src/librustc_mir/borrow_check/nll/type_check/relate_tys.rs

use crate::borrow_check::nll::constraints::OutlivesConstraint;
use crate::borrow_check::nll::type_check::{BorrowCheckContext, Locations};
use rustc::infer::nll_relate::{TypeRelating, TypeRelatingDelegate, NormalizationStrategy};
use rustc::infer::{InferCtxt, NLLRegionVariableOrigin};
use rustc::mir::ConstraintCategory;
use rustc::traits::query::Fallible;
use rustc::traits::DomainGoal;
use rustc::ty::relate::TypeRelation;
use rustc::ty::{self, Ty};

/// Adds sufficient constraints to ensure that `a R b` where `R` depends on `v`:
///
/// - "Covariant" `a <: b`
/// - "Invariant" `a == b`
/// - "Contravariant" `a :> b`
///
/// N.B., the type `a` is permitted to have unresolved inference
/// variables, but not the type `b`.
pub(super) fn relate_types<'tcx>(
    infcx: &InferCtxt<'_, 'tcx>,
    a: Ty<'tcx>,
    v: ty::Variance,
    b: Ty<'tcx>,
    locations: Locations,
    category: ConstraintCategory,
    borrowck_context: Option<&mut BorrowCheckContext<'_, 'tcx>>,
) -> Fallible<()> {
    debug!("eq_types(a={:?}, b={:?}, locations={:?})", a, b, locations);
    TypeRelating::new(
        infcx,
        NllTypeRelatingDelegate::new(infcx, borrowck_context, locations, category),
        v
    ).relate(&a, &b)?;
    Ok(())
}

struct NllTypeRelatingDelegate<'me, 'bccx, 'tcx> {
    infcx: &'me InferCtxt<'me, 'tcx>,
    borrowck_context: Option<&'me mut BorrowCheckContext<'bccx, 'tcx>>,

    /// Where (and why) is this relation taking place?
    locations: Locations,

    /// What category do we assign the resulting `'a: 'b` relationships?
    category: ConstraintCategory,
}

impl NllTypeRelatingDelegate<'me, 'bccx, 'tcx> {
    fn new(
        infcx: &'me InferCtxt<'me, 'tcx>,
        borrowck_context: Option<&'me mut BorrowCheckContext<'bccx, 'tcx>>,
        locations: Locations,
        category: ConstraintCategory,
    ) -> Self {
        Self {
            infcx,
            borrowck_context,
            locations,
            category,
        }
    }
}

impl TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx> {
    fn create_next_universe(&mut self) -> ty::UniverseIndex {
        self.infcx.create_next_universe()
    }

    fn next_existential_region_var(&mut self, from_forall: bool) -> ty::Region<'tcx> {
        if let Some(_) = &mut self.borrowck_context {
            let origin = NLLRegionVariableOrigin::Existential { from_forall };
            self.infcx.next_nll_region_var(origin)
        } else {
            self.infcx.tcx.lifetimes.re_erased
        }
    }

    fn next_placeholder_region(
        &mut self,
        placeholder: ty::PlaceholderRegion
    ) -> ty::Region<'tcx> {
        if let Some(borrowck_context) = &mut self.borrowck_context {
            borrowck_context.constraints.placeholder_region(self.infcx, placeholder)
        } else {
            self.infcx.tcx.lifetimes.re_erased
        }
    }

    fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx> {
        self.infcx
            .next_nll_region_var_in_universe(NLLRegionVariableOrigin::Existential {
                from_forall: false
            }, universe)
    }

    fn push_outlives(&mut self, sup: ty::Region<'tcx>, sub: ty::Region<'tcx>) {
        if let Some(borrowck_context) = &mut self.borrowck_context {
            let sub = borrowck_context.universal_regions.to_region_vid(sub);
            let sup = borrowck_context.universal_regions.to_region_vid(sup);
            borrowck_context
                .constraints
                .outlives_constraints
                .push(OutlivesConstraint {
                    sup,
                    sub,
                    locations: self.locations,
                    category: self.category,
                });
        }
    }

    fn push_domain_goal(&mut self, _: DomainGoal<'tcx>) {
        bug!("should never be invoked with eager normalization")
    }

    fn normalization() -> NormalizationStrategy {
        NormalizationStrategy::Eager
    }

    fn forbid_inference_vars() -> bool {
        true
    }
}