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`].
5 use rustc_infer::infer::canonical::{self, Canonical};
6 use rustc_infer::infer::outlives::components::{push_outlives_components, Component};
7 use rustc_infer::infer::TyCtxtInferExt;
8 use rustc_infer::traits::query::OutlivesBound;
9 use rustc_middle::ty::query::Providers;
10 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
11 use rustc_span::def_id::CRATE_DEF_ID;
12 use rustc_span::source_map::DUMMY_SP;
13 use rustc_trait_selection::infer::InferCtxtBuilderExt;
14 use rustc_trait_selection::traits::query::{CanonicalTyGoal, Fallible, NoSolution};
15 use rustc_trait_selection::traits::wf;
16 use rustc_trait_selection::traits::ObligationCtxt;
17 use smallvec::{smallvec, SmallVec};
19 pub(crate) fn provide(p: &mut Providers) {
20 *p = Providers { implied_outlives_bounds, ..*p };
23 fn implied_outlives_bounds<'tcx>(
25 goal: CanonicalTyGoal<'tcx>,
27 &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
30 tcx.infer_ctxt().enter_canonical_trait_query(&goal, |ocx, key| {
31 let (param_env, ty) = key.into_parts();
32 compute_implied_outlives_bounds(ocx, param_env, ty)
36 fn compute_implied_outlives_bounds<'tcx>(
37 ocx: &ObligationCtxt<'_, 'tcx>,
38 param_env: ty::ParamEnv<'tcx>,
40 ) -> Fallible<Vec<OutlivesBound<'tcx>>> {
41 let tcx = ocx.infcx.tcx;
43 // Sometimes when we ask what it takes for T: WF, we get back that
44 // U: WF is required; in that case, we push U onto this stack and
45 // process it next. Because the resulting predicates aren't always
46 // guaranteed to be a subset of the original type, so we need to store the
47 // WF args we've computed in a set.
48 let mut checked_wf_args = rustc_data_structures::fx::FxHashSet::default();
49 let mut wf_args = vec![ty.into()];
51 let mut outlives_bounds: Vec<ty::OutlivesPredicate<ty::GenericArg<'tcx>, ty::Region<'tcx>>> =
54 while let Some(arg) = wf_args.pop() {
55 if !checked_wf_args.insert(arg) {
59 // Compute the obligations for `arg` to be well-formed. If `arg` is
60 // an unresolved inference variable, just substituted an empty set
61 // -- because the return type here is going to be things we *add*
62 // to the environment, it's always ok for this set to be smaller
63 // than the ultimate set. (Note: normally there won't be
64 // unresolved inference variables here anyway, but there might be
65 // during typeck under some circumstances.)
67 // FIXME(@lcnr): It's not really "always fine", having fewer implied
68 // bounds can be backward incompatible, e.g. #101951 was caused by
69 // us not dealing with inference vars in `TypeOutlives` predicates.
70 let obligations = wf::obligations(ocx.infcx, param_env, CRATE_DEF_ID, 0, arg, DUMMY_SP)
73 // While these predicates should all be implied by other parts of
74 // the program, they are still relevant as they may constrain
75 // inference variables, which is necessary to add the correct
76 // implied bounds in some cases, mostly when dealing with projections.
77 ocx.register_obligations(
78 obligations.iter().filter(|o| o.predicate.has_non_region_infer()).cloned(),
81 // From the full set of obligations, just filter down to the
82 // region relationships.
83 outlives_bounds.extend(obligations.into_iter().filter_map(|obligation| {
84 assert!(!obligation.has_escaping_bound_vars());
85 match obligation.predicate.kind().no_bound_vars() {
87 Some(pred) => match pred {
88 ty::PredicateKind::Clause(ty::Clause::Trait(..))
89 | ty::PredicateKind::Subtype(..)
90 | ty::PredicateKind::Coerce(..)
91 | ty::PredicateKind::Clause(ty::Clause::Projection(..))
92 | ty::PredicateKind::ClosureKind(..)
93 | ty::PredicateKind::ObjectSafe(..)
94 | ty::PredicateKind::ConstEvaluatable(..)
95 | ty::PredicateKind::ConstEquate(..)
96 | ty::PredicateKind::Ambiguous
97 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
98 ty::PredicateKind::WellFormed(arg) => {
103 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(
104 ty::OutlivesPredicate(r_a, r_b),
105 )) => Some(ty::OutlivesPredicate(r_a.into(), r_b)),
107 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
110 ))) => Some(ty::OutlivesPredicate(ty_a.into(), r_b)),
116 // This call to `select_all_or_error` is necessary to constrain inference variables, which we
117 // use further down when computing the implied bounds.
118 match ocx.select_all_or_error().as_slice() {
120 _ => return Err(NoSolution),
123 // We lazily compute the outlives components as
124 // `select_all_or_error` constrains inference variables.
125 let implied_bounds = outlives_bounds
127 .flat_map(|ty::OutlivesPredicate(a, r_b)| match a.unpack() {
128 ty::GenericArgKind::Lifetime(r_a) => vec![OutlivesBound::RegionSubRegion(r_b, r_a)],
129 ty::GenericArgKind::Type(ty_a) => {
130 let ty_a = ocx.infcx.resolve_vars_if_possible(ty_a);
131 let mut components = smallvec![];
132 push_outlives_components(tcx, ty_a, &mut components);
133 implied_bounds_from_components(r_b, components)
135 ty::GenericArgKind::Const(_) => unreachable!(),
142 /// When we have an implied bound that `T: 'a`, we can further break
143 /// this down to determine what relationships would have to hold for
144 /// `T: 'a` to hold. We get to assume that the caller has validated
145 /// those relationships.
146 fn implied_bounds_from_components<'tcx>(
147 sub_region: ty::Region<'tcx>,
148 sup_components: SmallVec<[Component<'tcx>; 4]>,
149 ) -> Vec<OutlivesBound<'tcx>> {
152 .filter_map(|component| {
154 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
155 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
156 Component::Alias(p) => Some(OutlivesBound::RegionSubAlias(sub_region, p)),
157 Component::EscapingAlias(_) =>
158 // If the projection has escaping regions, don't
159 // try to infer any implied bounds even for its
160 // free components. This is conservative, because
161 // the caller will still have to prove that those
162 // free components outlive `sub_region`. But the
163 // idea is that the WAY that the caller proves
164 // that may change in the future and we want to
165 // give ourselves room to get smarter here.
169 Component::UnresolvedInferenceVariable(..) => None,