compiler/rustc_trait_selection/src/traits/vtable.rs - third_party/rust - Git at Google

 use std::fmt::Debug;
 use std::ops::ControlFlow;

 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::util::PredicateSet;
 use rustc_middle::bug;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{
     self, GenericArgs, GenericParamDefKind, Ty, TyCtxt, TypeVisitableExt, Upcast, VtblEntry,
 };
 use rustc_span::DUMMY_SP;
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;

 use crate::traits::{impossible_predicates, is_vtable_safe_method};

 #[derive(Clone, Debug)]
 pub enum VtblSegment<'tcx> {
     MetadataDSA,
     TraitOwnEntries { trait_ref: ty::TraitRef<'tcx>, emit_vptr: bool },
 }

 /// Prepare the segments for a vtable
 // FIXME: This should take a `PolyExistentialTraitRef`, since we don't care
 // about our `Self` type here.
 pub fn prepare_vtable_segments<'tcx, T>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
     segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>,
 ) -> Option<T> {
     prepare_vtable_segments_inner(tcx, trait_ref, segment_visitor).break_value()
 }

 /// Helper for [`prepare_vtable_segments`] that returns `ControlFlow`,
 /// such that we can use `?` in the body.
 fn prepare_vtable_segments_inner<'tcx, T>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
     mut segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>,
 ) -> ControlFlow<T> {
     // The following constraints holds for the final arrangement.
     // 1. The whole virtual table of the first direct super trait is included as the
     //    the prefix. If this trait doesn't have any super traits, then this step
     //    consists of the dsa metadata.
     // 2. Then comes the proper pointer metadata(vptr) and all own methods for all
     //    other super traits except those already included as part of the first
     //    direct super trait virtual table.
     // 3. finally, the own methods of this trait.

     // This has the advantage that trait upcasting to the first direct super trait on each level
     // is zero cost, and to another trait includes only replacing the pointer with one level indirection,
     // while not using too much extra memory.

     // For a single inheritance relationship like this,
     //   D --> C --> B --> A
     // The resulting vtable will consists of these segments:
     //  DSA, A, B, C, D

     // For a multiple inheritance relationship like this,
     //   D --> C --> A
     //           \-> B
     // The resulting vtable will consists of these segments:
     //  DSA, A, B, B-vptr, C, D

     // For a diamond inheritance relationship like this,
     //   D --> B --> A
     //     \-> C -/
     // The resulting vtable will consists of these segments:
     //  DSA, A, B, C, C-vptr, D

     // For a more complex inheritance relationship like this:
     //   O --> G --> C --> A
     //     \     \     \-> B
     //     |     |-> F --> D
     //     |           \-> E
     //     |-> N --> J --> H
     //           \     \-> I
     //           |-> M --> K
     //                 \-> L
     // The resulting vtable will consists of these segments:
     //  DSA, A, B, B-vptr, C, D, D-vptr, E, E-vptr, F, F-vptr, G,
     //  H, H-vptr, I, I-vptr, J, J-vptr, K, K-vptr, L, L-vptr, M, M-vptr,
     //  N, N-vptr, O

     // emit dsa segment first.
     segment_visitor(VtblSegment::MetadataDSA)?;

     let mut emit_vptr_on_new_entry = false;
     let mut visited = PredicateSet::new(tcx);
     let predicate = trait_ref.upcast(tcx);
     let mut stack: SmallVec<[(ty::TraitRef<'tcx>, _, _); 5]> =
         smallvec![(trait_ref, emit_vptr_on_new_entry, maybe_iter(None))];
     visited.insert(predicate);

     // the main traversal loop:
     // basically we want to cut the inheritance directed graph into a few non-overlapping slices of nodes
     // such that each node is emitted after all its descendants have been emitted.
     // so we convert the directed graph into a tree by skipping all previously visited nodes using a visited set.
     // this is done on the fly.
     // Each loop run emits a slice - it starts by find a "childless" unvisited node, backtracking upwards, and it
     // stops after it finds a node that has a next-sibling node.
     // This next-sibling node will used as the starting point of next slice.

