rust/src/librustc_mir/borrow_check/nll/constraint_generation.rs

use crate::borrow_check::borrow_set::BorrowSet;
use crate::borrow_check::location::LocationTable;
use crate::borrow_check::nll::ToRegionVid;
use crate::borrow_check::nll::facts::AllFacts;
use crate::borrow_check::nll::region_infer::values::LivenessValues;
use rustc::infer::InferCtxt;
use rustc::mir::visit::TyContext;
use rustc::mir::visit::Visitor;
use rustc::mir::{BasicBlock, BasicBlockData, Location, Mir, Place, PlaceBase, Rvalue};
use rustc::mir::{SourceInfo, Statement, Terminator};
use rustc::mir::UserTypeProjection;
use rustc::ty::fold::TypeFoldable;
use rustc::ty::{self, ClosureSubsts, GeneratorSubsts, RegionVid};
use rustc::ty::subst::SubstsRef;

pub(super) fn generate_constraints<'cx, 'gcx, 'tcx>(
    infcx: &InferCtxt<'cx, 'gcx, 'tcx>,
    liveness_constraints: &mut LivenessValues<RegionVid>,
    all_facts: &mut Option<AllFacts>,
    location_table: &LocationTable,
    mir: &Mir<'tcx>,
    borrow_set: &BorrowSet<'tcx>,
) {
    let mut cg = ConstraintGeneration {
        borrow_set,
        infcx,
        liveness_constraints,
        location_table,
        all_facts,
    };

    for (bb, data) in mir.basic_blocks().iter_enumerated() {
        cg.visit_basic_block_data(bb, data);
    }
}

/// 'cg = the duration of the constraint generation process itself.
struct ConstraintGeneration<'cg, 'cx: 'cg, 'gcx: 'tcx, 'tcx: 'cx> {
    infcx: &'cg InferCtxt<'cx, 'gcx, 'tcx>,
    all_facts: &'cg mut Option<AllFacts>,
    location_table: &'cg LocationTable,
    liveness_constraints: &'cg mut LivenessValues<RegionVid>,
    borrow_set: &'cg BorrowSet<'tcx>,
}

impl<'cg, 'cx, 'gcx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'gcx, 'tcx> {
    fn visit_basic_block_data(&mut self, bb: BasicBlock, data: &BasicBlockData<'tcx>) {
        self.super_basic_block_data(bb, data);
    }

    /// We sometimes have `substs` within an rvalue, or within a
    /// call. Make them live at the location where they appear.
    fn visit_substs(&mut self, substs: &SubstsRef<'tcx>, location: Location) {
        self.add_regular_live_constraint(*substs, location);
        self.super_substs(substs);
    }

    /// We sometimes have `region` within an rvalue, or within a
    /// call. Make them live at the location where they appear.
    fn visit_region(&mut self, region: &ty::Region<'tcx>, location: Location) {
        self.add_regular_live_constraint(*region, location);
        self.super_region(region);
    }

    /// We sometimes have `ty` within an rvalue, or within a
    /// call. Make them live at the location where they appear.
    fn visit_ty(&mut self, ty: &ty::Ty<'tcx>, ty_context: TyContext) {
        match ty_context {
            TyContext::ReturnTy(SourceInfo { span, .. })
            | TyContext::YieldTy(SourceInfo { span, .. })
            | TyContext::UserTy(span)
            | TyContext::LocalDecl { source_info: SourceInfo { span, .. }, .. } => {
                span_bug!(
                    span,
                    "should not be visiting outside of the CFG: {:?}",
                    ty_context
                );
            }
            TyContext::Location(location) => {
                self.add_regular_live_constraint(*ty, location);
            }
        }

        self.super_ty(ty);
    }

    /// We sometimes have `generator_substs` within an rvalue, or within a
    /// call. Make them live at the location where they appear.
    fn visit_generator_substs(&mut self, substs: &GeneratorSubsts<'tcx>, location: Location) {
        self.add_regular_live_constraint(*substs, location);
        self.super_generator_substs(substs);
    }

    /// We sometimes have `closure_substs` within an rvalue, or within a
    /// call. Make them live at the location where they appear.
    fn visit_closure_substs(&mut self, substs: &ClosureSubsts<'tcx>, location: Location) {
        self.add_regular_live_constraint(*substs, location);
        self.super_closure_substs(substs);
    }

    fn visit_statement(
        &mut self,
        statement: &Statement<'tcx>,
        location: Location,
    ) {
        if let Some(all_facts) = self.all_facts {
            all_facts.cfg_edge.push((
                self.location_table.start_index(location),
                self.location_table.mid_index(location),
            ));

            all_facts.cfg_edge.push((
                self.location_table.mid_index(location),
                self.location_table
                    .start_index(location.successor_within_block()),
            ));
        }

        self.super_statement(statement, location);
    }

    fn visit_assign(
        &mut self,
        place: &Place<'tcx>,
        rvalue: &Rvalue<'tcx>,
        location: Location,
    ) {
        // When we see `X = ...`, then kill borrows of
        // `(*X).foo` and so forth.
        if let Some(all_facts) = self.all_facts {
            if let Place::Base(PlaceBase::Local(temp)) = place {
                if let Some(borrow_indices) = self.borrow_set.local_map.get(temp) {
                    all_facts.killed.reserve(borrow_indices.len());
                    for &borrow_index in borrow_indices {
                        let location_index = self.location_table.mid_index(location);
                        all_facts.killed.push((borrow_index, location_index));
                    }
                }
            }
        }

        self.super_assign(place, rvalue, location);
    }

    fn visit_terminator(
        &mut self,
        terminator: &Terminator<'tcx>,
        location: Location,
    ) {
        if let Some(all_facts) = self.all_facts {
            all_facts.cfg_edge.push((
                self.location_table.start_index(location),
                self.location_table.mid_index(location),
            ));

            let successor_blocks = terminator.successors();
            all_facts.cfg_edge.reserve(successor_blocks.size_hint().0);
            for successor_block in successor_blocks {
                all_facts.cfg_edge.push((
                    self.location_table.mid_index(location),
                    self.location_table
                        .start_index(successor_block.start_location()),
                ));
            }
        }

        self.super_terminator(terminator, location);
    }

    fn visit_ascribe_user_ty(
        &mut self,
        _place: &Place<'tcx>,
        _variance: &ty::Variance,
        _user_ty: &UserTypeProjection,
        _location: Location,
    ) {
    }
}

impl<'cx, 'cg, 'gcx, 'tcx> ConstraintGeneration<'cx, 'cg, 'gcx, 'tcx> {
    /// Some variable with type `live_ty` is "regular live" at
    /// `location` -- i.e., it may be used later. This means that all
    /// regions appearing in the type `live_ty` must be live at
    /// `location`.
    fn add_regular_live_constraint<T>(&mut self, live_ty: T, location: Location)
    where
        T: TypeFoldable<'tcx>,
    {
        debug!(
            "add_regular_live_constraint(live_ty={:?}, location={:?})",
            live_ty, location
        );

        self.infcx
            .tcx
            .for_each_free_region(&live_ty, |live_region| {
                let vid = live_region.to_region_vid();
                self.liveness_constraints.add_element(vid, location);
            });
    }
}