src/librustc_trait_selection/traits/util.rs - third_party/rust - Git at Google

 use rustc_errors::DiagnosticBuilder;
 use rustc_span::Span;
 use smallvec::smallvec;
 use smallvec::SmallVec;

 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, WithConstness};

 use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
 pub use rustc_infer::traits::util::*;

 ///////////////////////////////////////////////////////////////////////////
 // `TraitAliasExpander` iterator
 ///////////////////////////////////////////////////////////////////////////

 /// "Trait alias expansion" is the process of expanding a sequence of trait
 /// references into another sequence by transitively following all trait
 /// aliases. e.g. If you have bounds like `Foo + Send`, a trait alias
 /// `trait Foo = Bar + Sync;`, and another trait alias
 /// `trait Bar = Read + Write`, then the bounds would expand to
 /// `Read + Write + Sync + Send`.
 /// Expansion is done via a DFS (depth-first search), and the `visited` field
 /// is used to avoid cycles.
 pub struct TraitAliasExpander<'tcx> {
     tcx: TyCtxt<'tcx>,
     stack: Vec<TraitAliasExpansionInfo<'tcx>>,
 }

 /// Stores information about the expansion of a trait via a path of zero or more trait aliases.
 #[derive(Debug, Clone)]
 pub struct TraitAliasExpansionInfo<'tcx> {
     pub path: SmallVec<[(ty::PolyTraitRef<'tcx>, Span); 4]>,
 }

 impl<'tcx> TraitAliasExpansionInfo<'tcx> {
     fn new(trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
         Self { path: smallvec![(trait_ref, span)] }
     }

     /// Adds diagnostic labels to `diag` for the expansion path of a trait through all intermediate
     /// trait aliases.
     pub fn label_with_exp_info(
         &self,
         diag: &mut DiagnosticBuilder<'_>,
         top_label: &str,
         use_desc: &str,
     ) {
         diag.span_label(self.top().1, top_label);
         if self.path.len() > 1 {
             for (_, sp) in self.path.iter().rev().skip(1).take(self.path.len() - 2) {
                 diag.span_label(*sp, format!("referenced here ({})", use_desc));
             }
         }
         diag.span_label(
             self.bottom().1,
             format!("trait alias used in trait object type ({})", use_desc),
         );
     }

     pub fn trait_ref(&self) -> &ty::PolyTraitRef<'tcx> {
         &self.top().0
     }

     pub fn top(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
         self.path.last().unwrap()
     }

     pub fn bottom(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
         self.path.first().unwrap()
     }

     fn clone_and_push(&self, trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
         let mut path = self.path.clone();
         path.push((trait_ref, span));

         Self { path }
     }
 }

 pub fn expand_trait_aliases<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_refs: impl Iterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
 ) -> TraitAliasExpander<'tcx> {
     let items: Vec<_> =
         trait_refs.map(|(trait_ref, span)| TraitAliasExpansionInfo::new(trait_ref, span)).collect();
     TraitAliasExpander { tcx, stack: items }
 }

 impl<'tcx> TraitAliasExpander<'tcx> {
     /// If `item` is a trait alias and its predicate has not yet been visited, then expands `item`
     /// to the definition, pushes the resulting expansion onto `self.stack`, and returns `false`.
     /// Otherwise, immediately returns `true` if `item` is a regular trait, or `false` if it is a
     /// trait alias.
     /// The return value indicates whether `item` should be yielded to the user.
     fn expand(&mut self, item: &TraitAliasExpansionInfo<'tcx>) -> bool {
         let tcx = self.tcx;
         let trait_ref = item.trait_ref();
         let pred = trait_ref.without_const().to_predicate(tcx);

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

         // Don't recurse if this bound is not a trait alias.
         let is_alias = tcx.is_trait_alias(trait_ref.def_id());
         if !is_alias {
             return true;
         }

         // Don't recurse if this trait alias is already on the stack for the DFS search.
         let anon_pred = anonymize_predicate(tcx, pred);
         if item.path.iter().rev().skip(1).any(|(tr, _)| {
             anonymize_predicate(tcx, tr.without_const().to_predicate(tcx)) == anon_pred
         }) {
             return false;
         }