     // Example:
     // For a diamond inheritance relationship like this,
     //   D#1 --> B#0 --> A#0
     //     \-> C#1 -/

     // Starting point 0 stack [D]
     // Loop run #0: Stack after diving in is [D B A], A is "childless"
     // after this point, all newly visited nodes won't have a vtable that equals to a prefix of this one.
     // Loop run #0: Emitting the slice [B A] (in reverse order), B has a next-sibling node, so this slice stops here.
     // Loop run #0: Stack after exiting out is [D C], C is the next starting point.
     // Loop run #1: Stack after diving in is [D C], C is "childless", since its child A is skipped(already emitted).
     // Loop run #1: Emitting the slice [D C] (in reverse order). No one has a next-sibling node.
     // Loop run #1: Stack after exiting out is []. Now the function exits.

     'outer: loop {
         // dive deeper into the stack, recording the path
         'diving_in: loop {
             let &(inner_most_trait_ref, _, _) = stack.last().unwrap();

             let mut direct_super_traits_iter = tcx
                 .explicit_super_predicates_of(inner_most_trait_ref.def_id)
                 .iter_identity_copied()
                 .filter_map(move |(pred, _)| {
                     pred.instantiate_supertrait(tcx, ty::Binder::dummy(inner_most_trait_ref))
                         .as_trait_clause()
                 })
                 .map(move |pred| {
                     tcx.normalize_erasing_late_bound_regions(
                         ty::TypingEnv::fully_monomorphized(),
                         pred,
                     )
                     .trait_ref
                 });

             // Find an unvisited supertrait
             match direct_super_traits_iter
                 .find(|&super_trait| visited.insert(super_trait.upcast(tcx)))
             {
                 // Push it to the stack for the next iteration of 'diving_in to pick up
                 Some(next_super_trait) => stack.push((
                     next_super_trait,
                     emit_vptr_on_new_entry,
                     maybe_iter(Some(direct_super_traits_iter)),
                 )),

                 // There are no more unvisited direct super traits, dive-in finished
                 None => break 'diving_in,
             }
         }

         // emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level.
         while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
             let has_entries = has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id);

             segment_visitor(VtblSegment::TraitOwnEntries {
                 trait_ref: inner_most_trait_ref,
                 emit_vptr: emit_vptr && has_entries && !tcx.sess.opts.unstable_opts.no_trait_vptr,
             })?;

             // If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,
             // we'll need to emit vptrs from now on.
             emit_vptr_on_new_entry |= has_entries;

             if let Some(next_inner_most_trait_ref) =
                 siblings.find(|&sibling| visited.insert(sibling.upcast(tcx)))
             {
                 stack.push((next_inner_most_trait_ref, emit_vptr_on_new_entry, siblings));

                 // just pushed a new trait onto the stack, so we need to go through its super traits
                 continue 'outer;
             }
         }

         // the stack is empty, all done
         return ControlFlow::Continue(());
     }
 }

 /// Turns option of iterator into an iterator (this is just flatten)
 fn maybe_iter<I: Iterator>(i: Option<I>) -> impl Iterator<Item = I::Item> {
     // Flatten is bad perf-vise, we could probably implement a special case here that is better
     i.into_iter().flatten()
 }

 fn has_own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
     own_existential_vtable_entries_iter(tcx, trait_def_id).next().is_some()
 }

 fn own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &[DefId] {
     tcx.arena.alloc_from_iter(own_existential_vtable_entries_iter(tcx, trait_def_id))
 }

 fn own_existential_vtable_entries_iter(
     tcx: TyCtxt<'_>,
     trait_def_id: DefId,
 ) -> impl Iterator<Item = DefId> {
     let trait_methods =
         tcx.associated_items(trait_def_id).in_definition_order().filter(|item| item.is_fn());

     // Now list each method's DefId (for within its trait).
     let own_entries = trait_methods.filter_map(move |&trait_method| {
         debug!("own_existential_vtable_entry: 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_def_id, trait_method) {
             debug!("own_existential_vtable_entry: not vtable safe");
             return None;
         }

