relate.rs

//! Generalized type relating mechanism.
//!
//! A type relation `R` relates a pair of values `(A, B)`. `A and B` are usually
//! types or regions but can be other things. Examples of type relations are
//! subtyping, type equality, etc.

use crate::hir::def_id::DefId;
use crate::ty::subst::{Kind, UnpackedKind, SubstsRef};
use crate::ty::{self, Ty, TyCtxt, TypeFoldable};
use crate::ty::error::{ExpectedFound, TypeError};
use crate::mir::interpret::{GlobalId, ConstValue, Scalar};
use crate::util::common::ErrorReported;
use syntax_pos::DUMMY_SP;
use std::rc::Rc;
use std::iter;
use rustc_target::spec::abi;
use crate::hir as ast;
use crate::traits;

pub type RelateResult<'tcx, T> = Result<T, TypeError<'tcx>>;

#[derive(Clone, Debug)]
pub enum Cause {
    ExistentialRegionBound, // relating an existential region bound
}

pub trait TypeRelation<'a, 'gcx: 'a+'tcx, 'tcx: 'a> : Sized {
    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx>;

    /// Returns a static string we can use for printouts.
    fn tag(&self) -> &'static str;

    /// Returns `true` if the value `a` is the "expected" type in the
    /// relation. Just affects error messages.
    fn a_is_expected(&self) -> bool;

    fn with_cause<F,R>(&mut self, _cause: Cause, f: F) -> R
        where F: FnOnce(&mut Self) -> R
    {
        f(self)
    }

    /// Generic relation routine suitable for most anything.
    fn relate<T: Relate<'tcx>>(&mut self, a: &T, b: &T) -> RelateResult<'tcx, T> {
        Relate::relate(self, a, b)
    }

    /// Relate the two substitutions for the given item. The default
    /// is to look up the variance for the item and proceed
    /// accordingly.
    fn relate_item_substs(&mut self,
                          item_def_id: DefId,
                          a_subst: SubstsRef<'tcx>,
                          b_subst: SubstsRef<'tcx>)
                          -> RelateResult<'tcx, SubstsRef<'tcx>>
    {
        debug!("relate_item_substs(item_def_id={:?}, a_subst={:?}, b_subst={:?})",
               item_def_id,
               a_subst,
               b_subst);

        let opt_variances = self.tcx().variances_of(item_def_id);
        relate_substs(self, Some(opt_variances), a_subst, b_subst)
    }

    /// Switch variance for the purpose of relating `a` and `b`.
    fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                             variance: ty::Variance,
                                             a: &T,
                                             b: &T)
                                             -> RelateResult<'tcx, T>;

    // Overrideable relations. You shouldn't typically call these
    // directly, instead call `relate()`, which in turn calls
    // these. This is both more uniform but also allows us to add
    // additional hooks for other types in the future if needed
    // without making older code, which called `relate`, obsolete.

    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>>;

    fn regions(
        &mut self,
        a: ty::Region<'tcx>,
        b: ty::Region<'tcx>
    ) -> RelateResult<'tcx, ty::Region<'tcx>>;

    fn consts(
        &mut self,
        a: &'tcx ty::Const<'tcx>,
        b: &'tcx ty::Const<'tcx>
    ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>;

    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
                  -> RelateResult<'tcx, ty::Binder<T>>
        where T: Relate<'tcx>;
}

pub trait Relate<'tcx>: TypeFoldable<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R, a: &Self, b: &Self)
                           -> RelateResult<'tcx, Self>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a;
}

///////////////////////////////////////////////////////////////////////////
// Relate impls

impl<'tcx> Relate<'tcx> for ty::TypeAndMut<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::TypeAndMut<'tcx>,
                           b: &ty::TypeAndMut<'tcx>)
                           -> RelateResult<'tcx, ty::TypeAndMut<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        debug!("{}.mts({:?}, {:?})",
               relation.tag(),
               a,
               b);
        if a.mutbl != b.mutbl {
            Err(TypeError::Mutability)
        } else {
            let mutbl = a.mutbl;
            let variance = match mutbl {
                ast::Mutability::MutImmutable => ty::Covariant,
                ast::Mutability::MutMutable => ty::Invariant,
            };
            let ty = relation.relate_with_variance(variance, &a.ty, &b.ty)?;
            Ok(ty::TypeAndMut { ty, mutbl })
        }
    }
}

pub fn relate_substs<'a, 'gcx, 'tcx, R>(relation: &mut R,
                                        variances: Option<&[ty::Variance]>,
                                        a_subst: SubstsRef<'tcx>,
                                        b_subst: SubstsRef<'tcx>)
                                        -> RelateResult<'tcx, SubstsRef<'tcx>>
    where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
{
    let tcx = relation.tcx();

    let params = a_subst.iter().zip(b_subst).enumerate().map(|(i, (a, b))| {
        let variance = variances.map_or(ty::Invariant, |v| v[i]);
        relation.relate_with_variance(variance, a, b)
    });