         // Get components of trait alias.
         let predicates = tcx.super_predicates_of(trait_ref.def_id());

         let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
             pred.subst_supertrait(tcx, &trait_ref)
                 .to_opt_poly_trait_ref()
                 .map(|trait_ref| item.clone_and_push(trait_ref, *span))
         });
         debug!("expand_trait_aliases: items={:?}", items.clone());

         self.stack.extend(items);

         false
     }
 }

 impl<'tcx> Iterator for TraitAliasExpander<'tcx> {
     type Item = TraitAliasExpansionInfo<'tcx>;

     fn size_hint(&self) -> (usize, Option<usize>) {
         (self.stack.len(), None)
     }

     fn next(&mut self) -> Option<TraitAliasExpansionInfo<'tcx>> {
         while let Some(item) = self.stack.pop() {
             if self.expand(&item) {
                 return Some(item);
             }
         }
         None
     }
 }

 ///////////////////////////////////////////////////////////////////////////
 // Iterator over def-IDs of supertraits
 ///////////////////////////////////////////////////////////////////////////

 pub struct SupertraitDefIds<'tcx> {
     tcx: TyCtxt<'tcx>,
     stack: Vec<DefId>,
     visited: FxHashSet<DefId>,
 }

 pub 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<'tcx> {
     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.to_opt_poly_trait_ref())
                 .map(|trait_ref| trait_ref.def_id())
                 .filter(|&super_def_id| visited.insert(super_def_id)),
         );
         Some(def_id)
     }
 }

 ///////////////////////////////////////////////////////////////////////////
 // Other
 ///////////////////////////////////////////////////////////////////////////

 /// Instantiate all bound parameters of the impl with the given substs,
 /// returning the resulting trait ref and all obligations that arise.
 /// The obligations are closed under normalization.
 pub fn impl_trait_ref_and_oblig<'a, 'tcx>(
     selcx: &mut SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_substs: SubstsRef<'tcx>,
 ) -> (ty::TraitRef<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
     let impl_trait_ref = selcx.tcx().impl_trait_ref(impl_def_id).unwrap();
     let impl_trait_ref = impl_trait_ref.subst(selcx.tcx(), impl_substs);
     let Normalized { value: impl_trait_ref, obligations: normalization_obligations1 } =
         super::normalize(selcx, param_env, ObligationCause::dummy(), &impl_trait_ref);

     let predicates = selcx.tcx().predicates_of(impl_def_id);
     let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
     let Normalized { value: predicates, obligations: normalization_obligations2 } =
         super::normalize(selcx, param_env, ObligationCause::dummy(), &predicates);
     let impl_obligations =
         predicates_for_generics(ObligationCause::dummy(), 0, param_env, predicates);

     let impl_obligations = impl_obligations
         .chain(normalization_obligations1.into_iter())
         .chain(normalization_obligations2.into_iter());

     (impl_trait_ref, impl_obligations)
 }

 pub fn predicates_for_generics<'tcx>(
     cause: ObligationCause<'tcx>,
     recursion_depth: usize,
     param_env: ty::ParamEnv<'tcx>,
     generic_bounds: ty::InstantiatedPredicates<'tcx>,
 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
     debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);

     generic_bounds.predicates.into_iter().map(move |predicate| Obligation {
         cause: cause.clone(),
         recursion_depth,
         param_env,
         predicate,
     })
 }

 pub fn predicate_for_trait_ref<'tcx>(
     tcx: TyCtxt<'tcx>,
     cause: ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
     recursion_depth: usize,
 ) -> PredicateObligation<'tcx> {
     Obligation {
         cause,
         param_env,
         recursion_depth,
         predicate: trait_ref.without_const().to_predicate(tcx),
     }
 }

 pub fn predicate_for_trait_def(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     cause: ObligationCause<'tcx>,
     trait_def_id: DefId,
     recursion_depth: usize,
     self_ty: Ty<'tcx>,
     params: &[GenericArg<'tcx>],
 ) -> PredicateObligation<'tcx> {
     let trait_ref =
         ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(self_ty, params) };
     predicate_for_trait_ref(tcx, cause, param_env, trait_ref, recursion_depth)
 }

