use crate::infer::{InferCtxt, TyOrConstInferVar}; use rustc_data_structures::obligation_forest::ProcessResult; use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome}; use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor}; use rustc_infer::traits::ProjectionCacheKey; use rustc_infer::traits::{SelectionError, TraitEngine, TraitObligation}; use rustc_middle::mir::interpret::ErrorHandled; use rustc_middle::ty::abstract_const::NotConstEvaluatable; use rustc_middle::ty::error::{ExpectedFound, TypeError}; use rustc_middle::ty::subst::SubstsRef; use rustc_middle::ty::{self, Binder, Const, TypeVisitableExt}; use std::marker::PhantomData; use super::const_evaluatable; use super::project::{self, ProjectAndUnifyResult}; use super::select::SelectionContext; use super::wf; use super::CodeAmbiguity; use super::CodeProjectionError; use super::CodeSelectionError; use super::EvaluationResult; use super::PredicateObligation; use super::Unimplemented; use super::{FulfillmentError, FulfillmentErrorCode}; use crate::traits::project::PolyProjectionObligation; use crate::traits::project::ProjectionCacheKeyExt as _; use crate::traits::query::evaluate_obligation::InferCtxtExt; impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> { /// Note that we include both the `ParamEnv` and the `Predicate`, /// as the `ParamEnv` can influence whether fulfillment succeeds /// or fails. type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>; fn as_cache_key(&self) -> Self::CacheKey { self.obligation.param_env.and(self.obligation.predicate) } } /// The fulfillment context is used to drive trait resolution. It /// consists of a list of obligations that must be (eventually) /// satisfied. The job is to track which are satisfied, which yielded /// errors, and which are still pending. At any point, users can call /// `select_where_possible`, and the fulfillment context will try to do /// selection, retaining only those obligations that remain /// ambiguous. This may be helpful in pushing type inference /// along. Once all type inference constraints have been generated, the /// method `select_all_or_error` can be used to report any remaining /// ambiguous cases as errors. pub struct FulfillmentContext<'tcx> { // A list of all obligations that have been registered with this // fulfillment context. predicates: ObligationForest>, // Is it OK to register obligations into this infcx inside // an infcx snapshot? // // The "primary fulfillment" in many cases in typeck lives // outside of any snapshot, so any use of it inside a snapshot // will lead to trouble and therefore is checked against, but // other fulfillment contexts sometimes do live inside of // a snapshot (they don't *straddle* a snapshot, so there // is no trouble there). usable_in_snapshot: bool, } #[derive(Clone, Debug)] pub struct PendingPredicateObligation<'tcx> { pub obligation: PredicateObligation<'tcx>, // This is far more often read than modified, meaning that we // should mostly optimize for reading speed, while modifying is not as relevant. // // For whatever reason using a boxed slice is slower than using a `Vec` here. pub stalled_on: Vec>, } // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger. #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))] static_assert_size!(PendingPredicateObligation<'_>, 72); impl<'a, 'tcx> FulfillmentContext<'tcx> { /// Creates a new fulfillment context. pub(super) fn new() -> FulfillmentContext<'tcx> { FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: false } } pub(super) fn new_in_snapshot() -> FulfillmentContext<'tcx> { FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: true } } /// Attempts to select obligations using `selcx`. fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec> { let span = debug_span!("select", obligation_forest_size = ?self.predicates.len()); let _enter = span.enter(); // Process pending obligations. let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut FulfillProcessor { selcx }); // FIXME: if we kept the original cache key, we could mark projection // obligations as complete for the projection cache here. let errors: Vec> = outcome.errors.into_iter().map(to_fulfillment_error).collect(); debug!( "select({} predicates remaining, {} errors) done", self.predicates.len(), errors.len() ); errors } } impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> { fn register_predicate_obligation( &mut self, infcx: &InferCtxt<'tcx>, obligation: PredicateObligation<'tcx>, ) { // this helps to reduce duplicate errors, as well as making // debug output much nicer to read and so on. let obligation = infcx.resolve_vars_if_possible(obligation); debug!