    Ok(tcx.mk_substs(params)?)
}

impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::FnSig<'tcx>,
                           b: &ty::FnSig<'tcx>)
                           -> RelateResult<'tcx, ty::FnSig<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        let tcx = relation.tcx();

        if a.c_variadic != b.c_variadic {
            return Err(TypeError::VariadicMismatch(
                expected_found(relation, &a.c_variadic, &b.c_variadic)));
        }
        let unsafety = relation.relate(&a.unsafety, &b.unsafety)?;
        let abi = relation.relate(&a.abi, &b.abi)?;

        if a.inputs().len() != b.inputs().len() {
            return Err(TypeError::ArgCount);
        }

        let inputs_and_output = a.inputs().iter().cloned()
            .zip(b.inputs().iter().cloned())
            .map(|x| (x, false))
            .chain(iter::once(((a.output(), b.output()), true)))
            .map(|((a, b), is_output)| {
                if is_output {
                    relation.relate(&a, &b)
                } else {
                    relation.relate_with_variance(ty::Contravariant, &a, &b)
                }
            });
        Ok(ty::FnSig {
            inputs_and_output: tcx.mk_type_list(inputs_and_output)?,
            c_variadic: a.c_variadic,
            unsafety,
            abi,
        })
    }
}

impl<'tcx> Relate<'tcx> for ast::Unsafety {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ast::Unsafety,
                           b: &ast::Unsafety)
                           -> RelateResult<'tcx, ast::Unsafety>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        if a != b {
            Err(TypeError::UnsafetyMismatch(expected_found(relation, a, b)))
        } else {
            Ok(*a)
        }
    }
}

impl<'tcx> Relate<'tcx> for abi::Abi {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &abi::Abi,
                           b: &abi::Abi)
                           -> RelateResult<'tcx, abi::Abi>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        if a == b {
            Ok(*a)
        } else {
            Err(TypeError::AbiMismatch(expected_found(relation, a, b)))
        }
    }
}

impl<'tcx> Relate<'tcx> for ty::ProjectionTy<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::ProjectionTy<'tcx>,
                           b: &ty::ProjectionTy<'tcx>)
                           -> RelateResult<'tcx, ty::ProjectionTy<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        if a.item_def_id != b.item_def_id {
            Err(TypeError::ProjectionMismatched(
                expected_found(relation, &a.item_def_id, &b.item_def_id)))
        } else {
            let substs = relation.relate(&a.substs, &b.substs)?;
            Ok(ty::ProjectionTy {
                item_def_id: a.item_def_id,
                substs: &substs,
            })
        }
    }
}

impl<'tcx> Relate<'tcx> for ty::ExistentialProjection<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::ExistentialProjection<'tcx>,
                           b: &ty::ExistentialProjection<'tcx>)
                           -> RelateResult<'tcx, ty::ExistentialProjection<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        if a.item_def_id != b.item_def_id {
            Err(TypeError::ProjectionMismatched(
                expected_found(relation, &a.item_def_id, &b.item_def_id)))
        } else {
            let ty = relation.relate(&a.ty, &b.ty)?;
            let substs = relation.relate(&a.substs, &b.substs)?;
            Ok(ty::ExistentialProjection {
                item_def_id: a.item_def_id,
                substs,
                ty,
            })
        }
    }
}

impl<'tcx> Relate<'tcx> for Vec<ty::PolyExistentialProjection<'tcx>> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &Vec<ty::PolyExistentialProjection<'tcx>>,
                           b: &Vec<ty::PolyExistentialProjection<'tcx>>)
                           -> RelateResult<'tcx, Vec<ty::PolyExistentialProjection<'tcx>>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        // To be compatible, `a` and `b` must be for precisely the
        // same set of traits and item names. We always require that
        // projection bounds lists are sorted by trait-def-id and item-name,
        // so we can just iterate through the lists pairwise, so long as they are the
        // same length.
        if a.len() != b.len() {
            Err(TypeError::ProjectionBoundsLength(expected_found(relation, &a.len(), &b.len())))
        } else {
            a.iter()
             .zip(b)
             .map(|(a, b)| relation.relate(a, b))
             .collect()
        }
    }
}