 /// Casts a trait reference into a reference to one of its super
 /// traits; returns `None` if `target_trait_def_id` is not a
 /// supertrait.
 pub fn upcast_choices(
     tcx: TyCtxt<'tcx>,
     source_trait_ref: ty::PolyTraitRef<'tcx>,
     target_trait_def_id: DefId,
 ) -> Vec<ty::PolyTraitRef<'tcx>> {
     if source_trait_ref.def_id() == target_trait_def_id {
         return vec![source_trait_ref]; // Shortcut the most common case.
     }

     supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
 }

 /// Given a trait `trait_ref`, returns the number of vtable entries
 /// that come from `trait_ref`, excluding its supertraits. Used in
 /// computing the vtable base for an upcast trait of a trait object.
 pub fn count_own_vtable_entries(tcx: TyCtxt<'tcx>, trait_ref: ty::PolyTraitRef<'tcx>) -> usize {
     let mut entries = 0;
     // Count number of methods and add them to the total offset.
     // Skip over associated types and constants.
     for trait_item in tcx.associated_items(trait_ref.def_id()).in_definition_order() {
         if trait_item.kind == ty::AssocKind::Fn {
             entries += 1;
         }
     }
     entries
 }

 /// 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<N>(
     tcx: TyCtxt<'tcx>,
     object: &super::ImplSourceObjectData<'tcx, N>,
     method_def_id: DefId,
 ) -> usize {
     // Count number of methods preceding the one we are selecting and
     // add them to the total offset.
     // Skip over associated types and constants, as those aren't stored in the vtable.
     let mut entries = object.vtable_base;
     for trait_item in tcx.associated_items(object.upcast_trait_ref.def_id()).in_definition_order() {
         if trait_item.def_id == method_def_id {
             // The item with the ID we were given really ought to be a method.
             assert_eq!(trait_item.kind, ty::AssocKind::Fn);
             return entries;
         }
         if trait_item.kind == ty::AssocKind::Fn {
             entries += 1;
         }
     }

     bug!("get_vtable_index_of_object_method: {:?} was not found", method_def_id);
 }

 pub fn closure_trait_ref_and_return_type(
     tcx: TyCtxt<'tcx>,
     fn_trait_def_id: DefId,
     self_ty: Ty<'tcx>,
     sig: ty::PolyFnSig<'tcx>,
     tuple_arguments: TupleArgumentsFlag,
 ) -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>)> {
     let arguments_tuple = match tuple_arguments {
         TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
         TupleArgumentsFlag::Yes => tcx.intern_tup(sig.skip_binder().inputs()),
     };
     let trait_ref = ty::TraitRef {
         def_id: fn_trait_def_id,
         substs: tcx.mk_substs_trait(self_ty, &[arguments_tuple.into()]),
     };
     ty::Binder::bind((trait_ref, sig.skip_binder().output()))
 }

 pub fn generator_trait_ref_and_outputs(
     tcx: TyCtxt<'tcx>,
     fn_trait_def_id: DefId,
     self_ty: Ty<'tcx>,
     sig: ty::PolyGenSig<'tcx>,
 ) -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)> {
     let trait_ref = ty::TraitRef {
         def_id: fn_trait_def_id,
         substs: tcx.mk_substs_trait(self_ty, &[sig.skip_binder().resume_ty.into()]),
     };
     ty::Binder::bind((trait_ref, sig.skip_binder().yield_ty, sig.skip_binder().return_ty))
 }

 pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
     assoc_item.defaultness.is_final() && tcx.impl_defaultness(assoc_item.container.id()).is_final()
 }

 pub enum TupleArgumentsFlag {
     Yes,
     No,
 }
	use rustc_errors::DiagnosticBuilder;
	use rustc_span::Span;
	use smallvec::smallvec;
	use smallvec::SmallVec;

	use rustc_data_structures::fx::FxHashSet;
	use rustc_hir::def_id::DefId;
	use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
	use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, WithConstness};

	use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
	pub use rustc_infer::traits::util::*;