(?obligation, "register_predicate_obligation"); assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot); self.predicates .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] }); } fn collect_remaining_errors(&mut self) -> Vec> { self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect() } fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec> { let selcx = SelectionContext::new(infcx); self.select(selcx) } fn drain_unstalled_obligations( &mut self, infcx: &InferCtxt<'tcx>, ) -> Vec> { let mut processor = DrainProcessor { removed_predicates: Vec::new(), infcx }; let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut processor); assert!(outcome.errors.is_empty()); return processor.removed_predicates; struct DrainProcessor<'a, 'tcx> { infcx: &'a InferCtxt<'tcx>, removed_predicates: Vec>, } impl<'tcx> ObligationProcessor for DrainProcessor<'_, 'tcx> { type Obligation = PendingPredicateObligation<'tcx>; type Error = !; type OUT = Outcome; fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool { pending_obligation .stalled_on .iter() .any(|&var| self.infcx.ty_or_const_infer_var_changed(var)) } fn process_obligation( &mut self, pending_obligation: &mut PendingPredicateObligation<'tcx>, ) -> ProcessResult, !> { assert!(self.needs_process_obligation(pending_obligation)); self.removed_predicates.push(pending_obligation.obligation.clone()); ProcessResult::Changed(vec![]) } fn process_backedge<'c, I>( &mut self, cycle: I, _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>, ) -> Result<(), !> where I: Clone + Iterator>, { self.removed_predicates.extend(cycle.map(|c| c.obligation.clone())); Ok(()) } } } fn pending_obligations(&self) -> Vec> { self.predicates.map_pending_obligations(|o| o.obligation.clone()) } } struct FulfillProcessor<'a, 'tcx> { selcx: SelectionContext<'a, 'tcx>, } fn mk_pending(os: Vec>) -> Vec> { os.into_iter() .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] }) .collect() } impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> { type Obligation = PendingPredicateObligation<'tcx>; type Error = FulfillmentErrorCode<'tcx>; type OUT = Outcome; /// Identifies whether a predicate obligation needs processing. /// /// This is always inlined because it has a single callsite and it is /// called *very* frequently. Be careful modifying this code! Several /// compile-time benchmarks are very sensitive to even small changes. #[inline(always)] fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool { // If we were stalled on some unresolved variables, first check whether // any of them have been resolved; if not, don't bother doing more work // yet. let stalled_on = &pending_obligation.stalled_on; match stalled_on.len() { // This case is the hottest most of the time, being hit up to 99% // of the time. `keccak` and `cranelift-codegen-0.82.1` are // benchmarks that particularly stress this path. 1 => self.selcx.infcx.ty_or_const_infer_var_changed(stalled_on[0]), // In this case we haven't changed, but wish to make a change. Note // that this is a special case, and is not equivalent to the `_` // case below, which would return `false` for an empty `stalled_on` // vector. // // This case is usually hit only 1% of the time or less, though it // reaches 20% in `wasmparser-0.101.0`. 0 => true, // This case is usually hit only 1% of the time or less, though it // reaches 95% in `mime-0.3.16`, 64% in `wast-54.0.0`, and 12% in // `inflate-0.4.5`. // // The obvious way of writing this, with a call to `any()` and no // closure, is currently slower than this version. _ => (|| { for &infer_var in stalled_on { if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) { return true; } } false })(), } } /// Processes a predicate obligation and returns either: /// - `Changed(v)` if the predicate is true, presuming that `v` are also true /// - `Unchanged` if we don't have enough info to be sure /// - `Error(e)` if the predicate does not hold /// /// This is called much less often than `needs_process_obligation`, so we /// never inline it. #[inline(never)] #[instrument(level = "debug", skip(self, pending_obligation))] fn process_obligation( &mut self, pending_obligation: &mut PendingPredicateObligation<'tcx>, ) -> ProcessResult, FulfillmentErrorCode<'tcx>> { pending_obligation.stalled_on.truncate(0); let obligation = &mut pending_obligation.obligation; debug!