path: root/compiler/rustc_borrowck/src/type_check/free_region_relations.rs

use rustc_data_structures::frozen::Frozen;
use rustc_data_structures::transitive_relation::{TransitiveRelation, TransitiveRelationBuilder};
use rustc_infer::infer::canonical::QueryRegionConstraints;
use rustc_infer::infer::outlives;
use rustc_infer::infer::outlives::env::RegionBoundPairs;
use rustc_infer::infer::region_constraints::GenericKind;
use rustc_infer::infer::InferCtxt;
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::traits::query::OutlivesBound;
use rustc_middle::ty::{self, RegionVid, Ty};
use rustc_span::Span;
use rustc_trait_selection::traits::query::type_op::{self, TypeOp};
use std::rc::Rc;
use type_op::TypeOpOutput;

use crate::{
    type_check::constraint_conversion,
    type_check::{Locations, MirTypeckRegionConstraints},
    universal_regions::UniversalRegions,
};

#[derive(Debug)]
pub(crate) struct UniversalRegionRelations<'tcx> {
    universal_regions: Rc<UniversalRegions<'tcx>>,

    /// Stores the outlives relations that are known to hold from the
    /// implied bounds, in-scope where-clauses, and that sort of
    /// thing.
    outlives: TransitiveRelation<RegionVid>,

    /// This is the `<=` relation; that is, if `a: b`, then `b <= a`,
    /// and we store that here. This is useful when figuring out how
    /// to express some local region in terms of external regions our
    /// caller will understand.
    inverse_outlives: TransitiveRelation<RegionVid>,
}

/// As part of computing the free region relations, we also have to
/// normalize the input-output types, which we then need later. So we
/// return those. This vector consists of first the input types and
/// then the output type as the last element.
type NormalizedInputsAndOutput<'tcx> = Vec<Ty<'tcx>>;

pub(crate) struct CreateResult<'tcx> {
    pub(crate) universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
    pub(crate) region_bound_pairs: RegionBoundPairs<'tcx>,
    pub(crate) normalized_inputs_and_output: NormalizedInputsAndOutput<'tcx>,
}

pub(crate) fn create<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    implicit_region_bound: ty::Region<'tcx>,
    universal_regions: &Rc<UniversalRegions<'tcx>>,
    constraints: &mut MirTypeckRegionConstraints<'tcx>,
) -> CreateResult<'tcx> {
    UniversalRegionRelationsBuilder {
        infcx,
        param_env,
        implicit_region_bound,
        constraints,
        universal_regions: universal_regions.clone(),
        region_bound_pairs: Default::default(),
        outlives: Default::default(),
        inverse_outlives: Default::default(),
    }
    .create()
}

impl UniversalRegionRelations<'_> {
    /// Given two universal regions, returns the postdominating
    /// upper-bound (effectively the least upper bound).
    ///
    /// (See `TransitiveRelation::postdom_upper_bound` for details on
    /// the postdominating upper bound in general.)
    pub(crate) fn postdom_upper_bound(&self, fr1: RegionVid, fr2: RegionVid) -> RegionVid {
        assert!(self.universal_regions.is_universal_region(fr1));
        assert!(self.universal_regions.is_universal_region(fr2));
        self.inverse_outlives
            .postdom_upper_bound(fr1, fr2)
            .unwrap_or(self.universal_regions.fr_static)
    }

    /// Finds an "upper bound" for `fr` that is not local. In other
    /// words, returns the smallest (*) known region `fr1` that (a)
    /// outlives `fr` and (b) is not local.
    ///
    /// (*) If there are multiple competing choices, we return all of them.
    pub(crate) fn non_local_upper_bounds(&self, fr: RegionVid) -> Vec<RegionVid> {
        debug!("non_local_upper_bound(fr={:?})", fr);
        let res = self.non_local_bounds(&self.inverse_outlives, fr);
        assert!(!res.is_empty(), "can't find an upper bound!?");
        res
    }

    /// Finds a "lower bound" for `fr` that is not local. In other
    /// words, returns the largest (*) known region `fr1` that (a) is
    /// outlived by `fr` and (b) is not local.
    ///
    /// (*) If there are multiple competing choices, we pick the "postdominating"
    /// one. See `TransitiveRelation::postdom_upper_bound` for details.
    pub(crate) fn non_local_lower_bound(&self, fr: RegionVid) -> Option<RegionVid> {
        debug!("non_local_lower_bound(fr={:?})", fr);
        let lower_bounds = self.non_local_bounds(&self.outlives, fr);