         Some(def_id)
     });

     own_entries
 }

 /// Given a trait `trait_ref`, iterates the vtable entries
 /// that come from `trait_ref`, including its supertraits.
 fn vtable_entries<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
 ) -> &'tcx [VtblEntry<'tcx>] {
     debug_assert!(!trait_ref.has_non_region_infer() && !trait_ref.has_non_region_param());
     debug_assert_eq!(
         tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), trait_ref),
         trait_ref,
         "vtable trait ref should be normalized"
     );

     debug!("vtable_entries({:?})", trait_ref);

     let mut entries = vec![];

     let vtable_segment_callback = |segment| -> ControlFlow<()> {
         match segment {
             VtblSegment::MetadataDSA => {
                 entries.extend(TyCtxt::COMMON_VTABLE_ENTRIES);
             }
             VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
                 let existential_trait_ref = ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref);

                 // Lookup the shape of vtable for the trait.
                 let own_existential_entries =
                     tcx.own_existential_vtable_entries(existential_trait_ref.def_id);

                 let own_entries = own_existential_entries.iter().copied().map(|def_id| {
                     debug!("vtable_entries: trait_method={:?}", def_id);

                     // The method may have some early-bound lifetimes; add regions for those.
                     // FIXME: Is this normalize needed?
                     let args = tcx.normalize_erasing_regions(
                         ty::TypingEnv::fully_monomorphized(),
                         GenericArgs::for_item(tcx, def_id, |param, _| match param.kind {
                             GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
                             GenericParamDefKind::Type { .. }
                             | GenericParamDefKind::Const { .. } => {
                                 trait_ref.args[param.index as usize]
                             }
                         }),
                     );

                     // It's possible that the method relies on where-clauses that
                     // do not hold for this particular set of type parameters.
                     // Note that this method could then never be called, so we
                     // do not want to try and codegen it, in that case (see #23435).
                     let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, args);
                     if impossible_predicates(
                         tcx,
                         predicates.map(|(predicate, _)| predicate).collect(),
                     ) {
                         debug!("vtable_entries: predicates do not hold");
                         return VtblEntry::Vacant;
                     }

                     let instance = ty::Instance::expect_resolve_for_vtable(
                         tcx,
                         ty::TypingEnv::fully_monomorphized(),
                         def_id,
                         args,
                         DUMMY_SP,
                     );

                     VtblEntry::Method(instance)
                 });

                 entries.extend(own_entries);

                 if emit_vptr {
                     entries.push(VtblEntry::TraitVPtr(trait_ref));
                 }
             }
         }

         ControlFlow::Continue(())
     };

     let _ = prepare_vtable_segments(tcx, trait_ref, vtable_segment_callback);

     tcx.arena.alloc_from_iter(entries)
 }

 // Given a `dyn Subtrait: Supertrait` trait ref, find corresponding first slot
 // for `Supertrait`'s methods in the vtable of `Subtrait`.
 pub(crate) fn first_method_vtable_slot<'tcx>(tcx: TyCtxt<'tcx>, key: ty::TraitRef<'tcx>) -> usize {
     debug_assert!(!key.has_non_region_infer() && !key.has_non_region_param());
     debug_assert_eq!(
         tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), key),
         key,
         "vtable trait ref should be normalized"
     );

     let ty::Dynamic(source, _, _) = *key.self_ty().kind() else {
         bug!();
     };
     let source_principal = tcx.instantiate_bound_regions_with_erased(
         source.principal().unwrap().with_self_ty(tcx, key.self_ty()),
     );

     // We're monomorphizing a call to a dyn trait object that can never be constructed.
     if tcx.instantiate_and_check_impossible_predicates((
         source_principal.def_id,
         source_principal.args,
     )) {
         return 0;
     }

     let target_principal = ty::ExistentialTraitRef::erase_self_ty(tcx, key);

     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: vtable_principal, emit_vptr } => {
                     if ty::ExistentialTraitRef::erase_self_ty(tcx, vtable_principal)
                         == target_principal
                     {
                         return ControlFlow::Break(vptr_offset);
                     }

                     vptr_offset +=
                         tcx.own_existential_vtable_entries(vtable_principal.def_id).len();

                     if emit_vptr {
                         vptr_offset += 1;
                     }
                 }
             }
             ControlFlow::Continue(())
         }
     };

     prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap()
 }

 /// Given a `dyn Subtrait` and `dyn Supertrait` trait object, find the slot of
 /// the trait vptr in the subtrait's vtable.
 ///
 /// A return value of `None` means that the original vtable can be reused.
 pub(crate) fn supertrait_vtable_slot<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: (
         Ty<'tcx>, // Source -- `dyn Subtrait`.
         Ty<'tcx>, // Target -- `dyn Supertrait` being coerced to.
     ),
 ) -> Option<usize> {
     debug_assert!(!key.has_non_region_infer() && !key.has_non_region_param());
     debug_assert_eq!(
         tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), key),
         key,
         "upcasting trait refs should be normalized"
     );

     let (source, target) = key;