(?obligation, "pre-resolve"); if obligation.predicate.has_non_region_infer() { obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate); } let obligation = &pending_obligation.obligation; let infcx = self.selcx.infcx; if obligation.predicate.has_projections() { let mut obligations = Vec::new(); let predicate = crate::traits::project::try_normalize_with_depth_to( &mut self.selcx, obligation.param_env, obligation.cause.clone(), obligation.recursion_depth + 1, obligation.predicate, &mut obligations, ); if predicate != obligation.predicate { obligations.push(obligation.with(infcx.tcx, predicate)); return ProcessResult::Changed(mk_pending(obligations)); } } let binder = obligation.predicate.kind(); match binder.no_bound_vars() { None => match binder.skip_binder() { // Evaluation will discard candidates using the leak check. // This means we need to pass it the bound version of our // predicate. ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => { let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref)); self.process_trait_obligation( obligation, trait_obligation, &mut pending_obligation.stalled_on, ) } ty::PredicateKind::Clause(ty::Clause::Projection(data)) => { let project_obligation = obligation.with(infcx.tcx, binder.rebind(data)); self.process_projection_obligation( obligation, project_obligation, &mut pending_obligation.stalled_on, ) } ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_)) | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_)) | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..)) | ty::PredicateKind::WellFormed(_) | ty::PredicateKind::ObjectSafe(_) | ty::PredicateKind::ClosureKind(..) | ty::PredicateKind::Subtype(_) | ty::PredicateKind::Coerce(_) | ty::PredicateKind::ConstEvaluatable(..) | ty::PredicateKind::ConstEquate(..) => { let pred = ty::Binder::dummy(infcx.instantiate_binder_with_placeholders(binder)); ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)])) } ty::PredicateKind::Ambiguous => ProcessResult::Unchanged, ty::PredicateKind::TypeWellFormedFromEnv(..) => { bug!("TypeWellFormedFromEnv is only used for Chalk") } ty::PredicateKind::AliasEq(..) => { bug!("AliasEq is only used for new solver") } }, Some(pred) => match pred { ty::PredicateKind::Clause(ty::Clause::Trait(data)) => { let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data)); self.process_trait_obligation( obligation, trait_obligation, &mut pending_obligation.stalled_on, ) } ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => { if infcx.considering_regions { infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)); } ProcessResult::Changed(vec![]) } ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate( t_a, r_b, ))) => { if infcx.considering_regions { infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause); } ProcessResult::Changed(vec![]) } ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => { let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data)); self.process_projection_obligation( obligation, project_obligation, &mut pending_obligation.stalled_on, ) } ty::PredicateKind::ObjectSafe(trait_def_id) => { if !self.selcx.tcx().check_is_object_safe(trait_def_id) { ProcessResult::Error(CodeSelectionError(Unimplemented)) } else { ProcessResult::Changed(vec![]) } } ty::PredicateKind::ClosureKind(_, closure_substs, kind) => { match self.selcx.infcx.closure_kind(closure_substs) { Some(closure_kind) => { if closure_kind.extends(kind) { ProcessResult::Changed(vec![]) } else { ProcessResult::Error(CodeSelectionError(Unimplemented)) } } None => ProcessResult::Unchanged, } } ty::PredicateKind::WellFormed(arg) => { match wf::obligations( self.selcx.infcx, obligation.param_env, obligation.cause.body_id, obligation.recursion_depth + 1, arg, obligation.cause.span, ) { None => { pending_obligation.stalled_on = vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()]; ProcessResult::Unchanged } Some(os) => ProcessResult::Changed(mk_pending(os)), } } ty::PredicateKind::Subtype(subtype) => { match self.selcx.infcx.subtype_predicate( &obligation.cause, obligation.param_env, Binder::dummy(subtype), ) { Err((a, b)) => { // None means that both are unresolved. pending_obligation.stalled_on = vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)]; ProcessResult::Unchanged } Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)), Ok(Err(err)) => { let expected_found = ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b); ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError( expected_found, err, )) } } } ty::PredicateKind::Coerce(coerce) => { match