src/librustc_traits/implied_outlives_bounds.rs

   1 //! Provider for the `implied_outlives_bounds` query.
   2 //! Do not call this query directory. See [`rustc::traits::query::implied_outlives_bounds`].
   3
   4 use rustc::ty::outlives::Component;
   5 use rustc::ty::query::Providers;
   6 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
   7 use rustc_hir as hir;
   8 use rustc_infer::infer::canonical::{self, Canonical};
   9 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
  10 use rustc_infer::traits::query::outlives_bounds::OutlivesBound;
  11 use rustc_infer::traits::query::{CanonicalTyGoal, Fallible, NoSolution};
  12 use rustc_infer::traits::wf;
  13 use rustc_infer::traits::FulfillmentContext;
  14 use rustc_infer::traits::{TraitEngine, TraitEngineExt};
  15 use rustc_span::source_map::DUMMY_SP;
  16 use smallvec::{smallvec, SmallVec};
  17
  18 crate fn provide(p: &mut Providers<'_>) {
  19     *p = Providers { implied_outlives_bounds, ..*p };
  20 }
  21
  22 fn implied_outlives_bounds<'tcx>(
  23     tcx: TyCtxt<'tcx>,
  24     goal: CanonicalTyGoal<'tcx>,
  25 ) -> Result<
  26     &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
  27     NoSolution,
  28 > {
  29     tcx.infer_ctxt().enter_canonical_trait_query(&goal, |infcx, _fulfill_cx, key| {
  30         let (param_env, ty) = key.into_parts();
  31         compute_implied_outlives_bounds(&infcx, param_env, ty)
  32     })
  33 }
  34
  35 fn compute_implied_outlives_bounds<'tcx>(
  36     infcx: &InferCtxt<'_, 'tcx>,
  37     param_env: ty::ParamEnv<'tcx>,
  38     ty: Ty<'tcx>,
  39 ) -> Fallible<Vec<OutlivesBound<'tcx>>> {
  40     let tcx = infcx.tcx;
  41
  42     // Sometimes when we ask what it takes for T: WF, we get back that
  43     // U: WF is required; in that case, we push U onto this stack and
  44     // process it next. Currently (at least) these resulting
  45     // predicates are always guaranteed to be a subset of the original
  46     // type, so we need not fear non-termination.
  47     let mut wf_types = vec![ty];
  48
  49     let mut implied_bounds = vec![];
  50
  51     let mut fulfill_cx = FulfillmentContext::new();
  52
  53     while let Some(ty) = wf_types.pop() {
  54         // Compute the obligations for `ty` to be well-formed. If `ty` is
  55         // an unresolved inference variable, just substituted an empty set
  56         // -- because the return type here is going to be things we *add*
  57         // to the environment, it's always ok for this set to be smaller
  58         // than the ultimate set. (Note: normally there won't be
  59         // unresolved inference variables here anyway, but there might be
  60         // during typeck under some circumstances.)
  61         let obligations =
  62             wf::obligations(infcx, param_env, hir::DUMMY_HIR_ID, ty, DUMMY_SP).unwrap_or(vec![]);
  63
  64         // N.B., all of these predicates *ought* to be easily proven
  65         // true. In fact, their correctness is (mostly) implied by
  66         // other parts of the program. However, in #42552, we had
  67         // an annoying scenario where:
  68         //
  69         // - Some `T::Foo` gets normalized, resulting in a
  70         //   variable `_1` and a `T: Trait<Foo=_1>` constraint
  71         //   (not sure why it couldn't immediately get
  72         //   solved). This result of `_1` got cached.
  73         // - These obligations were dropped on the floor here,
  74         //   rather than being registered.
  75         // - Then later we would get a request to normalize
  76         //   `T::Foo` which would result in `_1` being used from
  77         //   the cache, but hence without the `T: Trait<Foo=_1>`
  78         //   constraint. As a result, `_1` never gets resolved,
  79         //   and we get an ICE (in dropck).
  80         //
  81         // Therefore, we register any predicates involving
  82         // inference variables. We restrict ourselves to those
  83         // involving inference variables both for efficiency and
  84         // to avoids duplicate errors that otherwise show up.
  85         fulfill_cx.register_predicate_obligations(
  86             infcx,
  87             obligations.iter().filter(|o| o.predicate.has_infer_types_or_consts()).cloned(),
  88         );
  89
  90         // From the full set of obligations, just filter down to the
  91         // region relationships.
  92         implied_bounds.extend(obligations.into_iter().flat_map(|obligation| {
  93             assert!(!obligation.has_escaping_bound_vars());
  94             match obligation.predicate {
  95                 ty::Predicate::Trait(..)
  96                 | ty::Predicate::Subtype(..)
  97                 | ty::Predicate::Projection(..)
  98                 | ty::Predicate::ClosureKind(..)
  99                 | ty::Predicate::ObjectSafe(..)
 100                 | ty::Predicate::ConstEvaluatable(..) => vec![],
 101
 102                 ty::Predicate::WellFormed(subty) => {
 103                     wf_types.push(subty);
 104                     vec![]
 105                 }
 106
 107                 ty::Predicate::RegionOutlives(ref data) => match data.no_bound_vars() {
 108                     None => vec![],
 109                     Some(ty::OutlivesPredicate(r_a, r_b)) => {
 110                         vec![OutlivesBound::RegionSubRegion(r_b, r_a)]
 111                     }
 112                 },
 113
 114                 ty::Predicate::TypeOutlives(ref data) => match data.no_bound_vars() {
 115                     None => vec![],
 116                     Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 117                         let ty_a = infcx.resolve_vars_if_possible(&ty_a);
 118                         let mut components = smallvec![];
 119                         tcx.push_outlives_components(ty_a, &mut components);
 120                         implied_bounds_from_components(r_b, components)
 121                     }
 122                 },
 123             }
 124         }));
 125     }
 126
 127     // Ensure that those obligations that we had to solve
 128     // get solved *here*.
 129     match fulfill_cx.select_all_or_error(infcx) {
 130         Ok(()) => Ok(implied_bounds),
 131         Err(_) => Err(NoSolution),
 132     }
 133 }
 134
 135 /// When we have an implied bound that `T: 'a`, we can further break
 136 /// this down to determine what relationships would have to hold for
 137 /// `T: 'a` to hold. We get to assume that the caller has validated
 138 /// those relationships.
 139 fn implied_bounds_from_components(
 140     sub_region: ty::Region<'tcx>,
 141     sup_components: SmallVec<[Component<'tcx>; 4]>,
 142 ) -> Vec<OutlivesBound<'tcx>> {
 143     sup_components
 144         .into_iter()
 145         .filter_map(|component| {
 146             match component {
 147                 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
 148                 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
 149                 Component::Projection(p) => Some(OutlivesBound::RegionSubProjection(sub_region, p)),
 150                 Component::EscapingProjection(_) =>
 151                 // If the projection has escaping regions, don't
 152                 // try to infer any implied bounds even for its
 153                 // free components. This is conservative, because
 154                 // the caller will still have to prove that those
 155                 // free components outlive `sub_region`. But the
 156                 // idea is that the WAY that the caller proves
 157                 // that may change in the future and we want to
 158                 // give ourselves room to get smarter here.
 159                 {
 160                     None
 161                 }
 162                 Component::UnresolvedInferenceVariable(..) => None,
 163             }
 164         })
 165         .collect()
 166 }