     // If the target principal is `None`, we can just return `None`.
     let ty::Dynamic(target_data, _, _) = *target.kind() else {
         bug!();
     };
     let target_principal = tcx.instantiate_bound_regions_with_erased(target_data.principal()?);

     // Given that we have a target principal, it is a bug for there not to be a source principal.
     let ty::Dynamic(source_data, _, _) = *source.kind() else {
         bug!();
     };
     let source_principal = tcx.instantiate_bound_regions_with_erased(
         source_data.principal().unwrap().with_self_ty(tcx, source),
     );

     // We're monomorphizing a dyn trait object upcast that can never be constructed.
     if tcx.instantiate_and_check_impossible_predicates((
         source_principal.def_id,
         source_principal.args,
     )) {
         return None;
     }

     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: vtable_principal, emit_vptr } => {
                     vptr_offset +=
                         tcx.own_existential_vtable_entries(vtable_principal.def_id).len();
                     if ty::ExistentialTraitRef::erase_self_ty(tcx, vtable_principal)
                         == target_principal
                     {
                         if emit_vptr {
                             return ControlFlow::Break(Some(vptr_offset));
                         } else {
                             return ControlFlow::Break(None);
                         }
                     }

                     if emit_vptr {
                         vptr_offset += 1;
                     }
                 }
             }
             ControlFlow::Continue(())
         }
     };

     prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap()
 }

 pub(super) fn provide(providers: &mut Providers) {
     *providers = Providers {
         own_existential_vtable_entries,
         vtable_entries,
         first_method_vtable_slot,
         supertrait_vtable_slot,
         ..*providers
     };
 }
	use std::fmt::Debug;
	use std::ops::ControlFlow;

	use rustc_hir::def_id::DefId;
	use rustc_infer::traits::util::PredicateSet;
	use rustc_middle::bug;
	use rustc_middle::query::Providers;
	use rustc_middle::ty::{
	self, GenericArgs, GenericParamDefKind, Ty, TyCtxt, TypeVisitableExt, Upcast, VtblEntry,
	};
	use rustc_span::DUMMY_SP;
	use smallvec::{SmallVec, smallvec};
	use tracing::debug;

	use crate::traits::{impossible_predicates, is_vtable_safe_method};

	#[derive(Clone, Debug)]
	pub enum VtblSegment<'tcx> {
	MetadataDSA,
	TraitOwnEntries { trait_ref: ty::TraitRef<'tcx>, emit_vptr: bool },
	}

	/// Prepare the segments for a vtable
	// FIXME: This should take a `PolyExistentialTraitRef`, since we don't care
	// about our `Self` type here.
	pub fn prepare_vtable_segments<'tcx, T>(
	tcx: TyCtxt<'tcx>,
	trait_ref: ty::TraitRef<'tcx>,
	segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>,
	) -> Option<T> {
	prepare_vtable_segments_inner(tcx, trait_ref, segment_visitor).break_value()
	}

	/// Helper for [`prepare_vtable_segments`] that returns `ControlFlow`,
	/// such that we can use `?` in the body.
	fn prepare_vtable_segments_inner<'tcx, T>(
	tcx: TyCtxt<'tcx>,
	trait_ref: ty::TraitRef<'tcx>,
	mut segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>,
	) -> ControlFlow<T> {
	// The following constraints holds for the final arrangement.
	// 1. The whole virtual table of the first direct super trait is included as the
	// the prefix. If this trait doesn't have any super traits, then this step
	// consists of the dsa metadata.
	// 2. Then comes the proper pointer metadata(vptr) and all own methods for all
	// other super traits except those already included as part of the first
	// direct super trait virtual table.
	// 3. finally, the own methods of this trait.

	// This has the advantage that trait upcasting to the first direct super trait on each level
	// is zero cost, and to another trait includes only replacing the pointer with one level indirection,
	// while not using too much extra memory.

