rustc_type_ir/

region_constraint.rs

//! The bulk of the logic for implementing `-Zassumptions-on-binders`

use derive_where::derive_where;
use indexmap::IndexSet;
#[cfg(feature = "nightly")]
use rustc_data_structures::transitive_relation::{TransitiveRelation, TransitiveRelationBuilder};
#[cfg(feature = "nightly")]
use rustc_macros::StableHash_NoContext;
use rustc_type_ir_macros::{GenericTypeVisitable, TypeFoldable_Generic, TypeVisitable_Generic};
use tracing::{debug, instrument};

use crate::Const;

// Workaround for TransitiveRelation being in rustc_data_structures which isn't accessible on stable
#[cfg(not(feature = "nightly"))]
#[derive(Default, Clone, Debug)]
pub struct TransitiveRelation<T>(T);
#[cfg(not(feature = "nightly"))]
impl<T> TransitiveRelation<T> {
    pub fn reachable_from(&self, _data: T) -> Vec<T> {
        unreachable!("-Zassumptions-on-binders is not supported for r-a")
    }

    pub fn base_edges(&self) -> impl Iterator<Item = (T, T)> {
        unreachable!("-Zassumptions-on-binders is not supported for r-a");

        #[allow(unreachable_code)]
        [].into_iter()
    }
}
#[derive(Clone, Debug)]
#[cfg(not(feature = "nightly"))]
pub struct TransitiveRelationBuilder<T>(T);
#[cfg(not(feature = "nightly"))]
impl<T> TransitiveRelationBuilder<T> {
    pub fn freeze(self) -> TransitiveRelation<T> {
        unreachable!("-Zassumptions-on-binders is not supported for r-a")
    }

    pub fn add(&mut self, _: T, _: T) {
        unreachable!("-Zassumptions-on-binders is not supported for r-a")
    }
}
#[cfg(not(feature = "nightly"))]
impl<T> Default for TransitiveRelationBuilder<T> {
    fn default() -> Self {
        unreachable!("-Zassumptions-on-binders is not supported for r-a")
    }
}

use smallvec::SmallVec;

use crate::data_structures::IndexMap;
use crate::fold::TypeSuperFoldable;
use crate::inherent::*;
use crate::relate::{Relate, RelateResult, TypeRelation, VarianceDiagInfo};
use crate::visit::TypeVisitableExt;
use crate::{
    AliasTy, Binder, BoundRegion, BoundRegionKind, BoundVar, BoundVariableKind, ClauseKind,
    DebruijnIndex, InferCtxtLike, Interner, IsRigid, OutlivesClause, Region, RegionKind, TyKind,
    TypeFoldable, TypeFolder, TypingMode, UniverseIndex, Variance, elaborate, max_universe,
    set_aliases_to_non_rigid,
};

#[derive_where(Clone; I: Interner)]
pub struct Assumptions<I: Interner> {
    pub type_outlives: Vec<Binder<I, OutlivesClause<I, I::Ty>>>,
    /// Known `'a: 'b` assumptions, stored as an edge from the outliving region to the
    /// outlived one, i.e. an edge `('a, 'b)` means `'a: 'b`. Constructors expect a relation
    /// with this direction, see [`regions_outlived_by`] and [`regions_outliving`] for how it
    /// is consumed.
    pub region_outlives: TransitiveRelation<Region<I>>,
    pub inverse_region_outlives: TransitiveRelation<Region<I>>,
}

impl<I: Interner> std::fmt::Debug for Assumptions<I> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        f.debug_struct("Assumptions")
            .field("type_outlives", &self.type_outlives)
            .field("region_graph", &"elided")
            .finish()
    }
}

impl<I: Interner> Assumptions<I> {
    pub fn empty() -> Self {
        Self {
            type_outlives: Vec::new(),
            region_outlives: TransitiveRelationBuilder::default().freeze(),
            inverse_region_outlives: TransitiveRelationBuilder::default().freeze(),
        }
    }

    /// Builds assumptions from `clauses`, elaborating them and keeping the outlives ones.
    ///
    /// Callers hand us their clauses straight from the environment, so we have to elaborate
    /// here to get at the implied outlives bounds:
    /// - a `Ty: 'a` clause tells us that every region component of `Ty` outlives `'a`, e.g.
    ///   `&'b u8: 'a` implies `'b: 'a`. Without it we'd fail to prove `'b: 'a` when leaving
    ///   the binder these assumptions belong to.
    /// - it also gives us the components as type outlives, e.g. `Vec<T>: 'a` implies `T: 'a`,
    ///   which we need for placeholder and alias outlives.
    /// - trait clauses imply their supertraits, so `T: Bound<'a>` where `trait Bound<'c>: 'c`
    ///   gives us `T: 'a`. This is why we take clauses rather than just the outlives ones:
    ///   filtering down to outlives before elaborating would throw those away.
    ///
    /// Only the clauses whose max universe is exactly `universe` are kept, which is what the
    /// solver wants when computing the assumptions of a single binder. This happens after
    /// elaboration on purpose, so a clause whose regions live in more than one universe still
    /// contributes its implied bounds to each of them: `(&'b u8, &'c u8): 'a` gives us
    /// `'c: 'a` in `'c`s universe even though the clause itself is in `'b`s.
    ///
    /// Use [`Assumptions::new_unelaborated`] when the caller needs the assumptions to be
    /// exactly the clauses it passed in.
    pub fn new(
        infcx: &impl InferCtxtLike<Interner = I>,
        clauses: impl IntoIterator<Item = I::Clause>,
        region_outlives: TransitiveRelation<Region<I>>,
        universe: UniverseIndex,
    ) -> Self {
        let mut type_outlives = vec![];
        let mut region_outlives_builder = TransitiveRelationBuilder::default();
        for (r1, r2) in region_outlives.base_edges() {
            region_outlives_builder.add(r1, r2);
        }

        let clauses = elaborate::elaborate(infcx.cx(), clauses)
            .filter(|clause| max_universe(infcx, *clause) == universe);
        for clause in clauses {
            match clause.kind().skip_binder() {
                // The type outlives assumptions are kept around as they are required for
                // proving placeholder and alias outlives.
                ClauseKind::TypeOutlives(_) => {
                    type_outlives.push(clause.as_type_outlives_clause().unwrap());
                }
                ClauseKind::RegionOutlives(OutlivesClause(r1, r2)) => {
                    // `elaborate` drops the components which are bound inside of the type and
                    // bails on `for<'a> Ty: 'a`, so both regions here are free even though the
                    // clause itself may still be under a binder.
                    debug_assert!(!r1.is_bound() && !r2.is_bound());
                    region_outlives_builder.add(r1, r2);
                }
                // Anything else can't be used as an outlives assumption.
                _ => (),
            }
        }