	///////////////////////////////////////////////////////////////////////////
	// `TraitAliasExpander` iterator
	///////////////////////////////////////////////////////////////////////////

	/// "Trait alias expansion" is the process of expanding a sequence of trait
	/// references into another sequence by transitively following all trait
	/// aliases. e.g. If you have bounds like `Foo + Send`, a trait alias
	/// `trait Foo = Bar + Sync;`, and another trait alias
	/// `trait Bar = Read + Write`, then the bounds would expand to
	/// `Read + Write + Sync + Send`.
	/// Expansion is done via a DFS (depth-first search), and the `visited` field
	/// is used to avoid cycles.
	pub struct TraitAliasExpander<'tcx> {
	tcx: TyCtxt<'tcx>,
	stack: Vec<TraitAliasExpansionInfo<'tcx>>,
	}

	/// Stores information about the expansion of a trait via a path of zero or more trait aliases.
	#[derive(Debug, Clone)]
	pub struct TraitAliasExpansionInfo<'tcx> {
	pub path: SmallVec<[(ty::PolyTraitRef<'tcx>, Span); 4]>,
	}

	impl<'tcx> TraitAliasExpansionInfo<'tcx> {
	fn new(trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
	Self { path: smallvec![(trait_ref, span)] }
	}

	/// Adds diagnostic labels to `diag` for the expansion path of a trait through all intermediate
	/// trait aliases.
	pub fn label_with_exp_info(
	&self,
	diag: &mut DiagnosticBuilder<'_>,
	top_label: &str,
	use_desc: &str,
	) {
	diag.span_label(self.top().1, top_label);
	if self.path.len() > 1 {
	for (_, sp) in self.path.iter().rev().skip(1).take(self.path.len() - 2) {
	diag.span_label(*sp, format!("referenced here ({})", use_desc));
	}
	}
	diag.span_label(
	self.bottom().1,
	format!("trait alias used in trait object type ({})", use_desc),
	);
	}

	pub fn trait_ref(&self) -> &ty::PolyTraitRef<'tcx> {
	&self.top().0
	}

	pub fn top(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
	self.path.last().unwrap()
	}

	pub fn bottom(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
	self.path.first().unwrap()
	}

	fn clone_and_push(&self, trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
	let mut path = self.path.clone();
	path.push((trait_ref, span));

	Self { path }
	}
	}

	pub fn expand_trait_aliases<'tcx>(
	tcx: TyCtxt<'tcx>,
	trait_refs: impl Iterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
	) -> TraitAliasExpander<'tcx> {
	let items: Vec<_> =
	trait_refs.map(\|(trait_ref, span)\| TraitAliasExpansionInfo::new(trait_ref, span)).collect();
	TraitAliasExpander { tcx, stack: items }
	}

	impl<'tcx> TraitAliasExpander<'tcx> {
	/// If `item` is a trait alias and its predicate has not yet been visited, then expands `item`
	/// to the definition, pushes the resulting expansion onto `self.stack`, and returns `false`.
	/// Otherwise, immediately returns `true` if `item` is a regular trait, or `false` if it is a
	/// trait alias.
	/// The return value indicates whether `item` should be yielded to the user.
	fn expand(&mut self, item: &TraitAliasExpansionInfo<'tcx>) -> bool {
	let tcx = self.tcx;
	let trait_ref = item.trait_ref();
	let pred = trait_ref.without_const().to_predicate(tcx);

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

	// Don't recurse if this bound is not a trait alias.
	let is_alias = tcx.is_trait_alias(trait_ref.def_id());
	if !is_alias {
	return true;
	}

	// Don't recurse if this trait alias is already on the stack for the DFS search.
	let anon_pred = anonymize_predicate(tcx, pred);
	if item.path.iter().rev().skip(1).any(\|(tr, _)\| {
	anonymize_predicate(tcx, tr.without_const().to_predicate(tcx)) == anon_pred
	}) {
	return false;
	}