        // In case we find more than one, reduce to one for
        // convenience. This is to prevent us from generating more
        // complex constraints, but it will cause spurious errors.
        let post_dom = self.outlives.mutual_immediate_postdominator(lower_bounds);

        debug!("non_local_bound: post_dom={:?}", post_dom);

        post_dom.and_then(|post_dom| {
            // If the mutual immediate postdom is not local, then
            // there is no non-local result we can return.
            if !self.universal_regions.is_local_free_region(post_dom) {
                Some(post_dom)
            } else {
                None
            }
        })
    }

    /// Helper for `non_local_upper_bounds` and `non_local_lower_bounds`.
    /// Repeatedly invokes `postdom_parent` until we find something that is not
    /// local. Returns `None` if we never do so.
    fn non_local_bounds(
        &self,
        relation: &TransitiveRelation<RegionVid>,
        fr0: RegionVid,
    ) -> Vec<RegionVid> {
        // This method assumes that `fr0` is one of the universally
        // quantified region variables.
        assert!(self.universal_regions.is_universal_region(fr0));

        let mut external_parents = vec![];
        let mut queue = vec![fr0];

        // Keep expanding `fr` into its parents until we reach
        // non-local regions.
        while let Some(fr) = queue.pop() {
            if !self.universal_regions.is_local_free_region(fr) {
                external_parents.push(fr);
                continue;
            }

            queue.extend(relation.parents(fr));
        }

        debug!("non_local_bound: external_parents={:?}", external_parents);

        external_parents
    }

    /// Returns `true` if fr1 is known to outlive fr2.
    ///
    /// This will only ever be true for universally quantified regions.
    pub(crate) fn outlives(&self, fr1: RegionVid, fr2: RegionVid) -> bool {
        self.outlives.contains(fr1, fr2)
    }

    /// Returns a vector of free regions `x` such that `fr1: x` is
    /// known to hold.
    pub(crate) fn regions_outlived_by(&self, fr1: RegionVid) -> Vec<RegionVid> {
        self.outlives.reachable_from(fr1)
    }

    /// Returns the _non-transitive_ set of known `outlives` constraints between free regions.
    pub(crate) fn known_outlives(&self) -> impl Iterator<Item = (RegionVid, RegionVid)> + '_ {
        self.outlives.base_edges()
    }
}

struct UniversalRegionRelationsBuilder<'this, 'tcx> {
    infcx: &'this InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    universal_regions: Rc<UniversalRegions<'tcx>>,
    implicit_region_bound: ty::Region<'tcx>,
    constraints: &'this mut MirTypeckRegionConstraints<'tcx>,

    // outputs:
    outlives: TransitiveRelationBuilder<RegionVid>,
    inverse_outlives: TransitiveRelationBuilder<RegionVid>,
    region_bound_pairs: RegionBoundPairs<'tcx>,
}

impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
    /// Records in the `outlives_relation` (and
    /// `inverse_outlives_relation`) that `fr_a: fr_b`.
    fn relate_universal_regions(&mut self, fr_a: RegionVid, fr_b: RegionVid) {
        debug!("relate_universal_regions: fr_a={:?} outlives fr_b={:?}", fr_a, fr_b);
        self.outlives.add(fr_a, fr_b);
        self.inverse_outlives.add(fr_b, fr_a);
    }

    #[instrument(level = "debug", skip(self))]
    pub(crate) fn create(mut self) -> CreateResult<'tcx> {
        let span = self.infcx.tcx.def_span(self.universal_regions.defining_ty.def_id());

        // Insert the facts we know from the predicates. Why? Why not.
        let param_env = self.param_env;
        self.add_outlives_bounds(outlives::explicit_outlives_bounds(param_env));

        // - outlives is reflexive, so `'r: 'r` for every region `'r`
        // - `'static: 'r` for every region `'r`
        // - `'r: 'fn_body` for every (other) universally quantified
        //   region `'r`, all of which are provided by our caller
        let fr_static = self.universal_regions.fr_static;
        let fr_fn_body = self.universal_regions.fr_fn_body;
        for fr in self.universal_regions.universal_regions() {
            debug!("build: relating free region {:?} to itself and to 'static", fr);
            self.relate_universal_regions(fr, fr);
            self.relate_universal_regions(fr_static, fr);
            self.relate_universal_regions(fr, fr_fn_body);
        }

        let unnormalized_input_output_tys = self
            .universal_regions
            .unnormalized_input_tys
            .iter()
            .cloned()
            .chain(Some(self.universal_regions.unnormalized_output_ty));