        Self::new_unelaborated(type_outlives, region_outlives_builder.freeze())
    }

    /// Builds assumptions from exactly the given clauses, see [`Assumptions::new`] for when
    /// the clauses should get elaborated instead.
    pub fn new_unelaborated(
        type_outlives: Vec<Binder<I, OutlivesClause<I, I::Ty>>>,
        region_outlives: TransitiveRelation<Region<I>>,
    ) -> Self {
        Self {
            inverse_region_outlives: {
                let mut builder = TransitiveRelationBuilder::default();
                for (r1, r2) in region_outlives.base_edges() {
                    builder.add(r2, r1);
                }
                builder.freeze()
            },
            type_outlives,
            region_outlives,
        }
    }
}

#[derive_where(Clone, Hash, PartialEq, Eq; I: Interner, S)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
#[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
pub enum LeafRegionConstraint<I: Interner, S: Clone = ()> {
    Ambiguity(S),
    RegionOutlives(Region<I>, Region<I>, S),
    /// Requirement that a (potentially higher ranked) alias outlives some (potentially higher ranked)
    /// region due to an assumption in the environment. This cannot be satisfied via component outlives
    /// or item bounds.
    ///
    /// We cannot eagerly look at assumptions as we are usually working with an incomplete set of assumptions
    /// and there may wind up being assumptions we can use to prove this when we're in a smaller universe.
    ///
    /// We eagerly destructure alias outlives requirements into region outlives requirements corresponding to
    /// component outlives & item bound outlives rules, leaving only param env candidates.
    AliasTyOutlivesViaEnv(Binder<I, (AliasTy<I>, Region<I>)>, S),
    /// This is an `I::Ty` for two reasons:
    /// 1. We need the type visitable impl to be able to `visit_ty` on this so canonicalization
    ///    knows about the placeholder
    /// 2. When exiting the trait solver there may be placeholder outlives corresponding to params
    ///    from the root universe. These need to be changed from a `Placeholder` to the original
    ///    `Param`.
    ///
    /// We cannot eagerly look at assumptions as we are usually working with an incomplete set of assumptions
    /// and there may wind up being assumptions we can use to prove this when we're in a smaller universe.
    PlaceholderTyOutlives(I::Ty, Region<I>, S),
}

impl<I: Interner, S: Clone> std::fmt::Debug for LeafRegionConstraint<I, S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        match self {
            Self::Ambiguity(_span) => f.write_str("ambiguity"),
            Self::RegionOutlives(r1, r2, _span) => {
                r1.fmt(f)?;
                f.write_str(": ")?;
                r2.fmt(f)
            }
            Self::AliasTyOutlivesViaEnv(bound_outlives, _span) => {
                f.write_str("for<")?;
                bound_outlives.bound_vars().fmt(f)?;
                f.write_str(">")?;
                let (ty, r) = bound_outlives.skip_binder();
                ty.fmt(f)?;
                f.write_str(": ")?;
                r.fmt(f)
            }
            Self::PlaceholderTyOutlives(ty, r, _span) => {
                ty.fmt(f)?;
                f.write_str(": ")?;
                r.fmt(f)
            }
        }
    }
}

impl<I: Interner> LeafRegionConstraint<I> {
    pub fn with_span<S: Clone + std::fmt::Debug + Eq + std::hash::Hash>(
        self,
        span: S,
    ) -> LeafRegionConstraint<I, S> {
        use LeafRegionConstraint::*;

        match self {
            Ambiguity(()) => Ambiguity(span),
            RegionOutlives(r1, r2, ()) => RegionOutlives(r1, r2, span),
            AliasTyOutlivesViaEnv(bound_outlives, ()) => {
                AliasTyOutlivesViaEnv(bound_outlives, span)
            }
            PlaceholderTyOutlives(ty, r, ()) => PlaceholderTyOutlives(ty, r, span),
        }
    }
}

impl<I: Interner, S: Clone + std::fmt::Debug + Eq + std::hash::Hash> LeafRegionConstraint<I, S> {
    pub fn without_span(self) -> LeafRegionConstraint<I> {
        use LeafRegionConstraint::*;

        match self {
            Ambiguity(_) => Ambiguity(()),
            RegionOutlives(r1, r2, _) => RegionOutlives(r1, r2, ()),
            AliasTyOutlivesViaEnv(bound_outlives, _) => AliasTyOutlivesViaEnv(bound_outlives, ()),
            PlaceholderTyOutlives(ty, r, _) => PlaceholderTyOutlives(ty, r, ()),
        }
    }

    pub fn span(&self) -> S {
        use LeafRegionConstraint::*;

        let (Ambiguity(s)
        | RegionOutlives(_, _, s)
        | AliasTyOutlivesViaEnv(_, s)
        | PlaceholderTyOutlives(_, _, s)) = self;
        s.clone()
    }
}

#[derive_where(Clone, Hash, PartialEq, Eq; I: Interner, S)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
#[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
/// An OR of AND of LEAF constraints. Always in "canonical form" meaning:
/// - No two ANDs are equivalent
/// - All ANDs are in canonical form
/// - If any AND is empty, i.e. trivially true, it is the only AND
///
/// FIXME(-Zassumptions-on-binders): We should consider a more general canonical form which also
/// drops any AND that is a superset of another AND. Proving the superset requires strictly more
/// than proving the subset, so it can never be the candidate which makes the OR hold. E.g.
/// `OR(AND('a: 'b), AND('a: 'b, 'b: 'c))` really ought to just be `OR(AND('a: 'b))`. Only keeping
/// an empty AND is the degenerate case of that rule.
pub struct Or<I: Interner, S: Clone + std::fmt::Debug = ()>(pub SmallVec<[And<I, S>; 1]>);
impl<I: Interner> Or<I> {
    pub fn with_spans<S: Clone + std::fmt::Debug + Eq + std::hash::Hash>(
        self,
        span: S,
    ) -> Or<I, S> {
        Or(self.0.into_iter().map(|and| and.with_spans(span.clone())).collect())
    }
}
impl<I: Interner, S: Clone + std::hash::Hash + std::fmt::Debug + Eq> Or<I, S> {
    pub fn new_true() -> Self {
        Self(SmallVec::from_buf([And::new_true()]))
    }

    pub fn is_true(&self) -> bool {
        // OR([AND([])])
        if let [and] = &*self.0
            && and.0.is_empty()
        {
            true
        } else {
            false
        }
    }

    pub fn new_false() -> Self {
        Self(SmallVec::new())
    }

    pub fn is_false(&self) -> bool {
        // OR([])
        self.0.is_empty()
    }

    pub fn new(i: impl IntoIterator<Item = And<I, S>>) -> Self {
        let mut new_ands: SmallVec<[And<I, S>; _]> = SmallVec::new();

        for and in i {
            // An empty AND is trivially true, which makes the whole OR true no matter what the
            // other candidates are. `And::new` discards leaf constraints which are trivially
            // true, so this is how e.g. a reflexive `'a: 'a` candidate discharges an OR.
            if and.0.is_empty() {
                return Self::new_true();
            }

