147,14 @@ fn object_safety_violations_for_trait(
         violations.push(ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans));
     }
 
+    if violations.is_empty() {
+        for item in tcx.associated_items(trait_def_id).in_definition_order() {
+            if let ty::AssocKind::Fn = item.kind {
+                check_receiver_correct(tcx, trait_def_id, *item);
+            }
+        }
+    }
+
     debug!(
         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
         trait_def_id, violations
@@ -493,59 +503,8 @@ 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, tcx.types.unit, 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 confirm that the `receiver_is_dispatchable` is accurate later,
+            // see `check_receiver_correct`. It should be kept in sync with this code.
         }
     }
 
@@ -606,6 +565,55 @@ fn virtual_call_violations_for_method<'tcx>(
     errors
 }
 
+/// This code checks that `receiver_is_dispatchable` is correctly implemented.
+///
+/// This check is outlined from the object safety check to avoid cycles with
+/// layout computation, which relies on knowing whether methods are object safe.
+pub fn check_receiver_correct<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId, method: ty::AssocItem) {
+    if !is_vtable_safe_method(tcx, trait_def_id, method) {
+        return;
+    }
+
+    let method_def_id = method.def_id;
+    let sig = tcx.fn_sig(method_def_id).instantiate_identity();
+    let param_env = tcx.param_env(method_def_id);
+    let receiver_ty = tcx.liberate_late_bound_regions(method_def_id, sig.input(0));
+
+    if receiver_ty == tcx.types.self_param {
+        // Assumed OK, may change later if unsized_locals permits `self: Self` as dispatchable.
+        return;
+    }
+
+    // e.g., `Rc<()>`
+    let unit_receiver_ty = receiver_for_self_ty(tcx, receiver_ty, tcx.types.unit, method_def_id);
+    match tcx.layout_of(param_env.and(unit_receiver_ty)).map(|l| l.abi) {
+        Ok(Abi::Scalar(..)) => (),
+        abi => {
+            tcx.dcx().span_delayed_bug(
+                tcx.def_span(method_def_id),
+                format!("receiver {unit_receiver_ty:?} 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 tcx.layout_of(param_env.and(trait_object_receiver)).map(|l| l.abi) {
+        Ok(Abi::ScalarPair(..)) => (),
+        abi => {
+            tcx.dcx().span_delayed_bug(
+                tcx.def_span(method_def_id),
+                format!(
+                    "receiver {trait_object_receiver:?} when `Self = {trait_object_ty}` should have a ScalarPair ABI; found {abi:?}"
+                ),
+            );
+        }
+    }
+}
+
 /// Performs a type instantiation to produce the version of `receiver_ty` when `Self = self_ty`.
 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 fn receiver_for_self_ty<'tcx>(

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -1004,7 +1004,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let a_auto_traits: FxIndexSet<DefId> = a_data
                         .auto_traits()
                         .chain(principal_def_id_a.into_iter().flat_map(|principal_def_id| {
-                            util::supertrait_def_ids(self.tcx(), principal_def_id)
+                            self.tcx()
+                                .supertrait_def_ids(principal_def_id)
                                 .filter(|def_id| self.tcx().trait_is_auto(*def_id))
                         }))
                         .collect();

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2591,8 +2591,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         let a_auto_traits: FxIndexSet<DefId> = a_data
             .auto_traits()
             .chain(a_data.principal_def_id().into_iter().flat_map(|principal_def_id| {
-                util::supertrait_def_ids(tcx, principal_def_id)
-                    .filter(|def_id| tcx.trait_is_auto(*def_id))
+                tcx.supertrait_def_ids(principal_def_id).filter(|def_id| tcx.trait_is_auto(*def_id))
             }))
             .collect();