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|
//! Provider for the `implied_outlives_bounds` query.
//! Do not call this query directory. See
//! [`rustc_trait_selection::traits::query::type_op::implied_outlives_bounds`].
use rustc_hir as hir;
use rustc_infer::infer::canonical::{self, Canonical};
use rustc_infer::infer::outlives::components::{push_outlives_components, Component};
use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
use rustc_infer::traits::query::OutlivesBound;
use rustc_infer::traits::TraitEngineExt as _;
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
use rustc_span::source_map::DUMMY_SP;
use rustc_trait_selection::infer::InferCtxtBuilderExt;
use rustc_trait_selection::traits::query::{CanonicalTyGoal, Fallible, NoSolution};
use rustc_trait_selection::traits::wf;
use rustc_trait_selection::traits::{TraitEngine, TraitEngineExt};
use smallvec::{smallvec, SmallVec};
pub(crate) fn provide(p: &mut Providers) {
*p = Providers { implied_outlives_bounds, ..*p };
}
fn implied_outlives_bounds<'tcx>(
tcx: TyCtxt<'tcx>,
goal: CanonicalTyGoal<'tcx>,
) -> Result<
&'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
NoSolution,
> {
tcx.infer_ctxt().enter_canonical_trait_query(&goal, |infcx, _fulfill_cx, key| {
let (param_env, ty) = key.into_parts();
compute_implied_outlives_bounds(&infcx, param_env, ty)
})
}
fn compute_implied_outlives_bounds<'tcx>(
infcx: &InferCtxt<'_, 'tcx>,
param_env: ty::ParamEnv<'tcx>,
ty: Ty<'tcx>,
) -> Fallible<Vec<OutlivesBound<'tcx>>> {
let tcx = infcx.tcx;
// Sometimes when we ask what it takes for T: WF, we get back that
// U: WF is required; in that case, we push U onto this stack and
// process it next. Because the resulting predicates aren't always
// guaranteed to be a subset of the original type, so we need to store the
// WF args we've computed in a set.
let mut checked_wf_args = rustc_data_structures::fx::FxHashSet::default();
let mut wf_args = vec![ty.into()];
let mut outlives_bounds: Vec<ty::OutlivesPredicate<ty::GenericArg<'tcx>, ty::Region<'tcx>>> =
vec![];
let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
while let Some(arg) = wf_args.pop() {
if !checked_wf_args.insert(arg) {
continue;
}
// Compute the obligations for `arg` to be well-formed. If `arg` is
// an unresolved inference variable, just substituted an empty set
// -- because the return type here is going to be things we *add*
// to the environment, it's always ok for this set to be smaller
// 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();
// 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(),
);
// From the full set of obligations, just filter down to the
// region relationships.
outlives_bounds.extend(obligations.into_iter().filter_map(|obligation| {
assert!(!obligation.has_escaping_bound_vars());
match obligation.predicate.kind().no_bound_vars() {
None => None,
Some(pred) => match pred {
ty::PredicateKind::Trait(..)
| ty::PredicateKind::Subtype(..)
| ty::PredicateKind::Coerce(..)
| ty::PredicateKind::Projection(..)
| ty::PredicateKind::ClosureKind(..)
| ty::PredicateKind::ObjectSafe(..)
| ty::PredicateKind::ConstEvaluatable(..)
| ty::PredicateKind::ConstEquate(..)
| ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
ty::PredicateKind::WellFormed(arg) => {
wf_args.push(arg);
None
}
ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => {
Some(ty::OutlivesPredicate(r_a.into(), r_b))
}
ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, r_b)) => {
Some(ty::OutlivesPredicate(ty_a.into(), r_b))
}
},
}
}));
}
// Ensure that those obligations that we had to solve
// get solved *here*.
match fulfill_cx.select_all_or_error(infcx).as_slice() {
[] => (),
_ => 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
/// this down to determine what relationships would have to hold for
/// `T: 'a` to hold. We get to assume that the caller has validated
/// those relationships.
fn implied_bounds_from_components<'tcx>(
sub_region: ty::Region<'tcx>,
sup_components: SmallVec<[Component<'tcx>; 4]>,
) -> Vec<OutlivesBound<'tcx>> {
sup_components
.into_iter()
.filter_map(|component| {
match component {
Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
Component::Projection(p) => Some(OutlivesBound::RegionSubProjection(sub_region, p)),
Component::EscapingProjection(_) =>
// If the projection has escaping regions, don't
// try to infer any implied bounds even for its
// free components. This is conservative, because
// the caller will still have to prove that those
// free components outlive `sub_region`. But the
// idea is that the WAY that the caller proves
// that may change in the future and we want to
// give ourselves room to get smarter here.
{
None
}
Component::UnresolvedInferenceVariable(..) => None,
}
})
.collect()
}
|