            // FIXME(-Zassumptions-on-binders): We only discard an AND which is equivalent to one
            // we already have. More generally we should discard any AND which is a superset of
            // another, as it requires strictly more to hold. E.g. the second AND in
            // `OR(AND('a: 'b), AND('a: 'b, 'b: 'c))` is never the one which makes the OR true.
            if new_ands.iter().all(|c| !c.is_and_equivalent_to(&and)) {
                new_ands.push(and)
            }
        }

        Self(new_ands)
    }

    pub fn new_ambig(s: S) -> Self {
        Or::new_leaf(LeafRegionConstraint::Ambiguity(s))
    }

    pub fn new_leaf(l: LeafRegionConstraint<I, S>) -> Self {
        Or::new([And::new([l])])
    }

    pub fn build_and(a: Or<I, S>, b: Or<I, S>) -> Self {
        // FIXME(-Zassumptions-on-binders): probably bad for perf, doing a lot of reallocating
        // and whatnot here :3

        // Important: keeps `ands` empty if either `b_and` or `a_and` is empty
        let mut ands = Vec::new();
        for b_and in b.0 {
            for a_and in a.0.clone().into_iter() {
                ands.push(And::new(a_and.0.into_iter().chain(b_and.0.clone())))
            }
        }

        Or::new(ands)
    }

    pub fn build_or(a: Or<I, S>, b: Or<I, S>) -> Self {
        Or::new(a.0.into_iter().chain(b.0))
    }

    pub fn without_spans(self) -> Or<I> {
        Or(self.0.into_iter().map(|and| and.without_spans()).collect())
    }
}

impl<I: Interner, S: Clone + std::fmt::Debug> std::fmt::Debug for Or<I, S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        if self.0.is_empty() {
            f.write_str("false")
        } else if let [single] = &*self.0 {
            single.fmt(f)
        } else {
            f.write_str("or ")?;
            f.debug_set().entries(&self.0).finish()
        }
    }
}

#[derive_where(Clone, Hash, PartialEq, Eq; I: Interner, S)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
#[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
/// An AND of leaf constraints. Always in "canonical form", meaning:
/// - No leaf constraints are present twice in this AND
/// - No leaf constraint is trivially true, i.e. a reflexive `'a: 'a`
pub struct And<I: Interner, S: Clone + std::fmt::Debug = ()>(
    pub SmallVec<[LeafRegionConstraint<I, S>; 1]>,
);
impl<I: Interner> And<I> {
    pub fn with_spans<S: Clone + std::fmt::Debug + Eq + std::hash::Hash>(
        self,
        span: S,
    ) -> And<I, S> {
        And(self.0.into_iter().map(|leaf| leaf.with_span(span.clone())).collect())
    }
}
impl<I: Interner, S: Clone + std::hash::Hash + std::fmt::Debug + Eq> And<I, S> {
    pub fn new_true() -> Self {
        Self(SmallVec::new())
    }

    pub fn new(i: impl IntoIterator<Item = LeafRegionConstraint<I, S>>) -> Self {
        let mut seen = IndexSet::new();
        And(i
            .into_iter()
            .filter(|leaf| {
                // Outlives is reflexive so a `'a: 'a` leaf is always true and carries no
                // information. Dropping it here keeps the rest of the code from having to special
                // case it, and is what lets an OR with a reflexive candidate be recognized as true.
                if let LeafRegionConstraint::RegionOutlives(r1, r2, _) = leaf
                    && r1 == r2
                {
                    return false;
                }

                if seen.contains(&leaf.clone().without_span()) {
                    false
                } else {
                    seen.insert(leaf.clone().without_span());
                    true
                }
            })
            .collect())
    }

    fn is_and_equivalent_to(&self, other: &And<I, S>) -> bool {
        let this = self.clone().0;
        let other = other.clone().0;

        this.iter()
            .all(|c1| other.iter().any(|c2| c1.clone().without_span() == c2.clone().without_span()))
            && other.iter().all(|c2| {
                this.iter().any(|c1| c1.clone().without_span() == c2.clone().without_span())
            })
    }

    pub fn without_spans(self) -> And<I> {
        And(self.0.into_iter().map(|leaf| leaf.without_span()).collect())
    }
}

impl<I: Interner, S: Clone + std::fmt::Debug> std::fmt::Debug for And<I, S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        if self.0.is_empty() {
            f.write_str("true")
        } else if let [single] = &*self.0 {
            single.fmt(f)
        } else {
            f.write_str("and ")?;
            f.debug_set().entries(&self.0).finish()
        }
    }
}

#[derive_where(Clone, Hash, PartialEq; I: Interner, S)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
#[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
/// An `And` and an `Or` constraint both in canonical forms, with two additional constraints:
/// - If the `or_constraint` is false then the `and_constraint` is empty
/// - The `or_constraint` does not have any constraints present in all of its inner `And`s
///    - i.e. `OR ( AND ('a: 'b, 'b: 'c), AND ('a: 'b, 'b: 'd))` is not a thing
/// - The OR constraint is in canonical form
/// - The AND constraint is in canonical form
///
/// This should be thought of as an AND consisting of a set of LEAF constraints as well
/// as a single OR constraint.
pub struct RegionConstraint<I: Interner, S: Clone + std::fmt::Debug = ()> {
    pub and_constraint: And<I, S>,
    pub or_constraint: Or<I, S>,
}

impl<I: Interner> RegionConstraint<I> {
    pub fn with_spans<S: Clone + std::fmt::Debug + Eq + std::hash::Hash>(
        self,
        span: S,
    ) -> RegionConstraint<I, S> {
        RegionConstraint {
            and_constraint: self.and_constraint.with_spans(span.clone()),
            or_constraint: self.or_constraint.with_spans(span.clone()),
        }
    }
}
impl<I: Interner, S: Clone + std::fmt::Debug + Eq + std::hash::Hash> RegionConstraint<I, S> {
    pub fn new_from_or(or: Or<I, S>) -> Self {
        let Some(fst) = or.0.get(0).clone() else {
            return RegionConstraint::new_false();
        };
        let mut and_constraint = fst.0.to_vec();

        for and in or.0.split_first().unwrap().1 {
            and_constraint.retain(|c| {
                and.0.iter().any(|c2| c.clone().without_span() == c2.clone().without_span())
            });
        }
        let and_constraint = And::new(and_constraint);

        let or_constraint = Or::new(or.0.into_iter().map(|and| {
            And::new(and.0.into_iter().filter(|c| {
                and_constraint
                    .0
                    .iter()
                    .all(|s_c| c.clone().without_span() != s_c.clone().without_span())
            }))
        }));