impl<'tcx> Relate<'tcx> for ty::TraitRef<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::TraitRef<'tcx>,
                           b: &ty::TraitRef<'tcx>)
                           -> RelateResult<'tcx, ty::TraitRef<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        // Different traits cannot be related
        if a.def_id != b.def_id {
            Err(TypeError::Traits(expected_found(relation, &a.def_id, &b.def_id)))
        } else {
            let substs = relate_substs(relation, None, a.substs, b.substs)?;
            Ok(ty::TraitRef { def_id: a.def_id, substs: substs })
        }
    }
}

impl<'tcx> Relate<'tcx> for ty::ExistentialTraitRef<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::ExistentialTraitRef<'tcx>,
                           b: &ty::ExistentialTraitRef<'tcx>)
                           -> RelateResult<'tcx, ty::ExistentialTraitRef<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        // Different traits cannot be related
        if a.def_id != b.def_id {
            Err(TypeError::Traits(expected_found(relation, &a.def_id, &b.def_id)))
        } else {
            let substs = relate_substs(relation, None, a.substs, b.substs)?;
            Ok(ty::ExistentialTraitRef { def_id: a.def_id, substs: substs })
        }
    }
}

#[derive(Debug, Clone)]
struct GeneratorWitness<'tcx>(&'tcx ty::List<Ty<'tcx>>);

TupleStructTypeFoldableImpl! {
    impl<'tcx> TypeFoldable<'tcx> for GeneratorWitness<'tcx> {
        a
    }
}

impl<'tcx> Relate<'tcx> for GeneratorWitness<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &GeneratorWitness<'tcx>,
                           b: &GeneratorWitness<'tcx>)
                           -> RelateResult<'tcx, GeneratorWitness<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        assert_eq!(a.0.len(), b.0.len());
        let tcx = relation.tcx();
        let types = tcx.mk_type_list(a.0.iter().zip(b.0).map(|(a, b)| relation.relate(a, b)))?;
        Ok(GeneratorWitness(types))
    }
}

impl<'tcx> Relate<'tcx> for Ty<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &Ty<'tcx>,
                           b: &Ty<'tcx>)
                           -> RelateResult<'tcx, Ty<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        relation.tys(a, b)
    }
}

/// The main "type relation" routine. Note that this does not handle
/// inference artifacts, so you should filter those out before calling
/// it.
pub fn super_relate_tys<'a, 'gcx, 'tcx, R>(relation: &mut R,
                                           a: Ty<'tcx>,
                                           b: Ty<'tcx>)
                                           -> RelateResult<'tcx, Ty<'tcx>>
    where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
{
    let tcx = relation.tcx();
    debug!("super_relate_tys: a={:?} b={:?}", a, b);
    match (&a.sty, &b.sty) {
        (&ty::Infer(_), _) |
        (_, &ty::Infer(_)) =>
        {
            // The caller should handle these cases!
            bug!("var types encountered in super_relate_tys")
        }

        (ty::Bound(..), _) | (_, ty::Bound(..)) => {
            bug!("bound types encountered in super_relate_tys")
        }

        (&ty::Error, _) | (_, &ty::Error) =>
        {
            Ok(tcx.types.err)
        }

        (&ty::Never, _) |
        (&ty::Char, _) |
        (&ty::Bool, _) |
        (&ty::Int(_), _) |
        (&ty::Uint(_), _) |
        (&ty::Float(_), _) |
        (&ty::Str, _)
            if a == b =>
        {
            Ok(a)
        }

        (&ty::Param(ref a_p), &ty::Param(ref b_p))
            if a_p.index == b_p.index =>
        {
            Ok(a)
        }

        (ty::Placeholder(p1), ty::Placeholder(p2)) if p1 == p2 => {
            Ok(a)
        }

        (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
            if a_def == b_def =>
        {
            let substs = relation.relate_item_substs(a_def.did, a_substs, b_substs)?;
            Ok(tcx.mk_adt(a_def, substs))
        }

        (&ty::Foreign(a_id), &ty::Foreign(b_id))
            if a_id == b_id =>
        {
            Ok(tcx.mk_foreign(a_id))
        }

        (&ty::Dynamic(ref a_obj, ref a_region), &ty::Dynamic(ref b_obj, ref b_region)) => {
            let region_bound = relation.with_cause(Cause::ExistentialRegionBound,
                                                       |relation| {
                                                           relation.relate_with_variance(
                                                               ty::Contravariant,
                                                               a_region,
                                                               b_region)
                                                       })?;
            Ok(tcx.mk_dynamic(relation.relate(a_obj, b_obj)?, region_bound))
        }

