Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1080 insertions, 0 deletions

diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
new file mode 100644
index 000000000..bab4f3e9e
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -0,0 +1,1080 @@
+//! This code is kind of an alternate way of doing subtyping,
+//! supertyping, and type equating, distinct from the `combine.rs`
+//! code but very similar in its effect and design. Eventually the two
+//! ought to be merged. This code is intended for use in NLL and chalk.
+//!
+//! Here are the key differences:
+//!
+//! - This code may choose to bypass some checks (e.g., the occurs check)
+//!   in the case where we know that there are no unbound type inference
+//!   variables. This is the case for NLL, because at NLL time types are fully
+//!   inferred up-to regions.
+//! - This code uses "universes" to handle higher-ranked regions and
+//!   not the leak-check. This is "more correct" than what rustc does
+//!   and we are generally migrating in this direction, but NLL had to
+//!   get there first.
+//!
+//! Also, this code assumes that there are no bound types at all, not even
+//! free ones. This is ok because:
+//! - we are not relating anything quantified over some type variable
+//! - we will have instantiated all the bound type vars already (the one
+//!   thing we relate in chalk are basically domain goals and their
+//!   constituents)
+
+use crate::infer::combine::ConstEquateRelation;
+use crate::infer::InferCtxt;
+use crate::infer::{ConstVarValue, ConstVariableValue};
+use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
+use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
+use rustc_span::Span;
+use std::fmt::Debug;
+use std::ops::ControlFlow;
+
+#[derive(PartialEq)]
+pub enum NormalizationStrategy {
+    Lazy,
+    Eager,
+}
+
+pub struct TypeRelating<'me, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    infcx: &'me InferCtxt<'me, 'tcx>,
+
+    /// Callback to use when we deduce an outlives relationship.
+    delegate: D,
+
+    /// How are we relating `a` and `b`?
+    ///
+    /// - Covariant means `a <: b`.
+    /// - Contravariant means `b <: a`.
+    /// - Invariant means `a == b.
+    /// - Bivariant means that it doesn't matter.
+    ambient_variance: ty::Variance,
+
+    ambient_variance_info: ty::VarianceDiagInfo<'tcx>,
+
+    /// When we pass through a set of binders (e.g., when looking into
+    /// a `fn` type), we push a new bound region scope onto here. This
+    /// will contain the instantiated region for each region in those
+    /// binders. When we then encounter a `ReLateBound(d, br)`, we can
+    /// use the De Bruijn index `d` to find the right scope, and then
+    /// bound region name `br` to find the specific instantiation from
+    /// within that scope. See `replace_bound_region`.
+    ///
+    /// This field stores the instantiations for late-bound regions in
+    /// the `a` type.
+    a_scopes: Vec<BoundRegionScope<'tcx>>,
+
+    /// Same as `a_scopes`, but for the `b` type.
+    b_scopes: Vec<BoundRegionScope<'tcx>>,
+}
+
+pub trait TypeRelatingDelegate<'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+    fn span(&self) -> Span;
+
+    /// Push a constraint `sup: sub` -- this constraint must be
+    /// satisfied for the two types to be related. `sub` and `sup` may
+    /// be regions from the type or new variables created through the
+    /// delegate.
+    fn push_outlives(
+        &mut self,
+        sup: ty::Region<'tcx>,
+        sub: ty::Region<'tcx>,
+        info: ty::VarianceDiagInfo<'tcx>,
+    );
+
+    fn const_equate(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
+    fn register_opaque_type(
+        &mut self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        a_is_expected: bool,
+    ) -> Result<(), TypeError<'tcx>>;
+
+    /// Creates a new universe index. Used when instantiating placeholders.
+    fn create_next_universe(&mut self) -> ty::UniverseIndex;
+
+    /// Creates a new region variable representing a higher-ranked
+    /// region that is instantiated existentially. This creates an
+    /// inference variable, typically.
+    ///
+    /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
+    /// we will invoke this method to instantiate `'a` with an
+    /// inference variable (though `'b` would be instantiated first,
+    /// as a placeholder).
+    fn next_existential_region_var(&mut self, was_placeholder: bool) -> ty::Region<'tcx>;
+
+    /// Creates a new region variable representing a
+    /// higher-ranked region that is instantiated universally.
+    /// This creates a new region placeholder, typically.
+    ///
+    /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
+    /// we will invoke this method to instantiate `'b` with a
+    /// placeholder region.