        Self {
            and_constraint: if or_constraint.is_false() { And::new_true() } else { and_constraint },
            or_constraint,
        }
    }

    pub fn splatted_and_constraints(&self) -> Or<I, S> {
        Or::new(self.or_constraint.0.iter().map(|and| {
            And::new(and.0.iter().cloned().chain(self.and_constraint.0.iter().cloned()))
        }))
    }

    pub fn build_and(a: RegionConstraint<I, S>, b: RegionConstraint<I, S>) -> Self {
        let and_constraint = And::new(a.and_constraint.0.into_iter().chain(b.and_constraint.0));
        let or_constraint = Or::build_and(a.or_constraint, b.or_constraint);

        Self {
            and_constraint: if or_constraint.is_false() { And::new_true() } else { and_constraint },
            or_constraint,
        }
    }

    pub fn build_or(a: RegionConstraint<I, S>, b: RegionConstraint<I, S>) -> Self {
        Self::new_from_or(Or::build_or(a.splatted_and_constraints(), b.splatted_and_constraints()))
    }

    pub fn new_true() -> Self {
        Self { and_constraint: And::new_true(), or_constraint: Or::new_true() }
    }

    pub fn is_true(&self) -> bool {
        self.and_constraint.0.is_empty() && self.or_constraint.is_true()
    }

    pub fn new_false() -> Self {
        Self { and_constraint: And::new_true(), or_constraint: Or::new_false() }
    }

    pub fn is_false(&self) -> bool {
        self.or_constraint.is_false()
    }

    pub fn new_ambig(span: S) -> Self {
        Self {
            and_constraint: And::new([LeafRegionConstraint::Ambiguity(span)]),
            or_constraint: Or::new_true(),
        }
    }

    pub fn is_ambig(&self) -> bool {
        if let [c] = &*self.and_constraint.0
            && c.is_ambig()
            && self.or_constraint.is_true()
        {
            true
        } else {
            false
        }
    }

    pub fn without_spans(self) -> RegionConstraint<I> {
        RegionConstraint {
            and_constraint: self.and_constraint.without_spans(),
            or_constraint: self.or_constraint.without_spans(),
        }
    }

    pub fn new_leaf(l: LeafRegionConstraint<I, S>) -> Self {
        RegionConstraint { and_constraint: And::new([l]), or_constraint: Or::new_true() }
    }
}

impl<I: Interner, S: Clone + std::fmt::Debug> std::fmt::Debug for RegionConstraint<I, S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        if let [single_or] = &*self.or_constraint.0
            && single_or.0.is_empty()
        {
            self.and_constraint.fmt(f)
        } else if self.and_constraint.0.is_empty() {
            self.or_constraint.fmt(f)
        } else {
            f.write_str("and ")?;
            f.debug_set().entries(&self.and_constraint.0).entry(&self.or_constraint).finish()
        }
    }
}

impl<I: Interner, S: Clone + std::fmt::Debug> LeafRegionConstraint<I, S> {
    pub fn is_ambig(&self) -> bool {
        matches!(self, Self::Ambiguity(_))
    }
}

/// Takes any constraints involving placeholders from the current universe and eagerly checks them.
/// This can be done a few ways:
/// - There's an assumption on the binder introducing the placeholder which means the constraint is satisfied (true)
/// - There's assumptions on the binder introducing the placeholder which allow us to rewrite the constraint in
///    terms of lower universe variables. For example given `for<'a> where('b: 'a) { prove(T: '!a_u1) }` we can
///    convert this constraint to `T: 'b` which no longer references anything from `u1`.
/// - There are no relevant assumptions so we can neither rewrite the constraint nor consider it satisfied (false)
/// - We failed to compute the full set of assumptions when entering the binder corresponding to `u`. (ambiguity)
///
/// After handling all of the region constraints in `u` we then evaluate the entire constraint as much as possible,
/// propagating true/false/ambiguity as close to the root of the constraint as we can. The returned constraint should
/// be checked for whether it is true/false/ambiguous as that should affect the result of whatever operation required
/// entering the binder corresponding to `u`.
#[instrument(level = "debug", skip(infcx), ret)]
pub fn eagerly_handle_placeholders_in_universe<Infcx: InferCtxtLike<Interner = I>, I: Interner>(
    infcx: &Infcx,
    constraint: RegionConstraint<I>,
    u: UniverseIndex,
) -> RegionConstraint<I> {
    assert!(u > UniverseIndex::ROOT, "eagerly handle placeholders called for root universe");

    let assumptions = infcx.get_placeholder_assumptions(u);

    // 1. rewrite type outlives constraints involving things from `u` into either region constraints
    //     involving things from `u` or type outlives constraints not involving things from `u`
    //
    //    IOW, we only want to encounter things from `u` as part of region out lives constraints.
    let constraint = rewrite_type_outlives_constraints_in_universe_for_eager_placeholder_handling(
        infcx,
        constraint,
        u,
        &assumptions,
    );

    // 2. compute transitive region outlives and get a new set of region outlives constraints by
    //     looking for every region which either a placeholder_u flows into it, or it flows into
    //     the placeholder.
    let constraint = compute_new_region_constraints(infcx, constraint, u);