        // For each of the input/output types:
        // - Normalize the type. This will create some region
        //   constraints, which we buffer up because we are
        //   not ready to process them yet.
        // - Then compute the implied bounds. This will adjust
        //   the `region_bound_pairs` and so forth.
        // - After this is done, we'll process the constraints, once
        //   the `relations` is built.
        let mut normalized_inputs_and_output =
            Vec::with_capacity(self.universal_regions.unnormalized_input_tys.len() + 1);
        let mut constraints = vec![];
        for ty in unnormalized_input_output_tys {
            debug!("build: input_or_output={:?}", ty);
            // We add implied bounds from both the unnormalized and normalized ty.
            // See issue #87748
            let constraints_unnorm = self.add_implied_bounds(ty);
            if let Some(c) = constraints_unnorm {
                constraints.push(c)
            }
            let TypeOpOutput { output: norm_ty, constraints: constraints_normalize, .. } = self
                .param_env
                .and(type_op::normalize::Normalize::new(ty))
                .fully_perform(self.infcx)
                .unwrap_or_else(|_| {
                    let guar = self
                        .infcx
                        .tcx
                        .sess
                        .delay_span_bug(span, &format!("failed to normalize {:?}", ty));
                    TypeOpOutput {
                        output: self.infcx.tcx.ty_error(guar),
                        constraints: None,
                        error_info: None,
                    }
                });
            if let Some(c) = constraints_normalize {
                constraints.push(c)
            }

            // Note: we need this in examples like
            // ```
            // trait Foo {
            //   type Bar;
            //   fn foo(&self) -> &Self::Bar;
            // }
            // impl Foo for () {
            //   type Bar = ();
            //   fn foo(&self) ->&() {}
            // }
            // ```
            // Both &Self::Bar and &() are WF
            if ty != norm_ty {
                let constraints_norm = self.add_implied_bounds(norm_ty);
                if let Some(c) = constraints_norm {
                    constraints.push(c)
                }
            }

            normalized_inputs_and_output.push(norm_ty);
        }

        for c in constraints {
            self.push_region_constraints(c, span);
        }

        CreateResult {
            universal_region_relations: Frozen::freeze(UniversalRegionRelations {
                universal_regions: self.universal_regions,
                outlives: self.outlives.freeze(),
                inverse_outlives: self.inverse_outlives.freeze(),
            }),
            region_bound_pairs: self.region_bound_pairs,
            normalized_inputs_and_output,
        }
    }

    #[instrument(skip(self, data), level = "debug")]
    fn push_region_constraints(&mut self, data: &QueryRegionConstraints<'tcx>, span: Span) {
        debug!("constraints generated: {:#?}", data);

        constraint_conversion::ConstraintConversion::new(
            self.infcx,
            &self.universal_regions,
            &self.region_bound_pairs,
            self.implicit_region_bound,
            self.param_env,
            Locations::All(span),
            span,
            ConstraintCategory::Internal,
            &mut self.constraints,
        )
        .convert_all(data);
    }

    /// Update the type of a single local, which should represent
    /// either the return type of the MIR or one of its arguments. At
    /// the same time, compute and add any implied bounds that come
    /// from this local.
    #[instrument(level = "debug", skip(self))]
    fn add_implied_bounds(&mut self, ty: Ty<'tcx>) -> Option<&'tcx QueryRegionConstraints<'tcx>> {
        let TypeOpOutput { output: bounds, constraints, .. } = self
            .param_env
            .and(type_op::implied_outlives_bounds::ImpliedOutlivesBounds { ty })
            .fully_perform(self.infcx)
            .unwrap_or_else(|_| bug!("failed to compute implied bounds {:?}", ty));
        debug!(?bounds, ?constraints);
        self.add_outlives_bounds(bounds);
        constraints
    }

    /// Registers the `OutlivesBound` items from `outlives_bounds` in
    /// the outlives relation as well as the region-bound pairs
    /// listing.
    fn add_outlives_bounds<I>(&mut self, outlives_bounds: I)
    where
        I: IntoIterator<Item = OutlivesBound<'tcx>>,
    {
        for outlives_bound in outlives_bounds {
            debug!("add_outlives_bounds(bound={:?})", outlives_bound);

            match outlives_bound {
                OutlivesBound::RegionSubRegion(r1, r2) => {
                    // The bound says that `r1 <= r2`; we store `r2: r1`.
                    let r1 = self.universal_regions.to_region_vid(r1);
                    let r2 = self.universal_regions.to_region_vid(r2);
                    self.relate_universal_regions(r2, r1);
                }

                OutlivesBound::RegionSubParam(r_a, param_b) => {
                    self.region_bound_pairs
                        .insert(ty::OutlivesPredicate(GenericKind::Param(param_b), r_a));
                }

                OutlivesBound::RegionSubAlias(r_a, alias_b) => {
                    self.region_bound_pairs
                        .insert(ty::OutlivesPredicate(GenericKind::Alias(alias_b), r_a));
                }
            }
        }
    }
}