rust/compiler/rustc_traits/src/implied_outlives_bounds.rs

//! 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::TyCtxtInferExt;
use rustc_infer::traits::query::OutlivesBound;
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::ObligationCtxt;
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, |ocx, key| {
        let (param_env, ty) = key.into_parts();
        compute_implied_outlives_bounds(ocx, param_env, ty)
    })
}

fn compute_implied_outlives_bounds<'tcx>(
    ocx: &ObligationCtxt<'_, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    ty: Ty<'tcx>,
) -> Fallible<Vec<OutlivesBound<'tcx>>> {
    let tcx = ocx.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![];

    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(ocx.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.
        ocx.register_obligations(
            obligations.iter().filter(|o| o.predicate.has_non_region_infer()).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::Clause(ty::Clause::Trait(..))
                    | ty::PredicateKind::Subtype(..)
                    | ty::PredicateKind::Coerce(..)
                    | ty::PredicateKind::Clause(ty::Clause::Projection(..))
                    | ty::PredicateKind::ClosureKind(..)
                    | ty::PredicateKind::ObjectSafe(..)
                    | ty::PredicateKind::ConstEvaluatable(..)
                    | ty::PredicateKind::ConstEquate(..)
                    | ty::PredicateKind::Ambiguous
                    | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                    ty::PredicateKind::WellFormed(arg) => {
                        wf_args.push(arg);
                        None
                    }

                    ty::PredicateKind::Clause(ty::Clause::RegionOutlives(
                        ty::OutlivesPredicate(r_a, r_b),
                    )) => Some(ty::OutlivesPredicate(r_a.into(), r_b)),

                    ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
                        ty_a,
                        r_b,
                    ))) => Some(ty::OutlivesPredicate(ty_a.into(), r_b)),
                },
            }
        }));
    }

    // This call to `select_all_or_error` is necessary to constrain inference variables, which we
    // use further down when computing the implied bounds.
    match ocx.select_all_or_error().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 = ocx.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::Alias(p) => Some(OutlivesBound::RegionSubAlias(sub_region, p)),
                Component::EscapingAlias(_) =>
                // 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()
}