    // 3. rewrite region outlives constraints (potentially to false/true)
    let constraint =
        pull_region_outlives_constraints_out_of_universe(infcx, constraint, u, &assumptions);

    constraint
}

/// Filter our region constraints to not include constraints between region variables from `u` and
/// other regions as those are always satisfied. This requires some care to handle correctly for example:
/// `'!a_u1: '?x_u1: '!b_u1` should result in us requiring `'!a_u1: '!b_u1` rather than dropping the two
/// constraints entirely.
///
/// The only constraints involving things from `u` should be region outlives constraints at this point. Type
/// outlives constraints should have been handled already either by destructuring into region outlives or by
/// being rewritten in terms of smaller universe variables.
#[instrument(level = "debug", skip(infcx), ret)]
fn compute_new_region_constraints<Infcx: InferCtxtLike<Interner = I>, I: Interner>(
    infcx: &Infcx,
    constraint: RegionConstraint<I>,
    u: UniverseIndex,
) -> RegionConstraint<I> {
    use LeafRegionConstraint::*;

    let extend_from_and = |builder: &mut TransitiveRelationBuilder<_>,
                           regions: &mut IndexSet<_>,
                           constraints: &mut Vec<_>,
                           and: &And<I>| {
        for c in &and.0 {
            match c {
                Ambiguity(()) | PlaceholderTyOutlives(..) | AliasTyOutlivesViaEnv(..) => {
                    constraints.push(c.clone())
                }
                RegionOutlives(r1, r2, ()) => {
                    regions.insert(*r1);
                    regions.insert(*r2);
                    builder.add(*r2, *r1);
                }
            }
        }
    };

    let mut base_region_flows_builder = TransitiveRelationBuilder::default();
    let mut base_regions = IndexSet::new();
    let mut base_constraints = Vec::new();
    extend_from_and(
        &mut base_region_flows_builder,
        &mut base_regions,
        &mut base_constraints,
        &constraint.and_constraint,
    );

    let mut new_ands = Vec::new();
    for and in &constraint.or_constraint.0 {
        let mut region_flows_builder = base_region_flows_builder.clone();
        let mut regions = base_regions.clone();
        let mut constraints = base_constraints.clone();
        extend_from_and(&mut region_flows_builder, &mut regions, &mut constraints, and);

        let region_flow = region_flows_builder.freeze();
        for r in regions.into_iter() {
            for ub in region_flow.reachable_from(r) {
                // we want to retain any region constraints between two "placeholder-likes" where for our
                // purposes a placeholder-like is either a placeholder or variable in a lower universe
                let is_placeholder_like = |r: Region<I>| match r.kind() {
                    RegionKind::ReLateParam(..)
                    | RegionKind::ReEarlyParam(..)
                    | RegionKind::RePlaceholder(..)
                    | RegionKind::ReStatic => true,
                    RegionKind::ReVar(..) => max_universe(infcx, r) < u,
                    RegionKind::ReError(..) => false,
                    RegionKind::ReErased | RegionKind::ReBound(..) => unreachable!(),
                };

                if is_placeholder_like(r) && is_placeholder_like(ub) {
                    constraints.push(RegionOutlives(ub, r, ()));
                }
            }
        }

        new_ands.push(Or::new([And::new(constraints)]))
    }

    // FIXME(-Zassumptions-on-binders): probably bad for perf!
    RegionConstraint::new_from_or(
        new_ands.into_iter().fold(Or::new_false(), |acc, c| Or::build_or(acc, c)),
    )
}

/// Handles converting region outlives constraints involving placeholders from `u` into OR constraints
/// involving regions from smaller universes with known relationships to the placeholder. For example:
/// ```ignore (not rust)
/// for<'a, 'b> where(
///     'c: 'b, 'd: 'b,
///     'a: 'e, 'a: 'f,
/// ) {
///     'a_u1: 'b_u1
/// }
/// ```
/// will get converted to:
/// ```ignore (not rust)
/// OR(
///     'e: 'c,
///     'e: 'd,
///     'f: 'c,
///     'f: 'd,
/// )
/// ```
/// if we are handling constraints in `u1`.
#[instrument(level = "debug", skip(infcx), ret)]
fn pull_region_outlives_constraints_out_of_universe<
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
>(
    infcx: &Infcx,
    constraint: RegionConstraint<I>,
    u: UniverseIndex,
    assumptions: &Assumptions<I>,
) -> RegionConstraint<I> {
    assert!(max_universe(infcx, constraint.clone()) <= u);

    // FIXME(-Zassumptions-on-binders): we don't lower universes of region variables when exiting `u`
    // this seems dubious/potentially wrong? we can't just blindly do this though as if we had something
    // like `!T_u -> ?x_u -> !U_u` then lowering `?x` to `u-1` when exiting `u` would be wrong.
    //
    // I'm not even sure this would be necessary given we filter out region constraints involving regions#
    // from the current universe and only retain those between placeholders.

    use LeafRegionConstraint::*;

    let pull_and = |and: And<I>| {
        let mut pulled_constraints = Vec::new();
        for c in and.0 {
            match c {
                Ambiguity(()) | PlaceholderTyOutlives(..) | AliasTyOutlivesViaEnv(..) => {
                    assert!(max_universe(infcx, c.clone()) < u);
                    pulled_constraints.push(Or::new_leaf(c.clone()));
                }
                RegionOutlives(region_1, region_2, ()) => {
                    let region_1_u = max_universe(infcx, region_1);
                    let region_2_u = max_universe(infcx, region_2);

                    if region_1_u != u && region_2_u != u {
                        pulled_constraints.push(Or::new_leaf(c));
                        continue;
                    }

                    // The constraint may already be entailed by the assumptions of the binder we are
                    // leaving, e.g. `for<'a, 'b> where 'b: 'a { 'b: 'a }`. There is nothing to lift into
                    // a smaller universe in that case, and looking for lower universe candidates would
                    // wrongly result in `Or([])` whenever the placeholders have no lower universe bounds.
                    if regions_outlived_by(region_1, assumptions).any(|r| r == region_2) {
                        continue;
                    }

                    let mut candidates = vec![];

                    for ub in regions_outlived_by(region_1, assumptions)
                        .filter(|r| max_universe(infcx, *r) < u)
                    {
                        // FIXME(-Zassumptions-on-binders): if `region_2` is in a smaller universe there'll be both
                        // `'region_2` and `'static` as lower bounds which seems... unfortunate and may cause us to
                        // add a bunch of duplicate `'ub: 'static` candidates the more binders we leave.
                        for lb in regions_outliving(region_2, assumptions, infcx.cx())
                            .filter(|r| max_universe(infcx, *r) < u)
                        {
                            // As long as any region outlived by `region_1` outlives any region region which
                            // `region_2` outlives, we know that `region_1: region_2` holds. In other words,
                            // there exists some set of 4 regions for which `'r1: 'i1` `'i1: 'i2` `'i2: 'r2`
                            candidates.push(RegionOutlives(ub, lb, ()));
                        }
                    }

                    pulled_constraints.push(Or::new(candidates.into_iter().map(|c| And::new([c]))));
                }
            };
        }

        pulled_constraints.into_iter().fold(Or::new_true(), |acc, c| Or::build_and(acc, c))
    };

    let and_constraint = pull_and(constraint.and_constraint);
    let or_constraint = constraint
        .or_constraint
        .0
        .into_iter()
        .fold(Or::new_false(), |acc, c| Or::build_or(acc, pull_and(c)));
    RegionConstraint::new_from_or(Or::build_and(and_constraint, or_constraint))
}

