Auto merge of #126505 - compiler-errors:no-vtable, r=lcnr

Only compute vtable information during codegen

This PR removes vtable information from the `Object` and `TraitUpcasting` candidate sources in the trait solvers, and defers the computation of relevant information to `Instance::resolve`. This is because vtables really aren't a thing in the trait world -- they're an implementation detail in codegen.

Previously it was just easiest to tangle this information together since we were already doing the work of looking at all the supertraits in the trait solver, and specifically because we use traits to represent when it's possible to call a method via a vtable (`Object` candidate) and do upcasting (`Unsize` candidate). but I am somewhat suspicious we're doing a *lot* of extra work, especially in polymorphic contexts, so let's see what perf says.

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -39,8 +39,7 @@ pub(crate) fn unsized_info<'tcx>(
             }
 
             // trait upcasting coercion
-            let vptr_entry_idx =
-                fx.tcx.vtable_trait_upcasting_coercion_new_vptr_slot((source, target));
+            let vptr_entry_idx = fx.tcx.supertrait_vtable_slot((source, target));
 
             if let Some(entry_idx) = vptr_entry_idx {
                 let entry_idx = u32::try_from(entry_idx).unwrap();

compiler/rustc_codegen_ssa/src/base.rs

@@ -163,8 +163,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
             // trait upcasting coercion
 
-            let vptr_entry_idx =
-                cx.tcx().vtable_trait_upcasting_coercion_new_vptr_slot((source, target));
+            let vptr_entry_idx = cx.tcx().supertrait_vtable_slot((source, target));
 
             if let Some(entry_idx) = vptr_entry_idx {
                 let ptr_size = bx.data_layout().pointer_size;

compiler/rustc_middle/src/query/keys.rs

@@ -360,6 +360,14 @@ impl<'tcx> Key for (ty::ParamEnv<'tcx>, ty::TraitRef<'tcx>) {
     }
 }
 
+impl<'tcx> Key for ty::TraitRef<'tcx> {
+    type Cache<V> = DefaultCache<Self, V>;
+
+    fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
+        tcx.def_span(self.def_id)
+    }
+}
+
 impl<'tcx> Key for ty::PolyTraitRef<'tcx> {
     type Cache<V> = DefaultCache<Self, V>;
 

compiler/rustc_middle/src/query/mod.rs

@@ -46,6 +46,7 @@ use crate::traits::{
 };
 use crate::ty::fast_reject::SimplifiedType;
 use crate::ty::layout::ValidityRequirement;
+use crate::ty::print::PrintTraitRefExt;
 use crate::ty::util::AlwaysRequiresDrop;
 use crate::ty::TyCtxtFeed;
 use crate::ty::{
@@ -1271,7 +1272,11 @@ rustc_queries! {
         desc { |tcx| "finding all vtable entries for trait `{}`", tcx.def_path_str(key.def_id()) }
     }
 
-    query vtable_trait_upcasting_coercion_new_vptr_slot(key: (Ty<'tcx>, Ty<'tcx>)) -> Option<usize> {
+    query first_method_vtable_slot(key: ty::TraitRef<'tcx>) -> usize {
+        desc { |tcx| "finding the slot within the vtable of `{}` for the implementation of `{}`", key.self_ty(), key.print_only_trait_name() }
+    }
+
+    query supertrait_vtable_slot(key: (Ty<'tcx>, Ty<'tcx>)) -> Option<usize> {
         desc { |tcx| "finding the slot within vtable for trait object `{}` vtable ptr during trait upcasting coercion from `{}` vtable",
             key.1, key.0 }
     }