        (&ty::Generator(a_id, a_substs, movability),
         &ty::Generator(b_id, b_substs, _))
            if a_id == b_id =>
        {
            // All Generator types with the same id represent
            // the (anonymous) type of the same generator expression. So
            // all of their regions should be equated.
            let substs = relation.relate(&a_substs, &b_substs)?;
            Ok(tcx.mk_generator(a_id, substs, movability))
        }

        (&ty::GeneratorWitness(a_types), &ty::GeneratorWitness(b_types)) =>
        {
            // Wrap our types with a temporary GeneratorWitness struct
            // inside the binder so we can related them
            let a_types = a_types.map_bound(GeneratorWitness);
            let b_types = b_types.map_bound(GeneratorWitness);
            // Then remove the GeneratorWitness for the result
            let types = relation.relate(&a_types, &b_types)?.map_bound(|witness| witness.0);
            Ok(tcx.mk_generator_witness(types))
        }

        (&ty::Closure(a_id, a_substs),
         &ty::Closure(b_id, b_substs))
            if a_id == b_id =>
        {
            // All Closure types with the same id represent
            // the (anonymous) type of the same closure expression. So
            // all of their regions should be equated.
            let substs = relation.relate(&a_substs, &b_substs)?;
            Ok(tcx.mk_closure(a_id, substs))
        }

        (&ty::RawPtr(ref a_mt), &ty::RawPtr(ref b_mt)) =>
        {
            let mt = relation.relate(a_mt, b_mt)?;
            Ok(tcx.mk_ptr(mt))
        }

        (&ty::Ref(a_r, a_ty, a_mutbl), &ty::Ref(b_r, b_ty, b_mutbl)) =>
        {
            let r = relation.relate_with_variance(ty::Contravariant, &a_r, &b_r)?;
            let a_mt = ty::TypeAndMut { ty: a_ty, mutbl: a_mutbl };
            let b_mt = ty::TypeAndMut { ty: b_ty, mutbl: b_mutbl };
            let mt = relation.relate(&a_mt, &b_mt)?;
            Ok(tcx.mk_ref(r, mt))
        }

        (&ty::Array(a_t, sz_a), &ty::Array(b_t, sz_b)) =>
        {
            let t = relation.relate(&a_t, &b_t)?;
            let to_u64 = |x: ty::Const<'tcx>| -> Result<u64, ErrorReported> {
                match x.val {
                    // FIXME(const_generics): this doesn't work right now,
                    // because it tries to relate an `Infer` to a `Param`.
                    ConstValue::Unevaluated(def_id, substs) => {
                        // FIXME(eddyb) get the right param_env.
                        let param_env = ty::ParamEnv::empty();
                        if let Some(substs) = tcx.lift_to_global(&substs) {
                            let instance = ty::Instance::resolve(
                                tcx.global_tcx(),
                                param_env,
                                def_id,
                                substs,
                            );
                            if let Some(instance) = instance {
                                let cid = GlobalId {
                                    instance,
                                    promoted: None,
                                };
                                if let Some(s) = tcx.const_eval(param_env.and(cid))
                                                    .ok()
                                                    .map(|c| c.unwrap_usize(tcx)) {
                                    return Ok(s)
                                }
                            }
                        }
                        tcx.sess.delay_span_bug(tcx.def_span(def_id),
                            "array length could not be evaluated");
                        Err(ErrorReported)
                    }
                    _ => x.assert_usize(tcx).ok_or_else(|| {
                        tcx.sess.delay_span_bug(DUMMY_SP,
                            "array length could not be evaluated");
                        ErrorReported
                    })
                }
            };
            match (to_u64(*sz_a), to_u64(*sz_b)) {
                (Ok(sz_a_u64), Ok(sz_b_u64)) => {
                    if sz_a_u64 == sz_b_u64 {
                        Ok(tcx.mk_ty(ty::Array(t, sz_a)))
                    } else {
                        Err(TypeError::FixedArraySize(
                            expected_found(relation, &sz_a_u64, &sz_b_u64)))
                    }
                }
                // We reported an error or will ICE, so we can return Error.
                (Err(ErrorReported), _) | (_, Err(ErrorReported)) => {
                    Ok(tcx.types.err)
                }
            }
        }