/// Converts type outlives constraints into region outlives constraints. This assumes the *complete* set of
/// assumptions are known. This should not be called until the end of type checking.
///
/// The returned region constraint will not have *any* PlaceholderTyOutlives or AliasTyOutlivesViaEnv constraints.
#[instrument(level = "debug", skip(infcx), ret)]
pub fn destructure_type_outlives_constraints_in_root<
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    S: Clone + std::fmt::Debug + Eq + std::hash::Hash,
>(
    infcx: &Infcx,
    constraint: RegionConstraint<I, S>,
    assumptions: &Assumptions<I>,
) -> RegionConstraint<I, S> {
    use LeafRegionConstraint::*;

    let destructure_and = |and: &And<I, S>| {
        debug!("rewriting and: {:?}", and);
        let mut destructured_constraints = Vec::new();
        for c in &and.0 {
            match c {
                Ambiguity(_) | RegionOutlives(..) => {
                    destructured_constraints.push(Or::new_leaf(c.clone()))
                }
                PlaceholderTyOutlives(ty, r, span) => destructured_constraints.push(Or::new(
                    regions_outlived_by_placeholder(*ty, assumptions, infcx.cx()).map(
                        move |assumption_r| {
                            And::new([RegionOutlives(assumption_r, *r, span.clone())])
                        },
                    ),
                )),
                AliasTyOutlivesViaEnv(bound_outlives, span) => {
                    destructured_constraints.push(
                        alias_outlives_candidates_from_assumptions(
                            infcx,
                            *bound_outlives,
                            assumptions,
                        )
                        .with_spans(span.clone()),
                    );
                }
            }
        }
        debug!(?destructured_constraints);
        let merged_constraints = destructured_constraints
            .into_iter()
            .fold(Or::new_true(), |acc, c| Or::build_and(acc, c));
        debug!(?merged_constraints);
        merged_constraints
    };

    let and_constraint = destructure_and(&constraint.and_constraint);
    let or_constraint = constraint
        .or_constraint
        .0
        .into_iter()
        .fold(Or::new_false(), |acc, c| Or::build_or(acc, destructure_and(&c)));

    RegionConstraint::new_from_or(Or::build_and(and_constraint, or_constraint))
}

/// Converts type outlives constraints into either region outlives constraints, or type outlives
/// constraints which do not contain anything from `u`.
///
/// This only works off assumptions associated with the binder corresponding to `u` both for
/// perf reasons and because the full set of region assumptions is not known during type checking
/// due to closure signature inference.
///
/// This only really causes problems for higher-ranked outlives assumptions, for example if we have
/// `where for<'a> <T as Trait<'a>>::Assoc: 'b` then we can't use that to prove `<T as Trait<'!c>>::Assoc: 'b`
/// until we are in the root context. See comments inside this function for more detail.
#[instrument(level = "debug", skip(infcx), ret)]
fn rewrite_type_outlives_constraints_in_universe_for_eager_placeholder_handling<
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
>(
    infcx: &Infcx,
    constraint: RegionConstraint<I>,
    u: UniverseIndex,
    assumptions: &Assumptions<I>,
) -> RegionConstraint<I> {
    use LeafRegionConstraint::*;

    assert!(
        max_universe(infcx, constraint.clone()) <= u,
        "constraint {:?} contains terms from a larger universe than {:?}",
        constraint.clone(),
        u
    );

    let rewrite_and = |and: And<I>| {
        let mut rewritten_constraints = Vec::new();
        for c in and.0 {
            match c {
                Ambiguity(()) | RegionOutlives(..) => rewritten_constraints.push(Or::new_leaf(c)),
                PlaceholderTyOutlives(ty, region, ()) => {
                    rewritten_constraints.push(rewrite_placeholder_ty_outlives_constraints_in_universe_for_eager_placeholder_handling(infcx, ty, region, u, assumptions));
                }
                AliasTyOutlivesViaEnv(bound_outlives, ()) => {
                    rewritten_constraints.push(rewrite_alias_ty_outlives_constraints_in_universe_for_eager_placeholder_handling(infcx, bound_outlives, u, assumptions));
                }
            }
        }
        rewritten_constraints.into_iter().fold(Or::new_true(), |acc, c| Or::build_and(acc, c))
    };

    let and_constraint = rewrite_and(constraint.and_constraint);
    let or_constraint = constraint
        .or_constraint
        .0
        .into_iter()
        .fold(Or::new_false(), |acc, c| Or::build_or(acc, rewrite_and(c)));

    RegionConstraint::new_from_or(Or::build_and(and_constraint, or_constraint))
}

#[instrument(level = "debug", skip(infcx), ret)]
fn rewrite_placeholder_ty_outlives_constraints_in_universe_for_eager_placeholder_handling<
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
>(
    infcx: &Infcx,
    ty: I::Ty,
    region: Region<I>,
    u: UniverseIndex,
    assumptions: &Assumptions<I>,
) -> Or<I> {
    use LeafRegionConstraint::*;

    let ty_u = max_universe(infcx, ty);
    let region_u = max_universe(infcx, region);

    if region_u != u && ty_u != u {
        return Or::new_leaf(PlaceholderTyOutlives(ty, region, ()));
    }

    let mut candidates = vec![];

    // There could be `!T: 'region` assumptions in the env even if `!T` is in a
    // smaller universe
    candidates.extend(
        regions_outlived_by_placeholder(ty, assumptions, infcx.cx())
            .map(move |assumption_r| RegionOutlives(assumption_r, region, ())),
    );

    // We can express `!T: 'region` as `!T: 'r` where `'r: 'region`. This is only necessary
    // if the placeholder type is in a smaller universe as otherwise we know all regions which
    // the placeholder outlives and can just destructure into an OR of RegionOutlives.
    if region_u == u && ty_u < u {
        candidates.extend(
            regions_outliving::<I>(region, assumptions, infcx.cx())
                .filter(|r| max_universe(infcx, *r) < u)
                .map(|r| PlaceholderTyOutlives(ty, r, ())),
        );
    }

    Or::new(candidates.into_iter().map(|c| And::new([c])))
}

#[instrument(level = "debug", skip(infcx), ret)]
fn rewrite_alias_ty_outlives_constraints_in_universe_for_eager_placeholder_handling<
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
>(
    infcx: &Infcx,
    bound_outlives: Binder<I, (AliasTy<I>, Region<I>)>,
    u: UniverseIndex,
    assumptions: &Assumptions<I>,
) -> Or<I> {
    use LeafRegionConstraint::*;

    if max_universe(infcx, bound_outlives) != u {
        return Or::new_leaf(AliasTyOutlivesViaEnv(bound_outlives, ()));
    }

    let mut candidates = Vec::new();