+    fn next_placeholder_region(&mut self, placeholder: ty::PlaceholderRegion) -> ty::Region<'tcx>;
+
+    /// Creates a new existential region in the given universe. This
+    /// is used when handling subtyping and type variables -- if we
+    /// have that `?X <: Foo<'a>`, for example, we would instantiate
+    /// `?X` with a type like `Foo<'?0>` where `'?0` is a fresh
+    /// existential variable created by this function. We would then
+    /// relate `Foo<'?0>` with `Foo<'a>` (and probably add an outlives
+    /// relation stating that `'?0: 'a`).
+    fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx>;
+
+    /// Define the normalization strategy to use, eager or lazy.
+    fn normalization() -> NormalizationStrategy;
+
+    /// Enables some optimizations if we do not expect inference variables
+    /// in the RHS of the relation.
+    fn forbid_inference_vars() -> bool;
+}
+
+#[derive(Clone, Debug, Default)]
+struct BoundRegionScope<'tcx> {
+    map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>,
+}
+
+#[derive(Copy, Clone)]
+struct UniversallyQuantified(bool);
+
+impl<'me, 'tcx, D> TypeRelating<'me, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    pub fn new(
+        infcx: &'me InferCtxt<'me, 'tcx>,
+        delegate: D,
+        ambient_variance: ty::Variance,
+    ) -> Self {
+        Self {
+            infcx,
+            delegate,
+            ambient_variance,
+            ambient_variance_info: ty::VarianceDiagInfo::default(),
+            a_scopes: vec![],
+            b_scopes: vec![],
+        }
+    }
+
+    fn ambient_covariance(&self) -> bool {
+        match self.ambient_variance {
+            ty::Variance::Covariant | ty::Variance::Invariant => true,
+            ty::Variance::Contravariant | ty::Variance::Bivariant => false,
+        }
+    }
+
+    fn ambient_contravariance(&self) -> bool {
+        match self.ambient_variance {
+            ty::Variance::Contravariant | ty::Variance::Invariant => true,
+            ty::Variance::Covariant | ty::Variance::Bivariant => false,
+        }
+    }
+
+    fn create_scope(
+        &mut self,
+        value: ty::Binder<'tcx, impl Relate<'tcx>>,
+        universally_quantified: UniversallyQuantified,
+    ) -> BoundRegionScope<'tcx> {
+        let mut scope = BoundRegionScope::default();
+
+        // Create a callback that creates (via the delegate) either an
+        // existential or placeholder region as needed.
+        let mut next_region = {
+            let delegate = &mut self.delegate;
+            let mut lazy_universe = None;
+            move |br: ty::BoundRegion| {
+                if universally_quantified.0 {
+                    // The first time this closure is called, create a
+                    // new universe for the placeholders we will make
+                    // from here out.
+                    let universe = lazy_universe.unwrap_or_else(|| {
+                        let universe = delegate.create_next_universe();
+                        lazy_universe = Some(universe);
+                        universe
+                    });
+
+                    let placeholder = ty::PlaceholderRegion { universe, name: br.kind };
+                    delegate.next_placeholder_region(placeholder)
+                } else {
+                    delegate.next_existential_region_var(true)
+                }
+            }
+        };
+
+        value.skip_binder().visit_with(&mut ScopeInstantiator {
+            next_region: &mut next_region,
+            target_index: ty::INNERMOST,
+            bound_region_scope: &mut scope,
+        });
+
+        scope
+    }
+
+    /// When we encounter binders during the type traversal, we record
+    /// the value to substitute for each of the things contained in
+    /// that binder. (This will be either a universal placeholder or
+    /// an existential inference variable.) Given the De Bruijn index
+    /// `debruijn` (and name `br`) of some binder we have now
+    /// encountered, this routine finds the value that we instantiated
+    /// the region with; to do so, it indexes backwards into the list
+    /// of ambient scopes `scopes`.
+    fn lookup_bound_region(
+        debruijn: ty::DebruijnIndex,
+        br: &ty::BoundRegion,
+        first_free_index: ty::DebruijnIndex,
+        scopes: &[BoundRegionScope<'tcx>],
+    ) -> ty::Region<'tcx> {
+        // The debruijn index is a "reverse index" into the
+        // scopes listing. So when we have INNERMOST (0), we
+        // want the *last* scope pushed, and so forth.
+        let debruijn_index = debruijn.index() - first_free_index.index();
+        let scope = &scopes[scopes.len() - debruijn_index - 1];
+
+        // Find this bound region in that scope to map to a
+        // particular region.
+        scope.map[br]
+    }
+
+    /// If `r` is a bound region, find the scope in which it is bound
+    /// (from `scopes`) and return the value that we instantiated it
+    /// with. Otherwise just return `r`.