	// For a single inheritance relationship like this,
	// D --> C --> B --> A
	// The resulting vtable will consists of these segments:
	// DSA, A, B, C, D

	// For a multiple inheritance relationship like this,
	// D --> C --> A
	// \-> B
	// The resulting vtable will consists of these segments:
	// DSA, A, B, B-vptr, C, D

	// For a diamond inheritance relationship like this,
	// D --> B --> A
	// \-> C -/
	// The resulting vtable will consists of these segments:
	// DSA, A, B, C, C-vptr, D

	// For a more complex inheritance relationship like this:
	// O --> G --> C --> A
	// \ \ \-> B
	// \| \|-> F --> D
	// \| \-> E
	// \|-> N --> J --> H
	// \ \-> I
	// \|-> M --> K
	// \-> L
	// The resulting vtable will consists of these segments:
	// DSA, A, B, B-vptr, C, D, D-vptr, E, E-vptr, F, F-vptr, G,
	// H, H-vptr, I, I-vptr, J, J-vptr, K, K-vptr, L, L-vptr, M, M-vptr,
	// N, N-vptr, O

	// emit dsa segment first.
	segment_visitor(VtblSegment::MetadataDSA)?;

	let mut emit_vptr_on_new_entry = false;
	let mut visited = PredicateSet::new(tcx);
	let predicate = trait_ref.upcast(tcx);
	let mut stack: SmallVec<[(ty::TraitRef<'tcx>, _, _); 5]> =
	smallvec![(trait_ref, emit_vptr_on_new_entry, maybe_iter(None))];
	visited.insert(predicate);

	// the main traversal loop:
	// basically we want to cut the inheritance directed graph into a few non-overlapping slices of nodes
	// such that each node is emitted after all its descendants have been emitted.
	// so we convert the directed graph into a tree by skipping all previously visited nodes using a visited set.
	// this is done on the fly.
	// Each loop run emits a slice - it starts by find a "childless" unvisited node, backtracking upwards, and it
	// stops after it finds a node that has a next-sibling node.
	// This next-sibling node will used as the starting point of next slice.

	// Example:
	// For a diamond inheritance relationship like this,
	// D#1 --> B#0 --> A#0
	// \-> C#1 -/

	// Starting point 0 stack [D]
	// Loop run #0: Stack after diving in is [D B A], A is "childless"
	// after this point, all newly visited nodes won't have a vtable that equals to a prefix of this one.
	// Loop run #0: Emitting the slice [B A] (in reverse order), B has a next-sibling node, so this slice stops here.
	// Loop run #0: Stack after exiting out is [D C], C is the next starting point.
	// Loop run #1: Stack after diving in is [D C], C is "childless", since its child A is skipped(already emitted).
	// Loop run #1: Emitting the slice [D C] (in reverse order). No one has a next-sibling node.
	// Loop run #1: Stack after exiting out is []. Now the function exits.

	'outer: loop {
	// dive deeper into the stack, recording the path
	'diving_in: loop {
	let &(inner_most_trait_ref, _, _) = stack.last().unwrap();

	let mut direct_super_traits_iter = tcx
	.explicit_super_predicates_of(inner_most_trait_ref.def_id)
	.iter_identity_copied()
	.filter_map(move \|(pred, _)\| {
	pred.instantiate_supertrait(tcx, ty::Binder::dummy(inner_most_trait_ref))
	.as_trait_clause()
	})
	.map(move \|pred\| {
	tcx.normalize_erasing_late_bound_regions(
	ty::TypingEnv::fully_monomorphized(),
	pred,
	)
	.trait_ref
	});

	// Find an unvisited supertrait
	match direct_super_traits_iter
	.find(\|&super_trait\| visited.insert(super_trait.upcast(tcx)))
	{
	// Push it to the stack for the next iteration of 'diving_in to pick up
	Some(next_super_trait) => stack.push((
	next_super_trait,
	emit_vptr_on_new_entry,
	maybe_iter(Some(direct_super_traits_iter)),
	)),

	// There are no more unvisited direct super traits, dive-in finished
	None => break 'diving_in,
	}
	}

	// emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level.
	while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
	let has_entries = has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id);

	segment_visitor(VtblSegment::TraitOwnEntries {
	trait_ref: inner_most_trait_ref,
	emit_vptr: emit_vptr && has_entries && !tcx.sess.opts.unstable_opts.no_trait_vptr,
	})?;