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -5,6 +5,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::LangItem;
 use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::util::supertraits;
 use rustc_middle::bug;
 use rustc_middle::traits::solve::inspect::ProbeKind;
 use rustc_middle::traits::solve::{Certainty, Goal, MaybeCause, QueryResult};
@@ -744,14 +745,14 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
         // a projection goal.
         if let Some(principal) = bounds.principal() {
             let principal_trait_ref = principal.with_self_ty(tcx, self_ty);
-            self.walk_vtable(principal_trait_ref, |ecx, assumption, vtable_base, _| {
+            for (idx, assumption) in supertraits(self.interner(), principal_trait_ref).enumerate() {
                 candidates.extend(G::probe_and_consider_object_bound_candidate(
-                    ecx,
-                    CandidateSource::BuiltinImpl(BuiltinImplSource::Object { vtable_base }),
+                    self,
+                    CandidateSource::BuiltinImpl(BuiltinImplSource::Object(idx)),
                     goal,
                     assumption.upcast(tcx),
                 ));
-            });
+            }
         }
     }
 

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -24,7 +24,6 @@ use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Gen
 use std::ops::ControlFlow;
 
 use crate::traits::coherence;
-use crate::traits::vtable::{count_own_vtable_entries, prepare_vtable_segments, VtblSegment};
 
 use super::inspect::ProofTreeBuilder;
 use super::{search_graph, GoalEvaluationKind, FIXPOINT_STEP_LIMIT};
@@ -1022,41 +1021,6 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
             }
         }
     }
-
-    /// Walk through the vtable of a principal trait ref, executing a `supertrait_visitor`
-    /// for every trait ref encountered (including the principal). Passes both the vtable
-    /// base and the (optional) vptr slot.
-    pub(super) fn walk_vtable(
-        &mut self,
-        principal: ty::PolyTraitRef<'tcx>,
-        mut supertrait_visitor: impl FnMut(&mut Self, ty::PolyTraitRef<'tcx>, usize, Option<usize>),
-    ) {
-        let tcx = self.interner();
-        let mut offset = 0;
-        prepare_vtable_segments::<()>(tcx, principal, |segment| {
-            match segment {
-                VtblSegment::MetadataDSA => {
-                    offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
-                }
-                VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
-                    let own_vtable_entries = count_own_vtable_entries(tcx, trait_ref);
-
-                    supertrait_visitor(
-                        self,
-                        trait_ref,
-                        offset,
-                        emit_vptr.then(|| offset + own_vtable_entries),
-                    );
-
-                    offset += own_vtable_entries;
-                    if emit_vptr {
-                        offset += 1;
-                    }
-                }
-            }
-            ControlFlow::Continue(())
-        });
-    }
 }
 
 /// Eagerly replace aliases with inference variables, emitting `AliasRelate`

compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs

@@ -114,8 +114,8 @@ fn candidate_should_be_dropped_in_favor_of<'tcx>(
         // In the old trait solver, we arbitrarily choose lower vtable candidates
         // over higher ones.
         (
-            CandidateSource::BuiltinImpl(BuiltinImplSource::Object { vtable_base: a }),
-            CandidateSource::BuiltinImpl(BuiltinImplSource::Object { vtable_base: b }),
+            CandidateSource::BuiltinImpl(BuiltinImplSource::Object(a)),
+            CandidateSource::BuiltinImpl(BuiltinImplSource::Object(b)),
         ) => a >= b,
         // Prefer dyn candidates over non-dyn candidates. This is necessary to
         // handle the unsoundness between `impl<T: ?Sized> Any for T` and `dyn Any: Any`.

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -9,6 +9,7 @@ use rustc_hir::{LangItem, Movability};
 use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::solve::MaybeCause;
+use rustc_infer::traits::util::supertraits;
 use rustc_middle::bug;
 use rustc_middle::traits::solve::inspect::ProbeKind;
 use rustc_middle::traits::solve::{CandidateSource, Certainty, Goal, QueryResult};
@@ -756,24 +757,19 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
                 a_data.principal(),
             ));
         } else if let Some(a_principal) = a_data.principal() {
-            self.walk_vtable(
-                a_principal.with_self_ty(tcx, a_ty),
-                |ecx, new_a_principal, _, vtable_vptr_slot| {
-                    responses.extend(ecx.consider_builtin_upcast_to_principal(
-                        goal,
-                        CandidateSource::BuiltinImpl(BuiltinImplSource::TraitUpcasting {
-                            vtable_vptr_slot,
-                        }),
-                        a_data,
-                        a_region,
-                        b_data,
-                        b_region,
-                        Some(new_a_principal.map_bound(|trait_ref| {
-                            ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
-                        })),
-                    ));
-                },
-            );
+            for new_a_principal in supertraits(tcx, a_principal.with_self_ty(tcx, a_ty)).skip(1) {
+                responses.extend(self.consider_builtin_upcast_to_principal(
+                    goal,
+                    CandidateSource::BuiltinImpl(BuiltinImplSource::TraitUpcasting),
+                    a_data,
+                    a_region,
+                    b_data,
+                    b_region,
+                    Some(new_a_principal.map_bound(|trait_ref| {
+                        ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+                    })),
+                ));
+            }
         }
 
