1 //! Provider for the `implied_outlives_bounds` query.
2 //! Do not call this query directory. See
3 //! [`rustc_trait_selection::traits::query::type_op::implied_outlives_bounds`].
6 use rustc_infer::infer::canonical::{self, Canonical};
7 use rustc_infer::infer::outlives::components::{push_outlives_components, Component};
8 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
9 use rustc_infer::traits::query::OutlivesBound;
10 use rustc_infer::traits::TraitEngineExt as _;
11 use rustc_middle::ty::query::Providers;
12 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
13 use rustc_span::source_map::DUMMY_SP;
14 use rustc_trait_selection::infer::InferCtxtBuilderExt;
15 use rustc_trait_selection::traits::query::{CanonicalTyGoal, Fallible, NoSolution};
16 use rustc_trait_selection::traits::wf;
17 use rustc_trait_selection::traits::{TraitEngine, TraitEngineExt};
18 use smallvec::{smallvec, SmallVec};
20 pub(crate) fn provide(p: &mut Providers) {
21 *p = Providers { implied_outlives_bounds, ..*p };
24 fn implied_outlives_bounds<'tcx>(
26 goal: CanonicalTyGoal<'tcx>,
28 &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
31 tcx.infer_ctxt().enter_canonical_trait_query(&goal, |ocx, key| {
32 let (param_env, ty) = key.into_parts();
33 compute_implied_outlives_bounds(&ocx.infcx, param_env, ty)
37 fn compute_implied_outlives_bounds<'tcx>(
38 infcx: &InferCtxt<'tcx>,
39 param_env: ty::ParamEnv<'tcx>,
41 ) -> Fallible<Vec<OutlivesBound<'tcx>>> {
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. Because the resulting predicates aren't always
47 // guaranteed to be a subset of the original type, so we need to store the
48 // WF args we've computed in a set.
49 let mut checked_wf_args = rustc_data_structures::fx::FxHashSet::default();
50 let mut wf_args = vec![ty.into()];
52 let mut outlives_bounds: Vec<ty::OutlivesPredicate<ty::GenericArg<'tcx>, ty::Region<'tcx>>> =
55 let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
57 while let Some(arg) = wf_args.pop() {
58 if !checked_wf_args.insert(arg) {
62 // Compute the obligations for `arg` to be well-formed. If `arg` is
63 // an unresolved inference variable, just substituted an empty set
64 // -- because the return type here is going to be things we *add*
65 // to the environment, it's always ok for this set to be smaller
66 // than the ultimate set. (Note: normally there won't be
67 // unresolved inference variables here anyway, but there might be
68 // during typeck under some circumstances.)
70 // FIXME(@lcnr): It's not really "always fine", having fewer implied
71 // bounds can be backward incompatible, e.g. #101951 was caused by
72 // us not dealing with inference vars in `TypeOutlives` predicates.
73 let obligations = wf::obligations(infcx, param_env, hir::CRATE_HIR_ID, 0, arg, DUMMY_SP)
76 // While these predicates should all be implied by other parts of
77 // the program, they are still relevant as they may constrain
78 // inference variables, which is necessary to add the correct
79 // implied bounds in some cases, mostly when dealing with projections.
80 fulfill_cx.register_predicate_obligations(
82 obligations.iter().filter(|o| o.predicate.has_non_region_infer()).cloned(),
85 // From the full set of obligations, just filter down to the
86 // region relationships.
87 outlives_bounds.extend(obligations.into_iter().filter_map(|obligation| {
88 assert!(!obligation.has_escaping_bound_vars());
89 match obligation.predicate.kind().no_bound_vars() {
91 Some(pred) => match pred {
92 ty::PredicateKind::Trait(..)
93 | ty::PredicateKind::Subtype(..)
94 | ty::PredicateKind::Coerce(..)
95 | ty::PredicateKind::Projection(..)
96 | ty::PredicateKind::ClosureKind(..)
97 | ty::PredicateKind::ObjectSafe(..)
98 | ty::PredicateKind::ConstEvaluatable(..)
99 | ty::PredicateKind::ConstEquate(..)
100 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
101 ty::PredicateKind::WellFormed(arg) => {
106 ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => {
107 Some(ty::OutlivesPredicate(r_a.into(), r_b))
110 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, r_b)) => {
111 Some(ty::OutlivesPredicate(ty_a.into(), r_b))
118 // Ensure that those obligations that we had to solve
119 // get solved *here*.
120 match fulfill_cx.select_all_or_error(infcx).as_slice() {
122 _ => return Err(NoSolution),
125 // We lazily compute the outlives components as
126 // `select_all_or_error` constrains inference variables.
127 let implied_bounds = outlives_bounds
129 .flat_map(|ty::OutlivesPredicate(a, r_b)| match a.unpack() {
130 ty::GenericArgKind::Lifetime(r_a) => vec![OutlivesBound::RegionSubRegion(r_b, r_a)],
131 ty::GenericArgKind::Type(ty_a) => {
132 let ty_a = infcx.resolve_vars_if_possible(ty_a);
133 let mut components = smallvec![];
134 push_outlives_components(tcx, ty_a, &mut components);
135 implied_bounds_from_components(r_b, components)
137 ty::GenericArgKind::Const(_) => unreachable!(),
144 /// When we have an implied bound that `T: 'a`, we can further break
145 /// this down to determine what relationships would have to hold for
146 /// `T: 'a` to hold. We get to assume that the caller has validated
147 /// those relationships.
148 fn implied_bounds_from_components<'tcx>(
149 sub_region: ty::Region<'tcx>,
150 sup_components: SmallVec<[Component<'tcx>; 4]>,
151 ) -> Vec<OutlivesBound<'tcx>> {
154 .filter_map(|component| {
156 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
157 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
158 Component::Projection(p) => Some(OutlivesBound::RegionSubProjection(sub_region, p)),
159 Component::Opaque(def_id, substs) => {
160 Some(OutlivesBound::RegionSubOpaque(sub_region, def_id, substs))
162 Component::EscapingProjection(_) =>
163 // If the projection has escaping regions, don't
164 // try to infer any implied bounds even for its
165 // free components. This is conservative, because
166 // the caller will still have to prove that those
167 // free components outlive `sub_region`. But the
168 // idea is that the WAY that the caller proves
169 // that may change in the future and we want to
170 // give ourselves room to get smarter here.
174 Component::UnresolvedInferenceVariable(..) => None,