+    fn replace_bound_region(
+        &self,
+        r: ty::Region<'tcx>,
+        first_free_index: ty::DebruijnIndex,
+        scopes: &[BoundRegionScope<'tcx>],
+    ) -> ty::Region<'tcx> {
+        debug!("replace_bound_regions(scopes={:?})", scopes);
+        if let ty::ReLateBound(debruijn, br) = *r {
+            Self::lookup_bound_region(debruijn, &br, first_free_index, scopes)
+        } else {
+            r
+        }
+    }
+
+    /// Push a new outlives requirement into our output set of
+    /// constraints.
+    fn push_outlives(
+        &mut self,
+        sup: ty::Region<'tcx>,
+        sub: ty::Region<'tcx>,
+        info: ty::VarianceDiagInfo<'tcx>,
+    ) {
+        debug!("push_outlives({:?}: {:?})", sup, sub);
+
+        self.delegate.push_outlives(sup, sub, info);
+    }
+
+    /// Relate a projection type and some value type lazily. This will always
+    /// succeed, but we push an additional `ProjectionEq` goal depending
+    /// on the value type:
+    /// - if the value type is any type `T` which is not a projection, we push
+    ///   `ProjectionEq(projection = T)`.
+    /// - if the value type is another projection `other_projection`, we create
+    ///   a new inference variable `?U` and push the two goals
+    ///   `ProjectionEq(projection = ?U)`, `ProjectionEq(other_projection = ?U)`.
+    fn relate_projection_ty(
+        &mut self,
+        projection_ty: ty::ProjectionTy<'tcx>,
+        value_ty: Ty<'tcx>,
+    ) -> Ty<'tcx> {
+        use rustc_span::DUMMY_SP;
+
+        match *value_ty.kind() {
+            ty::Projection(other_projection_ty) => {
+                let var = self.infcx.next_ty_var(TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::MiscVariable,
+                    span: DUMMY_SP,
+                });
+                // FIXME(lazy-normalization): This will always ICE, because the recursive
+                // call will end up in the _ arm below.
+                self.relate_projection_ty(projection_ty, var);
+                self.relate_projection_ty(other_projection_ty, var);
+                var
+            }
+
+            _ => bug!("should never be invoked with eager normalization"),
+        }
+    }
+
+    /// Relate a type inference variable with a value type. This works
+    /// by creating a "generalization" G of the value where all the
+    /// lifetimes are replaced with fresh inference values. This
+    /// generalization G becomes the value of the inference variable,
+    /// and is then related in turn to the value. So e.g. if you had
+    /// `vid = ?0` and `value = &'a u32`, we might first instantiate
+    /// `?0` to a type like `&'0 u32` where `'0` is a fresh variable,
+    /// and then relate `&'0 u32` with `&'a u32` (resulting in
+    /// relations between `'0` and `'a`).
+    ///
+    /// The variable `pair` can be either a `(vid, ty)` or `(ty, vid)`
+    /// -- in other words, it is always an (unresolved) inference
+    /// variable `vid` and a type `ty` that are being related, but the
+    /// vid may appear either as the "a" type or the "b" type,
+    /// depending on where it appears in the tuple. The trait
+    /// `VidValuePair` lets us work with the vid/type while preserving
+    /// the "sidedness" when necessary -- the sidedness is relevant in
+    /// particular for the variance and set of in-scope things.
+    fn relate_ty_var<PAIR: VidValuePair<'tcx>>(
+        &mut self,
+        pair: PAIR,
+    ) -> RelateResult<'tcx, Ty<'tcx>> {
+        debug!("relate_ty_var({:?})", pair);
+
+        let vid = pair.vid();
+        let value_ty = pair.value_ty();
+
+        // FIXME(invariance) -- this logic assumes invariance, but that is wrong.
+        // This only presently applies to chalk integration, as NLL
+        // doesn't permit type variables to appear on both sides (and
+        // doesn't use lazy norm).
+        match *value_ty.kind() {
+            ty::Infer(ty::TyVar(value_vid)) => {
+                // Two type variables: just equate them.
+                self.infcx.inner.borrow_mut().type_variables().equate(vid, value_vid);
+                return Ok(value_ty);
+            }
+
+            ty::Projection(projection_ty) if D::normalization() == NormalizationStrategy::Lazy => {
+                return Ok(self.relate_projection_ty(projection_ty, self.infcx.tcx.mk_ty_var(vid)));
+            }
+
+            _ => (),
+        }
+
+        let generalized_ty = self.generalize_value(value_ty, vid)?;
+        debug!("relate_ty_var: generalized_ty = {:?}", generalized_ty);
+
+        if D::forbid_inference_vars() {
+            // In NLL, we don't have type inference variables
+            // floating around, so we can do this rather imprecise
+            // variant of the occurs-check.