         responses

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -65,7 +65,7 @@ pub use self::structural_match::search_for_structural_match_violation;
 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::{impl_item_is_final, upcast_choices};
 pub use self::util::{supertraits, transitive_bounds, transitive_bounds_that_define_assoc_item};
 pub use self::util::{with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
 

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

@@ -22,10 +22,6 @@ use rustc_span::def_id::DefId;
 
 use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
 use crate::traits::util::{self, closure_trait_ref_and_return_type};
-use crate::traits::vtable::{
-    count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
-    VtblSegment,
-};
 use crate::traits::{
     ImplDerivedCause, ImplSource, ImplSourceUserDefinedData, Normalized, Obligation,
     ObligationCause, PolyTraitObligation, PredicateObligation, Selection, SelectionError,
@@ -689,13 +685,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?nested, "object nested obligations");
 
-        let vtable_base = vtable_trait_first_method_offset(
-            tcx,
-            unnormalized_upcast_trait_ref,
-            ty::Binder::dummy(object_trait_ref),
-        );
-
-        Ok(ImplSource::Builtin(BuiltinImplSource::Object { vtable_base: vtable_base }, nested))
+        Ok(ImplSource::Builtin(BuiltinImplSource::Object(index), nested))
     }
 
     fn confirm_fn_pointer_candidate(
@@ -1125,36 +1115,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             )?
             .expect("did not expect ambiguity during confirmation");
 
-        let vtable_segment_callback = {
-            let mut vptr_offset = 0;
-            move |segment| {
-                match segment {
-                    VtblSegment::MetadataDSA => {
-                        vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
-                    }
-                    VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
-                        vptr_offset += count_own_vtable_entries(tcx, trait_ref);
-                        if trait_ref == unnormalized_upcast_principal {
-                            if emit_vptr {
-                                return ControlFlow::Break(Some(vptr_offset));
-                            } else {
-                                return ControlFlow::Break(None);
-                            }
-                        }
-
-                        if emit_vptr {
-                            vptr_offset += 1;
-                        }
-                    }
-                }
-                ControlFlow::Continue(())
-            }
-        };
-
-        let vtable_vptr_slot =
-            prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap();
-
-        Ok(ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { vtable_vptr_slot }, nested))
+        Ok(ImplSource::Builtin(BuiltinImplSource::TraitUpcasting, nested))
     }
 
     fn confirm_builtin_unsize_candidate(

compiler/rustc_trait_selection/src/traits/util.rs

@@ -208,23 +208,6 @@ pub fn upcast_choices<'tcx>(
     supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
 }
 
-/// Given an upcast trait object described by `object`, returns the
-/// index of the method `method_def_id` (which should be part of
-/// `object.upcast_trait_ref`) within the vtable for `object`.
-pub fn get_vtable_index_of_object_method<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    vtable_base: usize,
-    method_def_id: DefId,
-) -> Option<usize> {
-    // Count number of methods preceding the one we are selecting and
-    // add them to the total offset.
-    tcx.own_existential_vtable_entries(tcx.parent(method_def_id))
-        .iter()
-        .copied()
-        .position(|def_id| def_id == method_def_id)
-        .map(|index| vtable_base + index)
-}
-
 pub fn closure_trait_ref_and_return_type<'tcx>(
     tcx: TyCtxt<'tcx>,
     fn_trait_def_id: DefId,