diff options
Diffstat (limited to 'compiler/rustc_traits/src/implied_outlives_bounds.rs')
| -rw-r--r-- | compiler/rustc_traits/src/implied_outlives_bounds.rs | 69 |
1 files changed, 36 insertions, 33 deletions
diff --git a/compiler/rustc_traits/src/implied_outlives_bounds.rs b/compiler/rustc_traits/src/implied_outlives_bounds.rs index e3e78f70b..691b79f10 100644 --- a/compiler/rustc_traits/src/implied_outlives_bounds.rs +++ b/compiler/rustc_traits/src/implied_outlives_bounds.rs @@ -49,7 +49,8 @@ fn compute_implied_outlives_bounds<'tcx>( let mut checked_wf_args = rustc_data_structures::fx::FxHashSet::default(); let mut wf_args = vec![ty.into()]; - let mut implied_bounds = vec![]; + let mut outlives_bounds: Vec<ty::OutlivesPredicate<ty::GenericArg<'tcx>, ty::Region<'tcx>>> = + vec![]; let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx); @@ -65,30 +66,17 @@ fn compute_implied_outlives_bounds<'tcx>( // than the ultimate set. (Note: normally there won't be // unresolved inference variables here anyway, but there might be // during typeck under some circumstances.) + // + // FIXME(@lcnr): It's not really "always fine", having fewer implied + // bounds can be backward incompatible, e.g. #101951 was caused by + // us not dealing with inference vars in `TypeOutlives` predicates. let obligations = wf::obligations(infcx, param_env, hir::CRATE_HIR_ID, 0, arg, DUMMY_SP) .unwrap_or_default(); - // N.B., all of these predicates *ought* to be easily proven - // true. In fact, their correctness is (mostly) implied by - // other parts of the program. However, in #42552, we had - // an annoying scenario where: - // - // - Some `T::Foo` gets normalized, resulting in a - // variable `_1` and a `T: Trait<Foo=_1>` constraint - // (not sure why it couldn't immediately get - // solved). This result of `_1` got cached. - // - These obligations were dropped on the floor here, - // rather than being registered. - // - Then later we would get a request to normalize - // `T::Foo` which would result in `_1` being used from - // the cache, but hence without the `T: Trait<Foo=_1>` - // constraint. As a result, `_1` never gets resolved, - // and we get an ICE (in dropck). - // - // Therefore, we register any predicates involving - // inference variables. We restrict ourselves to those - // involving inference variables both for efficiency and - // to avoids duplicate errors that otherwise show up. + // While these predicates should all be implied by other parts of + // the program, they are still relevant as they may constrain + // inference variables, which is necessary to add the correct + // implied bounds in some cases, mostly when dealing with projections. fulfill_cx.register_predicate_obligations( infcx, obligations.iter().filter(|o| o.predicate.has_infer_types_or_consts()).cloned(), @@ -96,10 +84,10 @@ fn compute_implied_outlives_bounds<'tcx>( // From the full set of obligations, just filter down to the // region relationships. - implied_bounds.extend(obligations.into_iter().flat_map(|obligation| { + outlives_bounds.extend(obligations.into_iter().filter_map(|obligation| { assert!(!obligation.has_escaping_bound_vars()); match obligation.predicate.kind().no_bound_vars() { - None => vec![], + None => None, Some(pred) => match pred { ty::PredicateKind::Trait(..) | ty::PredicateKind::Subtype(..) @@ -109,21 +97,18 @@ fn compute_implied_outlives_bounds<'tcx>( | ty::PredicateKind::ObjectSafe(..) | ty::PredicateKind::ConstEvaluatable(..) | ty::PredicateKind::ConstEquate(..) - | ty::PredicateKind::TypeWellFormedFromEnv(..) => vec![], + | ty::PredicateKind::TypeWellFormedFromEnv(..) => None, ty::PredicateKind::WellFormed(arg) => { wf_args.push(arg); - vec![] + None } ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => { - vec![OutlivesBound::RegionSubRegion(r_b, r_a)] + Some(ty::OutlivesPredicate(r_a.into(), r_b)) } ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, r_b)) => { - let ty_a = infcx.resolve_vars_if_possible(ty_a); - let mut components = smallvec![]; - push_outlives_components(tcx, ty_a, &mut components); - implied_bounds_from_components(r_b, components) + Some(ty::OutlivesPredicate(ty_a.into(), r_b)) } }, } @@ -133,9 +118,27 @@ fn compute_implied_outlives_bounds<'tcx>( // Ensure that those obligations that we had to solve // get solved *here*. match fulfill_cx.select_all_or_error(infcx).as_slice() { - [] => Ok(implied_bounds), - _ => Err(NoSolution), + [] => (), + _ => return Err(NoSolution), } + + // We lazily compute the outlives components as + // `select_all_or_error` constrains inference variables. + let implied_bounds = outlives_bounds + .into_iter() + .flat_map(|ty::OutlivesPredicate(a, r_b)| match a.unpack() { + ty::GenericArgKind::Lifetime(r_a) => vec![OutlivesBound::RegionSubRegion(r_b, r_a)], + ty::GenericArgKind::Type(ty_a) => { + let ty_a = infcx.resolve_vars_if_possible(ty_a); + let mut components = smallvec![]; + push_outlives_components(tcx, ty_a, &mut components); + implied_bounds_from_components(r_b, components) + } + ty::GenericArgKind::Const(_) => unreachable!(), + }) + .collect(); + + Ok(implied_bounds) } /// When we have an implied bound that `T: 'a`, we can further break |