        (&ty::Slice(a_t), &ty::Slice(b_t)) =>
        {
            let t = relation.relate(&a_t, &b_t)?;
            Ok(tcx.mk_slice(t))
        }

        (&ty::Tuple(as_), &ty::Tuple(bs)) =>
        {
            if as_.len() == bs.len() {
                Ok(tcx.mk_tup(as_.iter().zip(bs).map(|(a, b)| {
                    relation.relate(&a.expect_ty(), &b.expect_ty())
                }))?)
            } else if !(as_.is_empty() || bs.is_empty()) {
                Err(TypeError::TupleSize(
                    expected_found(relation, &as_.len(), &bs.len())))
            } else {
                Err(TypeError::Sorts(expected_found(relation, &a, &b)))
            }
        }

        (&ty::FnDef(a_def_id, a_substs), &ty::FnDef(b_def_id, b_substs))
            if a_def_id == b_def_id =>
        {
            let substs = relation.relate_item_substs(a_def_id, a_substs, b_substs)?;
            Ok(tcx.mk_fn_def(a_def_id, substs))
        }

        (&ty::FnPtr(a_fty), &ty::FnPtr(b_fty)) =>
        {
            let fty = relation.relate(&a_fty, &b_fty)?;
            Ok(tcx.mk_fn_ptr(fty))
        }

        (ty::UnnormalizedProjection(a_data), ty::UnnormalizedProjection(b_data)) => {
            let projection_ty = relation.relate(a_data, b_data)?;
            Ok(tcx.mk_ty(ty::UnnormalizedProjection(projection_ty)))
        }

        // these two are already handled downstream in case of lazy normalization
        (ty::Projection(a_data), ty::Projection(b_data)) => {
            let projection_ty = relation.relate(a_data, b_data)?;
            Ok(tcx.mk_projection(projection_ty.item_def_id, projection_ty.substs))
        }

        (&ty::Opaque(a_def_id, a_substs), &ty::Opaque(b_def_id, b_substs))
            if a_def_id == b_def_id =>
        {
            let substs = relate_substs(relation, None, a_substs, b_substs)?;
            Ok(tcx.mk_opaque(a_def_id, substs))
        }

        _ =>
        {
            Err(TypeError::Sorts(expected_found(relation, &a, &b)))
        }
    }
}

/// The main "const relation" routine. Note that this does not handle
/// inference artifacts, so you should filter those out before calling
/// it.
pub fn super_relate_consts<'a, 'gcx, 'tcx, R>(
    relation: &mut R,
    a: &'tcx ty::Const<'tcx>,
    b: &'tcx ty::Const<'tcx>
) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>
where
    R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
{
    let tcx = relation.tcx();

    // Currently, the values that can be unified are those that
    // implement both `PartialEq` and `Eq`, corresponding to
    // `structural_match` types.
    // FIXME(const_generics): check for `structural_match` synthetic attribute.
    match (a.val, b.val) {
        (ConstValue::Infer(_), _) | (_, ConstValue::Infer(_)) => {
            // The caller should handle these cases!
            bug!("var types encountered in super_relate_consts: {:?} {:?}", a, b)
        }
        (ConstValue::Param(a_p), ConstValue::Param(b_p)) if a_p.index == b_p.index => {
            Ok(a)
        }
        (ConstValue::Placeholder(p1), ConstValue::Placeholder(p2)) if p1 == p2 => {
            Ok(a)
        }
        (ConstValue::Scalar(Scalar::Bits { .. }), _) if a == b => {
            Ok(a)
        }
        (ConstValue::ByRef(..), _) => {
            bug!(
                "non-Scalar ConstValue encountered in super_relate_consts {:?} {:?}",
                a,
                b,
            );
        }

        // FIXME(const_generics): this is wrong, as it is a projection
        (ConstValue::Unevaluated(a_def_id, a_substs),
            ConstValue::Unevaluated(b_def_id, b_substs)) if a_def_id == b_def_id => {
                let substs =
                    relation.relate_with_variance(ty::Variance::Invariant, &a_substs, &b_substs)?;
                Ok(tcx.mk_const(ty::Const {
                    val: ConstValue::Unevaluated(a_def_id, &substs),
                    ty: a.ty,
                }))
            }