	// Get components of trait alias.
	let predicates = tcx.super_predicates_of(trait_ref.def_id());

	let items = predicates.predicates.iter().rev().filter_map(\|(pred, span)\| {
	pred.subst_supertrait(tcx, &trait_ref)
	.to_opt_poly_trait_ref()
	.map(\|trait_ref\| item.clone_and_push(trait_ref, *span))
	});
	debug!("expand_trait_aliases: items={:?}", items.clone());

	self.stack.extend(items);

	false
	}
	}

	impl<'tcx> Iterator for TraitAliasExpander<'tcx> {
	type Item = TraitAliasExpansionInfo<'tcx>;

	fn size_hint(&self) -> (usize, Option<usize>) {
	(self.stack.len(), None)
	}

	fn next(&mut self) -> Option<TraitAliasExpansionInfo<'tcx>> {
	while let Some(item) = self.stack.pop() {
	if self.expand(&item) {
	return Some(item);
	}
	}
	None
	}
	}

	///////////////////////////////////////////////////////////////////////////
	// Iterator over def-IDs of supertraits
	///////////////////////////////////////////////////////////////////////////

	pub struct SupertraitDefIds<'tcx> {
	tcx: TyCtxt<'tcx>,
	stack: Vec<DefId>,
	visited: FxHashSet<DefId>,
	}

	pub 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<'tcx> {
	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.to_opt_poly_trait_ref())
	.map(\|trait_ref\| trait_ref.def_id())
	.filter(\|&super_def_id\| visited.insert(super_def_id)),
	);
	Some(def_id)
	}
	}

	///////////////////////////////////////////////////////////////////////////
	// Other
	///////////////////////////////////////////////////////////////////////////

	/// Instantiate all bound parameters of the impl with the given substs,
	/// returning the resulting trait ref and all obligations that arise.
	/// The obligations are closed under normalization.
	pub fn impl_trait_ref_and_oblig<'a, 'tcx>(
	selcx: &mut SelectionContext<'a, 'tcx>,
	param_env: ty::ParamEnv<'tcx>,
	impl_def_id: DefId,
	impl_substs: SubstsRef<'tcx>,
	) -> (ty::TraitRef<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
	let impl_trait_ref = selcx.tcx().impl_trait_ref(impl_def_id).unwrap();
	let impl_trait_ref = impl_trait_ref.subst(selcx.tcx(), impl_substs);
	let Normalized { value: impl_trait_ref, obligations: normalization_obligations1 } =
	super::normalize(selcx, param_env, ObligationCause::dummy(), &impl_trait_ref);

	let predicates = selcx.tcx().predicates_of(impl_def_id);
	let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
	let Normalized { value: predicates, obligations: normalization_obligations2 } =
	super::normalize(selcx, param_env, ObligationCause::dummy(), &predicates);
	let impl_obligations =
	predicates_for_generics(ObligationCause::dummy(), 0, param_env, predicates);

	let impl_obligations = impl_obligations
	.chain(normalization_obligations1.into_iter())
	.chain(normalization_obligations2.into_iter());

	(impl_trait_ref, impl_obligations)
	}

	pub fn predicates_for_generics<'tcx>(
	cause: ObligationCause<'tcx>,
	recursion_depth: usize,
	param_env: ty::ParamEnv<'tcx>,
	generic_bounds: ty::InstantiatedPredicates<'tcx>,
	) -> impl Iterator<Item = PredicateObligation<'tcx>> {
	debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);

	generic_bounds.predicates.into_iter().map(move \|predicate\| Obligation {
	cause: cause.clone(),
	recursion_depth,
	param_env,
	predicate,
	})
	}

	pub fn predicate_for_trait_ref<'tcx>(
	tcx: TyCtxt<'tcx>,
	cause: ObligationCause<'tcx>,
	param_env: ty::ParamEnv<'tcx>,
	trait_ref: ty::TraitRef<'tcx>,
	recursion_depth: usize,
	) -> PredicateObligation<'tcx> {
	Obligation {
	cause,
	param_env,
	recursion_depth,
	predicate: trait_ref.without_const().to_predicate(tcx),
	}
	}

	pub fn predicate_for_trait_def(
	tcx: TyCtxt<'tcx>,
	param_env: ty::ParamEnv<'tcx>,
	cause: ObligationCause<'tcx>,
	trait_def_id: DefId,
	recursion_depth: usize,
	self_ty: Ty<'tcx>,
	params: &[GenericArg<'tcx>],
	) -> PredicateObligation<'tcx> {
	let trait_ref =
	ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(self_ty, params) };
	predicate_for_trait_ref(tcx, cause, param_env, trait_ref, recursion_depth)
	}