self.selcx.infcx.coerce_predicate( &obligation.cause, obligation.param_env, Binder::dummy(coerce), ) { Err((a, b)) => { // None means that both are unresolved. pending_obligation.stalled_on = vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)]; ProcessResult::Unchanged } Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)), Ok(Err(err)) => { let expected_found = ExpectedFound::new(false, coerce.a, coerce.b); ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError( expected_found, err, )) } } } ty::PredicateKind::ConstEvaluatable(uv) => { match const_evaluatable::is_const_evaluatable( self.selcx.infcx, uv, obligation.param_env, obligation.cause.span, ) { Ok(()) => ProcessResult::Changed(vec![]), Err(NotConstEvaluatable::MentionsInfer) => { pending_obligation.stalled_on.clear(); pending_obligation.stalled_on.extend( uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg), ); ProcessResult::Unchanged } Err( e @ NotConstEvaluatable::MentionsParam | e @ NotConstEvaluatable::Error(_), ) => ProcessResult::Error(CodeSelectionError( SelectionError::NotConstEvaluatable(e), )), } } ty::PredicateKind::ConstEquate(c1, c2) => { let tcx = self.selcx.tcx(); assert!( tcx.features().generic_const_exprs, "`ConstEquate` without a feature gate: {c1:?} {c2:?}", ); // FIXME: we probably should only try to unify abstract constants // if the constants depend on generic parameters. // // Let's just see where this breaks :shrug: { let c1 = tcx.expand_abstract_consts(c1); let c2 = tcx.expand_abstract_consts(c2); debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2); use rustc_hir::def::DefKind; use ty::ConstKind::Unevaluated; match (c1.kind(), c2.kind()) { (Unevaluated(a), Unevaluated(b)) if a.def.did == b.def.did && tcx.def_kind(a.def.did) == DefKind::AssocConst => { if let Ok(new_obligations) = infcx .at(&obligation.cause, obligation.param_env) .trace(c1, c2) .eq(a.substs, b.substs) { return ProcessResult::Changed(mk_pending( new_obligations.into_obligations(), )); } } (_, Unevaluated(_)) | (Unevaluated(_), _) => (), (_, _) => { if let Ok(new_obligations) = infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2) { return ProcessResult::Changed(mk_pending( new_obligations.into_obligations(), )); } } } } let stalled_on = &mut pending_obligation.stalled_on; let mut evaluate = |c: Const<'tcx>| { if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() { match self.selcx.infcx.try_const_eval_resolve( obligation.param_env, unevaluated, c.ty(), Some(obligation.cause.span), ) { Ok(val) => Ok(val), Err(e) => match e { ErrorHandled::TooGeneric => { stalled_on.extend( unevaluated.substs.iter().filter_map( TyOrConstInferVar::maybe_from_generic_arg, ), ); Err(ErrorHandled::TooGeneric) } _ => Err(e), }, } } else { Ok(c) } }; match (evaluate(c1), evaluate(c2)) { (Ok(c1), Ok(c2)) => { match self .selcx .infcx .at(&obligation.cause, obligation.param_env) .eq(c1, c2) { Ok(inf_ok) => { ProcessResult::Changed(mk_pending(inf_ok.into_obligations())) } Err(err) => ProcessResult::Error( FulfillmentErrorCode::CodeConstEquateError( ExpectedFound::new(true, c1, c2), err, ), ), } } (Err(ErrorHandled::Reported(reported)), _) | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error( CodeSelectionError(SelectionError::NotConstEvaluatable( NotConstEvaluatable::Error(reported), )), ), (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => { if c1.has_non_region_infer() || c2.has_non_region_infer() { ProcessResult::Unchanged } else { // Two different constants using generic parameters ~> error. let expected_found = ExpectedFound::new(true, c1, c2); ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError( expected_found, TypeError::ConstMismatch(expected_found), )) } } } } ty::PredicateKind::Ambiguous => ProcessResult::Unchanged, ty::PredicateKind::TypeWellFormedFromEnv(..) => { bug!("TypeWellFormedFromEnv is only used for Chalk") } ty::PredicateKind::AliasEq(..) => { bug!("AliasEq is only used for new solver") } ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => { match self .selcx .infcx .at(&obligation.cause, obligation.param_env) .eq(ct.ty(), ty) { Ok(inf_ok) => ProcessResult::Changed(mk_pending(inf_ok.into_obligations())), Err(_) => ProcessResult::Error(FulfillmentErrorCode::CodeSelectionError( SelectionError::Unimplemented, )), } } }, } } #[inline(never)] fn process_backedge<'c, I>( &mut self, cycle: I, _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>, ) -> Result<(), FulfillmentErrorCode<'tcx>> where I: Clone + Iterator>, { if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) { debug!