            _ => {
            Err(TypeError::ConstMismatch(expected_found(relation, &a, &b)))
        }
    }
}

impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &Self,
                           b: &Self)
        -> RelateResult<'tcx, Self>
            where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a {

        if a.len() != b.len() {
            return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
        }

        let tcx = relation.tcx();
        let v = a.iter().zip(b.iter()).map(|(ep_a, ep_b)| {
            use crate::ty::ExistentialPredicate::*;
            match (*ep_a, *ep_b) {
                (Trait(ref a), Trait(ref b)) => Ok(Trait(relation.relate(a, b)?)),
                (Projection(ref a), Projection(ref b)) => Ok(Projection(relation.relate(a, b)?)),
                (AutoTrait(ref a), AutoTrait(ref b)) if a == b => Ok(AutoTrait(*a)),
                _ => Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)))
            }
        });
        Ok(tcx.mk_existential_predicates(v)?)
    }
}

impl<'tcx> Relate<'tcx> for ty::ClosureSubsts<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::ClosureSubsts<'tcx>,
                           b: &ty::ClosureSubsts<'tcx>)
                           -> RelateResult<'tcx, ty::ClosureSubsts<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        let substs = relate_substs(relation, None, a.substs, b.substs)?;
        Ok(ty::ClosureSubsts { substs })
    }
}

impl<'tcx> Relate<'tcx> for ty::GeneratorSubsts<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::GeneratorSubsts<'tcx>,
                           b: &ty::GeneratorSubsts<'tcx>)
                           -> RelateResult<'tcx, ty::GeneratorSubsts<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        let substs = relate_substs(relation, None, a.substs, b.substs)?;
        Ok(ty::GeneratorSubsts { substs })
    }
}

impl<'tcx> Relate<'tcx> for SubstsRef<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &SubstsRef<'tcx>,
                           b: &SubstsRef<'tcx>)
                           -> RelateResult<'tcx, SubstsRef<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        relate_substs(relation, None, a, b)
    }
}

impl<'tcx> Relate<'tcx> for ty::Region<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::Region<'tcx>,
                           b: &ty::Region<'tcx>)
                           -> RelateResult<'tcx, ty::Region<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        relation.regions(*a, *b)
    }
}

impl<'tcx> Relate<'tcx> for &'tcx ty::Const<'tcx> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &&'tcx ty::Const<'tcx>,
                           b: &&'tcx ty::Const<'tcx>)
                           -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        relation.consts(*a, *b)
    }
}

impl<'tcx, T: Relate<'tcx>> Relate<'tcx> for ty::Binder<T> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &ty::Binder<T>,
                           b: &ty::Binder<T>)
                           -> RelateResult<'tcx, ty::Binder<T>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        relation.binders(a, b)
    }
}

impl<'tcx, T: Relate<'tcx>> Relate<'tcx> for Rc<T> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &Rc<T>,
                           b: &Rc<T>)
                           -> RelateResult<'tcx, Rc<T>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        let a: &T = a;
        let b: &T = b;
        Ok(Rc::new(relation.relate(a, b)?))
    }
}

impl<'tcx, T: Relate<'tcx>> Relate<'tcx> for Box<T> {
    fn relate<'a, 'gcx, R>(relation: &mut R,
                           a: &Box<T>,
                           b: &Box<T>)
                           -> RelateResult<'tcx, Box<T>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
    {
        let a: &T = a;
        let b: &T = b;
        Ok(Box::new(relation.relate(a, b)?))
    }
}

impl<'tcx> Relate<'tcx> for Kind<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &Kind<'tcx>,
        b: &Kind<'tcx>
    ) -> RelateResult<'tcx, Kind<'tcx>>
    where
        R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a,
    {
        match (a.unpack(), b.unpack()) {
            (UnpackedKind::Lifetime(a_lt), UnpackedKind::Lifetime(b_lt)) => {
                Ok(relation.relate(&a_lt, &b_lt)?.into())
            }
            (UnpackedKind::Type(a_ty), UnpackedKind::Type(b_ty)) => {
                Ok(relation.relate(&a_ty, &b_ty)?.into())
            }
            (UnpackedKind::Const(a_ct), UnpackedKind::Const(b_ct)) => {
                Ok(relation.relate(&a_ct, &b_ct)?.into())
            }
            (UnpackedKind::Lifetime(unpacked), x) => {
                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
            }
            (UnpackedKind::Type(unpacked), x) => {
                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
            }
            (UnpackedKind::Const(unpacked), x) => {
                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
            }
        }
    }
}

impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &ty::TraitPredicate<'tcx>,
        b: &ty::TraitPredicate<'tcx>
    ) -> RelateResult<'tcx, ty::TraitPredicate<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        Ok(ty::TraitPredicate {
            trait_ref: relation.relate(&a.trait_ref, &b.trait_ref)?,
        })
    }
}