    // given there can be higher ranked assumptions, e.g. `for<'a> <T as Trait<'a>>::Assoc: 'c`, that
    // means that it's actually *always* possible for an alias outlive to be satisfied in the root universe
    // which means there should *always* be atleast one candidate when destructuring alias outlives.
    //
    // we dont care about this for region outlives as `for<'a> 'a: 'b` can't exist as we don't elaborate
    // higher ranked type outlives assumptions into higher ranked region outlives assumptions. similarly,
    // we don't care about `for<'a> Foo<'a>: 'b` as we always destructure adts into their components and if
    // we dont equivalently elaborate the assumption into assumptions on the adt's components we just drop the
    // assumptions
    //
    // so actually only `for<'a, 'b> Alias<'a>: 'b` and `for<'a> T: 'a` are assumptions we actually need to
    // handle.
    let higher_ranked_candidate_for_outlived_region = |region| {
        let bound_outlives = bound_outlives.map_bound(|(alias, _)| (alias, region));

        let mut replacer = MakeHigherRankederer {
            cx: infcx,
            preexisting_var_count: bound_outlives.bound_vars().len(),
            regions: IndexMap::default(),
            universe: u,
            current_index: DebruijnIndex::ZERO,
        };
        let escaping_outlives = bound_outlives.skip_binder().fold_with(&mut replacer);
        let bound_vars = bound_outlives.bound_vars().iter().chain(
            core::mem::take(&mut replacer.regions)
                .into_iter()
                .map(|(_, bound_region)| BoundVariableKind::Region(bound_region.kind)),
        );
        let bound_outlives = Binder::bind_with_vars(
            escaping_outlives,
            I::BoundVarKinds::from_vars(infcx.cx(), bound_vars),
        );

        // `MakeHigherRankederer` only folds regions. A non-lifetime binder can leave
        // a placeholder type in `u`, so this type-outlives constraint cannot be
        // handled by the region-outlives-only eager placeholder machinery.
        if max_universe(infcx, bound_outlives) == u {
            return Or::new_ambig(());
        }

        debug!("higher ranked candidate: {bound_outlives:?}");
        Or::new_leaf(AliasTyOutlivesViaEnv(bound_outlives, ()))
    };

    // We elaborate something like `Alias<'a_u2>: 'b_u2` into alias outlives involving each
    // region which is known to outlive `'b_u2`. So e.g. if `'c_u2: 'b_u2` is known to hold
    // would wind up looking at `Alias<'a_u2>: 'c_u2` as well as `Alias<'a_u2>: 'b_u2`.
    //
    // This is important in two cases. First, if the outliving region is in a lower universe then
    // we've successfully lowered the universe of one of the terms in our alias outlives constraint.
    //
    // Secondly, if it's a region in the current universe *and* it's a region in the components of the
    // alias then we get a less general constraint after replacing with bound vars compared to if we
    // had just replaced the initial region. See `assumptions_on_binders/alias_outlives_with_repeated_placeholder.rs`
    let escaping_r = bound_outlives.skip_binder().1;
    if max_universe(infcx, escaping_r) < u || escaping_r.has_escaping_bound_vars() {
        // we don't know anything about lower universe regions, and we also don't bother
        // looking at the assumptions on the `AliasTyOutlivesViaEnv` binder.
        candidates.push(higher_ranked_candidate_for_outlived_region(escaping_r))
    } else {
        for r in regions_outliving(escaping_r, assumptions, infcx.cx()) {
            candidates.push(higher_ranked_candidate_for_outlived_region(r));
        }
    };

    // Actually look at the assumptions and matching our higher ranked alias outlives goal
    // against potentially higher ranked type outlives assumptions.
    candidates.push(alias_outlives_candidates_from_assumptions(infcx, bound_outlives, assumptions));

    // I'm not convinced our handling here is *complete* so for now
    // let's be conservative and not let alias outlives' cause NoSolution
    // in coherence
    match infcx.typing_mode_raw() {
        TypingMode::Coherence => candidates.push(Or::new_ambig(())),
        TypingMode::Typeck { .. }
        | TypingMode::Reflection
        | TypingMode::ErasedNotCoherence { .. }
        | TypingMode::PostTypeckUntilBorrowck { .. }
        | TypingMode::PostBorrowck { .. }
        | TypingMode::PostAnalysis
        | TypingMode::Codegen => (),
    };

    candidates.into_iter().fold(Or::new_false(), |acc, c| Or::build_or(acc, c))
}

/// Returns all regions `r2` for which `r: r2` is known to hold in
/// the universe associated with `assumptions`
pub fn regions_outlived_by<I: Interner>(
    r: Region<I>,
    assumptions: &Assumptions<I>,
) -> impl Iterator<Item = Region<I>> {
    assumptions.region_outlives.reachable_from(r).into_iter().chain([r])
}

/// Returns all regions `r2` for which `r2: r` is known to hold in
/// the universe associated with `assumptions`
pub fn regions_outliving<I: Interner>(
    r: Region<I>,
    assumptions: &Assumptions<I>,
    cx: I,
) -> impl Iterator<Item = Region<I>> {
    assumptions
        .inverse_region_outlives
        .reachable_from(r)
        .into_iter()
        // FIXME(-Zassumptions-on-binders): 'static may have been an input region canonicalized to something else is that important?
        // FIXME(-Zassumptions-on-binders): do we need to adding the reflexive edge here?
        .chain([r, Region::new_static(cx)])
}

/// Returns all regions `r` for which `!t: r` is known to hold in
/// the universe associated with `assumptions`
pub fn regions_outlived_by_placeholder<I: Interner>(
    t: I::Ty,
    assumptions: &Assumptions<I>,
    cx: I,
) -> impl Iterator<Item = Region<I>> {
    match t.kind() {
        TyKind::Placeholder(..) | TyKind::Param(..) => (),
        _ => unreachable!("non-placeholder in `regions_outlived_by_placeholder`: {t:?}"),
    }

    assumptions.type_outlives.iter().flat_map(move |binder| match binder.no_bound_vars() {
        Some(OutlivesClause(ty, r)) => (ty == t).then_some(r),
        None => Some(Region::new_static(cx)),
    })
}

pub struct MakeHigherRankederer<'a, I: Interner, Infcx: InferCtxtLike<Interner = I>> {
    cx: &'a Infcx,
    preexisting_var_count: usize,
    regions: IndexMap<Region<I>, BoundRegion<I>>,
    universe: UniverseIndex,
    current_index: DebruijnIndex,
}

