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::TraitEngineExt as _;
  11 use rustc_span::source_map::DUMMY_SP;
  12 use rustc_trait_selection::infer::InferCtxtBuilderExt;
  13 use rustc_trait_selection::traits::query::outlives_bounds::OutlivesBound;
  14 use rustc_trait_selection::traits::query::{CanonicalTyGoal, Fallible, NoSolution};
  15 use rustc_trait_selection::traits::wf;
  16 use rustc_trait_selection::traits::FulfillmentContext;
  17 use rustc_trait_selection::traits::TraitEngine;
  18 use smallvec::{smallvec, SmallVec};
  19
  20 crate fn provide(p: &mut Providers<'_>) {
  21     *p = Providers { implied_outlives_bounds, ..*p };
  22 }
  23
  24 fn implied_outlives_bounds<'tcx>(
  25     tcx: TyCtxt<'tcx>,
  26     goal: CanonicalTyGoal<'tcx>,
  27 ) -> Result<
  28     &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
  29     NoSolution,
  30 > {
  31     tcx.infer_ctxt().enter_canonical_trait_query(&goal, |infcx, _fulfill_cx, key| {
  32         let (param_env, ty) = key.into_parts();
  33         compute_implied_outlives_bounds(&infcx, param_env, ty)
  34     })
  35 }
  36
  37 fn compute_implied_outlives_bounds<'tcx>(
  38     infcx: &InferCtxt<'_, 'tcx>,
  39     param_env: ty::ParamEnv<'tcx>,
  40     ty: Ty<'tcx>,
  41 ) -> Fallible<Vec<OutlivesBound<'tcx>>> {
  42     let tcx = infcx.tcx;
  43
  44     // Sometimes when we ask what it takes for T: WF, we get back that
  45     // U: WF is required; in that case, we push U onto this stack and
  46     // process it next. Currently (at least) these resulting
  47     // predicates are always guaranteed to be a subset of the original
  48     // type, so we need not fear non-termination.
  49     let mut wf_types = vec![ty];
  50
  51     let mut implied_bounds = vec![];
  52
  53     let mut fulfill_cx = FulfillmentContext::new();
  54
  55     while let Some(ty) = wf_types.pop() {
  56         // Compute the obligations for `ty` to be well-formed. If `ty` is
  57         // an unresolved inference variable, just substituted an empty set
  58         // -- because the return type here is going to be things we *add*
  59         // to the environment, it's always ok for this set to be smaller
  60         // than the ultimate set. (Note: normally there won't be
  61         // unresolved inference variables here anyway, but there might be
  62         // during typeck under some circumstances.)
  63         let obligations =
  64             wf::obligations(infcx, param_env, hir::DUMMY_HIR_ID, ty, DUMMY_SP).unwrap_or(vec![]);
  65
  66         // N.B., all of these predicates *ought* to be easily proven
  67         // true. In fact, their correctness is (mostly) implied by
  68         // other parts of the program. However, in #42552, we had
  69         // an annoying scenario where:
  70         //
  71         // - Some `T::Foo` gets normalized, resulting in a
  72         //   variable `_1` and a `T: Trait<Foo=_1>` constraint
  73         //   (not sure why it couldn't immediately get
  74         //   solved). This result of `_1` got cached.
  75         // - These obligations were dropped on the floor here,
  76         //   rather than being registered.
  77         // - Then later we would get a request to normalize
  78         //   `T::Foo` which would result in `_1` being used from
  79         //   the cache, but hence without the `T: Trait<Foo=_1>`
  80         //   constraint. As a result, `_1` never gets resolved,
  81         //   and we get an ICE (in dropck).
  82         //
  83         // Therefore, we register any predicates involving
  84         // inference variables. We restrict ourselves to those
  85         // involving inference variables both for efficiency and
  86         // to avoids duplicate errors that otherwise show up.
  87         fulfill_cx.register_predicate_obligations(
  88             infcx,
  89             obligations.iter().filter(|o| o.predicate.has_infer_types_or_consts()).cloned(),
  90         );
  91
  92         // From the full set of obligations, just filter down to the
  93         // region relationships.
  94         implied_bounds.extend(obligations.into_iter().flat_map(|obligation| {
  95             assert!(!obligation.has_escaping_bound_vars());
  96             match obligation.predicate {
  97                 ty::Predicate::Trait(..)
  98                 | ty::Predicate::Subtype(..)
  99                 | ty::Predicate::Projection(..)
 100                 | ty::Predicate::ClosureKind(..)
 101                 | ty::Predicate::ObjectSafe(..)
 102                 | ty::Predicate::ConstEvaluatable(..) => vec![],
 103
 104                 ty::Predicate::WellFormed(subty) => {
 105                     wf_types.push(subty);
 106                     vec![]
 107                 }
 108
 109                 ty::Predicate::RegionOutlives(ref data) => match data.no_bound_vars() {
 110                     None => vec![],
 111                     Some(ty::OutlivesPredicate(r_a, r_b)) => {
 112                         vec![OutlivesBound::RegionSubRegion(r_b, r_a)]
 113                     }
 114                 },
 115
 116                 ty::Predicate::TypeOutlives(ref data) => match data.no_bound_vars() {
 117                     None => vec![],
 118                     Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 119                         let ty_a = infcx.resolve_vars_if_possible(&ty_a);
 120                         let mut components = smallvec![];
 121                         tcx.push_outlives_components(ty_a, &mut components);
 122                         implied_bounds_from_components(r_b, components)
 123                     }
 124                 },
 125             }
 126         }));
 127     }
 128
 129     // Ensure that those obligations that we had to solve
 130     // get solved *here*.
 131     match fulfill_cx.select_all_or_error(infcx) {
 132         Ok(()) => Ok(implied_bounds),
 133         Err(_) => Err(NoSolution),
 134     }
 135 }
 136
 137 /// When we have an implied bound that `T: 'a`, we can further break
 138 /// this down to determine what relationships would have to hold for
 139 /// `T: 'a` to hold. We get to assume that the caller has validated
 140 /// those relationships.
 141 fn implied_bounds_from_components(
 142     sub_region: ty::Region<'tcx>,
 143     sup_components: SmallVec<[Component<'tcx>; 4]>,
 144 ) -> Vec<OutlivesBound<'tcx>> {
 145     sup_components
 146         .into_iter()
 147         .filter_map(|component| {
 148             match component {
 149                 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
 150                 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
 151                 Component::Projection(p) => Some(OutlivesBound::RegionSubProjection(sub_region, p)),
 152                 Component::EscapingProjection(_) =>
 153                 // If the projection has escaping regions, don't
 154                 // try to infer any implied bounds even for its
 155                 // free components. This is conservative, because
 156                 // the caller will still have to prove that those
 157                 // free components outlive `sub_region`. But the
 158                 // idea is that the WAY that the caller proves
 159                 // that may change in the future and we want to
 160                 // give ourselves room to get smarter here.
 161                 {
 162                     None
 163                 }
 164                 Component::UnresolvedInferenceVariable(..) => None,
 165             }
 166         })
 167         .collect()
 168 }