	// If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,
	// we'll need to emit vptrs from now on.
	emit_vptr_on_new_entry \|= has_entries;

	if let Some(next_inner_most_trait_ref) =
	siblings.find(\|&sibling\| visited.insert(sibling.upcast(tcx)))
	{
	stack.push((next_inner_most_trait_ref, emit_vptr_on_new_entry, siblings));

	// just pushed a new trait onto the stack, so we need to go through its super traits
	continue 'outer;
	}
	}

	// the stack is empty, all done
	return ControlFlow::Continue(());
	}
	}

	/// Turns option of iterator into an iterator (this is just flatten)
	fn maybe_iter<I: Iterator>(i: Option<I>) -> impl Iterator<Item = I::Item> {
	// Flatten is bad perf-vise, we could probably implement a special case here that is better
	i.into_iter().flatten()
	}

	fn has_own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
	own_existential_vtable_entries_iter(tcx, trait_def_id).next().is_some()
	}

	fn own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &[DefId] {
	tcx.arena.alloc_from_iter(own_existential_vtable_entries_iter(tcx, trait_def_id))
	}

	fn own_existential_vtable_entries_iter(
	tcx: TyCtxt<'_>,
	trait_def_id: DefId,
	) -> impl Iterator<Item = DefId> {
	let trait_methods =
	tcx.associated_items(trait_def_id).in_definition_order().filter(\|item\| item.is_fn());

	// Now list each method's DefId (for within its trait).
	let own_entries = trait_methods.filter_map(move \|&trait_method\| {
	debug!("own_existential_vtable_entry: 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_def_id, trait_method) {
	debug!("own_existential_vtable_entry: not vtable safe");
	return None;
	}

	Some(def_id)
	});

	own_entries
	}

	/// Given a trait `trait_ref`, iterates the vtable entries
	/// that come from `trait_ref`, including its supertraits.
	fn vtable_entries<'tcx>(
	tcx: TyCtxt<'tcx>,
	trait_ref: ty::TraitRef<'tcx>,
	) -> &'tcx [VtblEntry<'tcx>] {
	debug_assert!(!trait_ref.has_non_region_infer() && !trait_ref.has_non_region_param());
	debug_assert_eq!(
	tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), trait_ref),
	trait_ref,
	"vtable trait ref should be normalized"
	);

	debug!("vtable_entries({:?})", trait_ref);

	let mut entries = vec![];

	let vtable_segment_callback = \|segment\| -> ControlFlow<()> {
	match segment {
	VtblSegment::MetadataDSA => {
	entries.extend(TyCtxt::COMMON_VTABLE_ENTRIES);
	}
	VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
	let existential_trait_ref = ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref);

	// Lookup the shape of vtable for the trait.
	let own_existential_entries =
	tcx.own_existential_vtable_entries(existential_trait_ref.def_id);

	let own_entries = own_existential_entries.iter().copied().map(\|def_id\| {
	debug!("vtable_entries: trait_method={:?}", def_id);

	// The method may have some early-bound lifetimes; add regions for those.
	// FIXME: Is this normalize needed?
	let args = tcx.normalize_erasing_regions(
	ty::TypingEnv::fully_monomorphized(),
	GenericArgs::for_item(tcx, def_id, \|param, _\| match param.kind {
	GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
	GenericParamDefKind::Type { .. }
	\| GenericParamDefKind::Const { .. } => {
	trait_ref.args[param.index as usize]
	}
	}),
	);

	// It's possible that the method relies on where-clauses that
	// do not hold for this particular set of type parameters.
	// Note that this method could then never be called, so we
	// do not want to try and codegen it, in that case (see #23435).
	let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, args);
	if impossible_predicates(
	tcx,
	predicates.map(\|(predicate, _)\| predicate).collect(),
	) {
	debug!("vtable_entries: predicates do not hold");
	return VtblEntry::Vacant;
	}

	let instance = ty::Instance::expect_resolve_for_vtable(
	tcx,
	ty::TypingEnv::fully_monomorphized(),
	def_id,
	args,
	DUMMY_SP,
	);