impl<'tcx> Relate<'tcx> for ty::ProjectionPredicate<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &ty::ProjectionPredicate<'tcx>,
        b: &ty::ProjectionPredicate<'tcx>,
    ) -> RelateResult<'tcx, ty::ProjectionPredicate<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        Ok(ty::ProjectionPredicate {
            projection_ty: relation.relate(&a.projection_ty, &b.projection_ty)?,
            ty: relation.relate(&a.ty, &b.ty)?,
        })
    }
}

impl<'tcx> Relate<'tcx> for traits::WhereClause<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::WhereClause<'tcx>,
        b: &traits::WhereClause<'tcx>
    ) -> RelateResult<'tcx, traits::WhereClause<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::WhereClause::*;
        match (a, b) {
            (Implemented(a_pred), Implemented(b_pred)) => {
                Ok(Implemented(relation.relate(a_pred, b_pred)?))
            }

            (ProjectionEq(a_pred), ProjectionEq(b_pred)) => {
                Ok(ProjectionEq(relation.relate(a_pred, b_pred)?))
            }

            (RegionOutlives(a_pred), RegionOutlives(b_pred)) => {
                Ok(RegionOutlives(ty::OutlivesPredicate(
                    relation.relate(&a_pred.0, &b_pred.0)?,
                    relation.relate(&a_pred.1, &b_pred.1)?,
                )))
            }

            (TypeOutlives(a_pred), TypeOutlives(b_pred)) => {
                Ok(TypeOutlives(ty::OutlivesPredicate(
                    relation.relate(&a_pred.0, &b_pred.0)?,
                    relation.relate(&a_pred.1, &b_pred.1)?,
                )))
            }

            _ =>  Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::WellFormed<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::WellFormed<'tcx>,
        b: &traits::WellFormed<'tcx>
    ) -> RelateResult<'tcx, traits::WellFormed<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::WellFormed::*;
        match (a, b) {
            (Trait(a_pred), Trait(b_pred)) => Ok(Trait(relation.relate(a_pred, b_pred)?)),
            (Ty(a_ty), Ty(b_ty)) => Ok(Ty(relation.relate(a_ty, b_ty)?)),
            _ =>  Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::FromEnv<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::FromEnv<'tcx>,
        b: &traits::FromEnv<'tcx>
    ) -> RelateResult<'tcx, traits::FromEnv<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::FromEnv::*;
        match (a, b) {
            (Trait(a_pred), Trait(b_pred)) => Ok(Trait(relation.relate(a_pred, b_pred)?)),
            (Ty(a_ty), Ty(b_ty)) => Ok(Ty(relation.relate(a_ty, b_ty)?)),
            _ =>  Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::DomainGoal<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::DomainGoal<'tcx>,
        b: &traits::DomainGoal<'tcx>
    ) -> RelateResult<'tcx, traits::DomainGoal<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::DomainGoal::*;
        match (a, b) {
            (Holds(a_wc), Holds(b_wc)) => Ok(Holds(relation.relate(a_wc, b_wc)?)),
            (WellFormed(a_wf), WellFormed(b_wf)) => Ok(WellFormed(relation.relate(a_wf, b_wf)?)),
            (FromEnv(a_fe), FromEnv(b_fe)) => Ok(FromEnv(relation.relate(a_fe, b_fe)?)),

            (Normalize(a_pred), Normalize(b_pred)) => {
                Ok(Normalize(relation.relate(a_pred, b_pred)?))
            }

            _ =>  Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::Goal<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::Goal<'tcx>,
        b: &traits::Goal<'tcx>
    ) -> RelateResult<'tcx, traits::Goal<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::GoalKind::*;
        match (a, b) {
            (Implies(a_clauses, a_goal), Implies(b_clauses, b_goal)) => {
                let clauses = relation.relate(a_clauses, b_clauses)?;
                let goal = relation.relate(a_goal, b_goal)?;
                Ok(relation.tcx().mk_goal(Implies(clauses, goal)))
            }

            (And(a_left, a_right), And(b_left, b_right)) => {
                let left = relation.relate(a_left, b_left)?;
                let right = relation.relate(a_right, b_right)?;
                Ok(relation.tcx().mk_goal(And(left, right)))
            }

            (Not(a_goal), Not(b_goal)) => {
                let goal = relation.relate(a_goal, b_goal)?;
                Ok(relation.tcx().mk_goal(Not(goal)))
            }