	/// Casts a trait reference into a reference to one of its super
	/// traits; returns `None` if `target_trait_def_id` is not a
	/// supertrait.
	pub fn upcast_choices(
	tcx: TyCtxt<'tcx>,
	source_trait_ref: ty::PolyTraitRef<'tcx>,
	target_trait_def_id: DefId,
	) -> Vec<ty::PolyTraitRef<'tcx>> {
	if source_trait_ref.def_id() == target_trait_def_id {
	return vec![source_trait_ref]; // Shortcut the most common case.
	}

	supertraits(tcx, source_trait_ref).filter(\|r\| r.def_id() == target_trait_def_id).collect()
	}

	/// Given a trait `trait_ref`, returns the number of vtable entries
	/// that come from `trait_ref`, excluding its supertraits. Used in
	/// computing the vtable base for an upcast trait of a trait object.
	pub fn count_own_vtable_entries(tcx: TyCtxt<'tcx>, trait_ref: ty::PolyTraitRef<'tcx>) -> usize {
	let mut entries = 0;
	// Count number of methods and add them to the total offset.
	// Skip over associated types and constants.
	for trait_item in tcx.associated_items(trait_ref.def_id()).in_definition_order() {
	if trait_item.kind == ty::AssocKind::Fn {
	entries += 1;
	}
	}
	entries
	}

	/// 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<N>(
	tcx: TyCtxt<'tcx>,
	object: &super::ImplSourceObjectData<'tcx, N>,
	method_def_id: DefId,
	) -> usize {
	// Count number of methods preceding the one we are selecting and
	// add them to the total offset.
	// Skip over associated types and constants, as those aren't stored in the vtable.
	let mut entries = object.vtable_base;
	for trait_item in tcx.associated_items(object.upcast_trait_ref.def_id()).in_definition_order() {
	if trait_item.def_id == method_def_id {
	// The item with the ID we were given really ought to be a method.
	assert_eq!(trait_item.kind, ty::AssocKind::Fn);
	return entries;
	}
	if trait_item.kind == ty::AssocKind::Fn {
	entries += 1;
	}
	}

	bug!("get_vtable_index_of_object_method: {:?} was not found", method_def_id);
	}

	pub fn closure_trait_ref_and_return_type(
	tcx: TyCtxt<'tcx>,
	fn_trait_def_id: DefId,
	self_ty: Ty<'tcx>,
	sig: ty::PolyFnSig<'tcx>,
	tuple_arguments: TupleArgumentsFlag,
	) -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>)> {
	let arguments_tuple = match tuple_arguments {
	TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
	TupleArgumentsFlag::Yes => tcx.intern_tup(sig.skip_binder().inputs()),
	};
	let trait_ref = ty::TraitRef {
	def_id: fn_trait_def_id,
	substs: tcx.mk_substs_trait(self_ty, &[arguments_tuple.into()]),
	};
	ty::Binder::bind((trait_ref, sig.skip_binder().output()))
	}

	pub fn generator_trait_ref_and_outputs(
	tcx: TyCtxt<'tcx>,
	fn_trait_def_id: DefId,
	self_ty: Ty<'tcx>,
	sig: ty::PolyGenSig<'tcx>,
	) -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)> {
	let trait_ref = ty::TraitRef {
	def_id: fn_trait_def_id,
	substs: tcx.mk_substs_trait(self_ty, &[sig.skip_binder().resume_ty.into()]),
	};
	ty::Binder::bind((trait_ref, sig.skip_binder().yield_ty, sig.skip_binder().return_ty))
	}

	pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
	assoc_item.defaultness.is_final() && tcx.impl_defaultness(assoc_item.container.id()).is_final()
	}

	pub enum TupleArgumentsFlag {
	Yes,
	No,
	}