	VtblEntry::Method(instance)
	});

	entries.extend(own_entries);

	if emit_vptr {
	entries.push(VtblEntry::TraitVPtr(trait_ref));
	}
	}
	}

	ControlFlow::Continue(())
	};

	let _ = prepare_vtable_segments(tcx, trait_ref, vtable_segment_callback);

	tcx.arena.alloc_from_iter(entries)
	}

	// Given a `dyn Subtrait: Supertrait` trait ref, find corresponding first slot
	// for `Supertrait`'s methods in the vtable of `Subtrait`.
	pub(crate) fn first_method_vtable_slot<'tcx>(tcx: TyCtxt<'tcx>, key: ty::TraitRef<'tcx>) -> usize {
	debug_assert!(!key.has_non_region_infer() && !key.has_non_region_param());
	debug_assert_eq!(
	tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), key),
	key,
	"vtable trait ref should be normalized"
	);

	let ty::Dynamic(source, _, _) = *key.self_ty().kind() else {
	bug!();
	};
	let source_principal = tcx.instantiate_bound_regions_with_erased(
	source.principal().unwrap().with_self_ty(tcx, key.self_ty()),
	);

	// We're monomorphizing a call to a dyn trait object that can never be constructed.
	if tcx.instantiate_and_check_impossible_predicates((
	source_principal.def_id,
	source_principal.args,
	)) {
	return 0;
	}

	let target_principal = ty::ExistentialTraitRef::erase_self_ty(tcx, key);

	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: vtable_principal, emit_vptr } => {
	if ty::ExistentialTraitRef::erase_self_ty(tcx, vtable_principal)
	== target_principal
	{
	return ControlFlow::Break(vptr_offset);
	}

	vptr_offset +=
	tcx.own_existential_vtable_entries(vtable_principal.def_id).len();

	if emit_vptr {
	vptr_offset += 1;
	}
	}
	}
	ControlFlow::Continue(())
	}
	};

	prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap()
	}

	/// Given a `dyn Subtrait` and `dyn Supertrait` trait object, find the slot of
	/// the trait vptr in the subtrait's vtable.
	///
	/// A return value of `None` means that the original vtable can be reused.
	pub(crate) fn supertrait_vtable_slot<'tcx>(
	tcx: TyCtxt<'tcx>,
	key: (
	Ty<'tcx>, // Source -- `dyn Subtrait`.
	Ty<'tcx>, // Target -- `dyn Supertrait` being coerced to.
	),
	) -> Option<usize> {
	debug_assert!(!key.has_non_region_infer() && !key.has_non_region_param());
	debug_assert_eq!(
	tcx.normalize_erasing_regions(ty::TypingEnv::fully_monomorphized(), key),
	key,
	"upcasting trait refs should be normalized"
	);

	let (source, target) = key;

	// If the target principal is `None`, we can just return `None`.
	let ty::Dynamic(target_data, _, _) = *target.kind() else {
	bug!();
	};
	let target_principal = tcx.instantiate_bound_regions_with_erased(target_data.principal()?);

	// Given that we have a target principal, it is a bug for there not to be a source principal.
	let ty::Dynamic(source_data, _, _) = *source.kind() else {
	bug!();
	};
	let source_principal = tcx.instantiate_bound_regions_with_erased(
	source_data.principal().unwrap().with_self_ty(tcx, source),
	);

	// We're monomorphizing a dyn trait object upcast that can never be constructed.
	if tcx.instantiate_and_check_impossible_predicates((
	source_principal.def_id,
	source_principal.args,
	)) {
	return None;
	}

	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: vtable_principal, emit_vptr } => {
	vptr_offset +=
	tcx.own_existential_vtable_entries(vtable_principal.def_id).len();
	if ty::ExistentialTraitRef::erase_self_ty(tcx, vtable_principal)
	== target_principal
	{
	if emit_vptr {
	return ControlFlow::Break(Some(vptr_offset));
	} else {
	return ControlFlow::Break(None);
	}
	}

	if emit_vptr {
	vptr_offset += 1;
	}
	}
	}
	ControlFlow::Continue(())
	}
	};

	prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap()
	}

	pub(super) fn provide(providers: &mut Providers) {
	*providers = Providers {
	own_existential_vtable_entries,
	vtable_entries,
	first_method_vtable_slot,
	supertrait_vtable_slot,
	..*providers
	};
	}