            (DomainGoal(a_goal), DomainGoal(b_goal)) => {
                let goal = relation.relate(a_goal, b_goal)?;
                Ok(relation.tcx().mk_goal(DomainGoal(goal)))
            }

            (Quantified(a_qkind, a_goal), Quantified(b_qkind, b_goal))
                if a_qkind == b_qkind =>
            {
                let goal = relation.relate(a_goal, b_goal)?;
                Ok(relation.tcx().mk_goal(Quantified(*a_qkind, goal)))
            }

            (CannotProve, CannotProve) => Ok(*a),

            _ => Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::Goals<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::Goals<'tcx>,
        b: &traits::Goals<'tcx>
    ) -> RelateResult<'tcx, traits::Goals<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        if a.len() != b.len() {
            return Err(TypeError::Mismatch);
        }

        let tcx = relation.tcx();
        let goals = a.iter().zip(b.iter()).map(|(a, b)| relation.relate(a, b));
        Ok(tcx.mk_goals(goals)?)
    }
}

impl<'tcx> Relate<'tcx> for traits::Clause<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::Clause<'tcx>,
        b: &traits::Clause<'tcx>
    ) -> RelateResult<'tcx, traits::Clause<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        use crate::traits::Clause::*;
        match (a, b) {
            (Implies(a_clause), Implies(b_clause)) => {
                let clause = relation.relate(a_clause, b_clause)?;
                Ok(Implies(clause))
            }

            (ForAll(a_clause), ForAll(b_clause)) => {
                let clause = relation.relate(a_clause, b_clause)?;
                Ok(ForAll(clause))
            }

            _ => Err(TypeError::Mismatch),
        }
    }
}

impl<'tcx> Relate<'tcx> for traits::Clauses<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::Clauses<'tcx>,
        b: &traits::Clauses<'tcx>
    ) -> RelateResult<'tcx, traits::Clauses<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        if a.len() != b.len() {
            return Err(TypeError::Mismatch);
        }

        let tcx = relation.tcx();
        let clauses = a.iter().zip(b.iter()).map(|(a, b)| relation.relate(a, b));
        Ok(tcx.mk_clauses(clauses)?)
    }
}

impl<'tcx> Relate<'tcx> for traits::ProgramClause<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::ProgramClause<'tcx>,
        b: &traits::ProgramClause<'tcx>
    ) -> RelateResult<'tcx, traits::ProgramClause<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        Ok(traits::ProgramClause {
            goal: relation.relate(&a.goal, &b.goal)?,
            hypotheses: relation.relate(&a.hypotheses, &b.hypotheses)?,
            category: traits::ProgramClauseCategory::Other,
        })
    }
}

impl<'tcx> Relate<'tcx> for traits::Environment<'tcx> {
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::Environment<'tcx>,
        b: &traits::Environment<'tcx>
    ) -> RelateResult<'tcx, traits::Environment<'tcx>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        Ok(traits::Environment {
            clauses: relation.relate(&a.clauses, &b.clauses)?,
        })
    }
}

impl<'tcx, G> Relate<'tcx> for traits::InEnvironment<'tcx, G>
    where G: Relate<'tcx>
{
    fn relate<'a, 'gcx, R>(
        relation: &mut R,
        a: &traits::InEnvironment<'tcx, G>,
        b: &traits::InEnvironment<'tcx, G>
    ) -> RelateResult<'tcx, traits::InEnvironment<'tcx, G>>
        where R: TypeRelation<'a, 'gcx, 'tcx>, 'gcx: 'tcx, 'tcx: 'a
    {
        Ok(traits::InEnvironment {
            environment: relation.relate(&a.environment, &b.environment)?,
            goal: relation.relate(&a.goal, &b.goal)?,
        })
    }
}

///////////////////////////////////////////////////////////////////////////
// Error handling

pub fn expected_found<'a, 'gcx, 'tcx, R, T>(relation: &mut R,
                                            a: &T,
                                            b: &T)
                                            -> ExpectedFound<T>
    where R: TypeRelation<'a, 'gcx, 'tcx>, T: Clone, 'gcx: 'a+'tcx, 'tcx: 'a
{
    expected_found_bool(relation.a_is_expected(), a, b)
}

pub fn expected_found_bool<T>(a_is_expected: bool,
                              a: &T,
                              b: &T)
                              -> ExpectedFound<T>
    where T: Clone
{
    let a = a.clone();
    let b = b.clone();
    if a_is_expected {
        ExpectedFound {expected: a, found: b}
    } else {
        ExpectedFound {expected: b, found: a}
    }
}