("process_child_obligations: coinductive match"); Ok(()) } else { let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect(); Err(FulfillmentErrorCode::CodeCycle(cycle)) } } } impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> { #[instrument(level = "debug", skip(self, obligation, stalled_on))] fn process_trait_obligation( &mut self, obligation: &PredicateObligation<'tcx>, trait_obligation: TraitObligation<'tcx>, stalled_on: &mut Vec>, ) -> ProcessResult, FulfillmentErrorCode<'tcx>> { let infcx = self.selcx.infcx; if obligation.predicate.is_global() { // no type variables present, can use evaluation for better caching. // FIXME: consider caching errors too. if infcx.predicate_must_hold_considering_regions(obligation) { debug!( "selecting trait at depth {} evaluated to holds", obligation.recursion_depth ); return ProcessResult::Changed(vec![]); } } match self.selcx.select(&trait_obligation) { Ok(Some(impl_source)) => { debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth); ProcessResult::Changed(mk_pending(impl_source.nested_obligations())) } Ok(None) => { debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth); // This is a bit subtle: for the most part, the // only reason we can fail to make progress on // trait selection is because we don't have enough // information about the types in the trait. stalled_on.clear(); stalled_on.extend(substs_infer_vars( &self.selcx, trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs), )); debug!( "process_predicate: pending obligation {:?} now stalled on {:?}", infcx.resolve_vars_if_possible(obligation.clone()), stalled_on ); ProcessResult::Unchanged } Err(selection_err) => { debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth); ProcessResult::Error(CodeSelectionError(selection_err)) } } } fn process_projection_obligation( &mut self, obligation: &PredicateObligation<'tcx>, project_obligation: PolyProjectionObligation<'tcx>, stalled_on: &mut Vec>, ) -> ProcessResult, FulfillmentErrorCode<'tcx>> { let tcx = self.selcx.tcx(); if obligation.predicate.is_global() { // no type variables present, can use evaluation for better caching. // FIXME: consider caching errors too. if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) { if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate( &mut self.selcx, project_obligation.predicate, ) { // If `predicate_must_hold_considering_regions` succeeds, then we've // evaluated all sub-obligations. We can therefore mark the 'root' // obligation as complete, and skip evaluating sub-obligations. self.selcx .infcx .inner .borrow_mut() .projection_cache() .complete(key, EvaluationResult::EvaluatedToOk); } return ProcessResult::Changed(vec![]); } else { debug!("Does NOT hold: {:?}", obligation); } } match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) { ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)), ProjectAndUnifyResult::FailedNormalization => { stalled_on.clear(); stalled_on.extend(substs_infer_vars( &self.selcx, project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs), )); ProcessResult::Unchanged } // Let the caller handle the recursion ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![ project_obligation.with(tcx, project_obligation.predicate), ])), ProjectAndUnifyResult::MismatchedProjectionTypes(e) => { ProcessResult::Error(CodeProjectionError(e)) } } } } /// Returns the set of inference variables contained in `substs`. fn substs_infer_vars<'a, 'tcx>( selcx: &SelectionContext<'a, 'tcx>, substs: ty::Binder<'tcx, SubstsRef<'tcx>>, ) -> impl Iterator> { selcx .infcx .resolve_vars_if_possible(substs) .skip_binder() // ok because this check doesn't care about regions .iter() .filter(|arg| arg.has_non_region_infer()) .flat_map(|arg| { let mut walker = arg.walk(); while let Some(c) = walker.next() { if !c.has_non_region_infer() { walker.visited.remove(&c); walker.skip_current_subtree(); } } walker.visited.into_iter() }) .filter_map(TyOrConstInferVar::maybe_from_generic_arg) } fn to_fulfillment_error<'tcx>( error: Error, FulfillmentErrorCode<'tcx>>, ) -> FulfillmentError<'tcx> { let mut iter = error.backtrace.into_iter(); let obligation = iter.next().unwrap().obligation; // The root obligation is the last item in the backtrace - if there's only // one item, then it's the same as the main obligation let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation); FulfillmentError::new(obligation, error.error, root_obligation) }