impl<'a, I: Interner, Infcx: InferCtxtLike<Interner = I>> MakeHigherRankederer<'a, I, Infcx> {
    fn existing_var_count(&self) -> usize {
        self.preexisting_var_count + self.regions.len()
    }
}

impl<'a, I: Interner, Infcx: InferCtxtLike<Interner = I>> TypeFolder<I>
    for MakeHigherRankederer<'a, I, Infcx>
{
    fn cx(&self) -> I {
        self.cx.cx()
    }

    fn fold_region(&mut self, r: Region<I>) -> Region<I> {
        let cx = self.cx();

        match r.kind() {
            RegionKind::RePlaceholder(p) if p.universe == self.universe => {
                let existing_var_count = self.existing_var_count();
                let mapped_var = self.regions.entry(r).or_insert(BoundRegion {
                    var: BoundVar::from_usize(existing_var_count),
                    kind: p.bound.kind,
                });
                Region::new_bound(cx, self.current_index, *mapped_var)
            }
            RegionKind::ReVar(inf) => {
                let u = self.cx.universe_of_region(inf).unwrap();
                if u == self.universe {
                    let existing_var_count = self.existing_var_count();
                    let mapped_var = self.regions.entry(r).or_insert(BoundRegion {
                        var: BoundVar::from_usize(existing_var_count),
                        kind: BoundRegionKind::Anon,
                    });
                    Region::new_bound(cx, self.current_index, *mapped_var)
                } else {
                    r
                }
            }
            _ => r,
        }
    }

    fn fold_binder<T: TypeFoldable<I>>(&mut self, b: Binder<I, T>) -> Binder<I, T> {
        self.current_index.shift_in(1);
        let b = b.super_fold_with(self);
        self.current_index.shift_out(1);
        b
    }
}

/// Converts an `AliasTyOutlivesViaEnv` constraint into an OR of region outlives constraints by
/// matching the alias against any `Alias: 'a` assumptions. This is somewhat tricky as we have a
/// potentially higher ranked alias being equated with a potentially higher ranked assumption and
/// we don't handle it correctly right now (though it is a somewhat reasonable halfway step).
#[instrument(level = "debug", skip(infcx), ret)]
fn alias_outlives_candidates_from_assumptions<Infcx: InferCtxtLike<Interner = I>, I: Interner>(
    infcx: &Infcx,
    bound_outlives: Binder<I, (AliasTy<I>, Region<I>)>,
    assumptions: &Assumptions<I>,
) -> Or<I> {
    let mut candidates = Vec::new();

    let prev_universe = infcx.universe();

    infcx.enter_forall_with_empty_assumptions(bound_outlives, |(alias, r)| {
        for bound_type_outlives in assumptions.type_outlives.iter() {
            let OutlivesClause(alias2, r2) =
                infcx.instantiate_binder_with_infer(*bound_type_outlives);

            let mut relation = HigherRankedAliasMatcher {
                infcx,
                region_constraints: vec![LeafRegionConstraint::RegionOutlives(r2, r, ())],
            };

            // FIXME(#155345): Both sides should be rigid in the future.
            // Currently we can't guarantee that.
            if let Ok(_) = relation.relate(
                alias.to_ty(infcx.cx(), IsRigid::No),
                set_aliases_to_non_rigid(infcx.cx(), alias2).skip_norm_wip(),
            ) {
                candidates.push(And::new(relation.region_constraints));
            }
        }
    });

    let constraint = RegionConstraint::new_from_or(Or::new(candidates));

    let largest_universe = infcx.universe();
    debug!(?prev_universe, ?largest_universe);

    let canonical_constraint = ((prev_universe.index() + 1)..=largest_universe.index())
        .map(|u| UniverseIndex::from_usize(u))
        .rev()
        .fold(constraint, |constraint, u| {
            eagerly_handle_placeholders_in_universe(infcx, constraint, u)
        });

    canonical_constraint.splatted_and_constraints()
}

struct HigherRankedAliasMatcher<'a, Infcx: InferCtxtLike<Interner = I>, I: Interner> {
    infcx: &'a Infcx,
    region_constraints: Vec<LeafRegionConstraint<I>>,
}

impl<'a, Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeRelation<I>
    for HigherRankedAliasMatcher<'a, Infcx, I>
{
    fn cx(&self) -> I {
        self.infcx.cx()
    }

    fn relate_ty_args(
        &mut self,
        a_ty: I::Ty,
        _b_ty: I::Ty,
        _ty_def_id: I::DefId,
        a_args: I::GenericArgs,
        b_args: I::GenericArgs,
        _mk: impl FnOnce(I::GenericArgs) -> I::Ty,
    ) -> RelateResult<I, I::Ty> {
        rustc_type_ir::relate::relate_args_invariantly(self, a_args, b_args)?;
        Ok(a_ty)
    }

    fn relate_with_variance<T: Relate<I>>(
        &mut self,
        _variance: Variance,
        _info: VarianceDiagInfo<I>,
        a: T,
        b: T,
    ) -> RelateResult<I, T> {
        // FIXME(-Zassumptions-on-binders): bivariance is important for opaque type args so
        // we should actually handle variance in some way here.
        self.relate(a, b)
    }

    fn tys(&mut self, a: I::Ty, b: I::Ty) -> RelateResult<I, I::Ty> {
        rustc_type_ir::relate::structurally_relate_tys(self, a, b)
    }

    fn regions(&mut self, a: Region<I>, b: Region<I>) -> RelateResult<I, Region<I>> {
        if a != b {
            self.region_constraints.push(LeafRegionConstraint::RegionOutlives(a, b, ()));
            self.region_constraints.push(LeafRegionConstraint::RegionOutlives(b, a, ()));
        }
        Ok(a)
    }

    fn consts(&mut self, a: Const<I>, b: Const<I>) -> RelateResult<I, Const<I>> {
        rustc_type_ir::relate::structurally_relate_consts(self, a, b)
    }

    fn binders<T>(&mut self, a: Binder<I, T>, b: Binder<I, T>) -> RelateResult<I, Binder<I, T>>
    where
        T: Relate<I>,
    {
        self.infcx.enter_forall_with_empty_assumptions(a, |a| {
            let u = self.infcx.universe();
            self.infcx.insert_placeholder_assumptions(u, Assumptions::empty());
            let b = self.infcx.instantiate_binder_with_infer(b);
            self.relate(a, b)
        })?;

        self.infcx.enter_forall_with_empty_assumptions(b, |b| {
            let u = self.infcx.universe();
            self.infcx.insert_placeholder_assumptions(u, Assumptions::empty());
            let a = self.infcx.instantiate_binder_with_infer(a);
            self.relate(a, b)
        })?;

        Ok(a)
    }
}