+            assert!(!generalized_ty.has_infer_types_or_consts());
+        }
+
+        self.infcx.inner.borrow_mut().type_variables().instantiate(vid, generalized_ty);
+
+        // The generalized values we extract from `canonical_var_values` have
+        // been fully instantiated and hence the set of scopes we have
+        // doesn't matter -- just to be sure, put an empty vector
+        // in there.
+        let old_a_scopes = std::mem::take(pair.vid_scopes(self));
+
+        // Relate the generalized kind to the original one.
+        let result = pair.relate_generalized_ty(self, generalized_ty);
+
+        // Restore the old scopes now.
+        *pair.vid_scopes(self) = old_a_scopes;
+
+        debug!("relate_ty_var: complete, result = {:?}", result);
+        result
+    }
+
+    fn generalize_value<T: Relate<'tcx>>(
+        &mut self,
+        value: T,
+        for_vid: ty::TyVid,
+    ) -> RelateResult<'tcx, T> {
+        let universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
+
+        let mut generalizer = TypeGeneralizer {
+            infcx: self.infcx,
+            delegate: &mut self.delegate,
+            first_free_index: ty::INNERMOST,
+            ambient_variance: self.ambient_variance,
+            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
+            universe,
+        };
+
+        generalizer.relate(value, value)
+    }
+}
+
+/// When we instantiate an inference variable with a value in
+/// `relate_ty_var`, we always have the pair of a `TyVid` and a `Ty`,
+/// but the ordering may vary (depending on whether the inference
+/// variable was found on the `a` or `b` sides). Therefore, this trait
+/// allows us to factor out common code, while preserving the order
+/// when needed.
+trait VidValuePair<'tcx>: Debug {
+    /// Extract the inference variable (which could be either the
+    /// first or second part of the tuple).
+    fn vid(&self) -> ty::TyVid;
+
+    /// Extract the value it is being related to (which will be the
+    /// opposite part of the tuple from the vid).
+    fn value_ty(&self) -> Ty<'tcx>;
+
+    /// Extract the scopes that apply to whichever side of the tuple
+    /// the vid was found on.  See the comment where this is called
+    /// for more details on why we want them.
+    fn vid_scopes<'r, D: TypeRelatingDelegate<'tcx>>(
+        &self,
+        relate: &'r mut TypeRelating<'_, 'tcx, D>,
+    ) -> &'r mut Vec<BoundRegionScope<'tcx>>;
+
+    /// Given a generalized type G that should replace the vid, relate
+    /// G to the value, putting G on whichever side the vid would have
+    /// appeared.
+    fn relate_generalized_ty<D>(
+        &self,
+        relate: &mut TypeRelating<'_, 'tcx, D>,
+        generalized_ty: Ty<'tcx>,
+    ) -> RelateResult<'tcx, Ty<'tcx>>
+    where
+        D: TypeRelatingDelegate<'tcx>;
+}
+
+impl<'tcx> VidValuePair<'tcx> for (ty::TyVid, Ty<'tcx>) {
+    fn vid(&self) -> ty::TyVid {
+        self.0
+    }
+
+    fn value_ty(&self) -> Ty<'tcx> {
+        self.1
+    }
+
+    fn vid_scopes<'r, D>(
+        &self,
+        relate: &'r mut TypeRelating<'_, 'tcx, D>,
+    ) -> &'r mut Vec<BoundRegionScope<'tcx>>
+    where
+        D: TypeRelatingDelegate<'tcx>,
+    {
+        &mut relate.a_scopes
+    }
+
+    fn relate_generalized_ty<D>(
+        &self,
+        relate: &mut TypeRelating<'_, 'tcx, D>,
+        generalized_ty: Ty<'tcx>,
+    ) -> RelateResult<'tcx, Ty<'tcx>>
+    where
+        D: TypeRelatingDelegate<'tcx>,
+    {
+        relate.relate(generalized_ty, self.value_ty())
+    }
+}
+
+// In this case, the "vid" is the "b" type.
+impl<'tcx> VidValuePair<'tcx> for (Ty<'tcx>, ty::TyVid) {
+    fn vid(&self) -> ty::TyVid {
+        self.1
+    }
+
+    fn value_ty(&self) -> Ty<'tcx> {
+        self.0
+    }
+
+    fn vid_scopes<'r, D>(
+        &self,
+        relate: &'r mut TypeRelating<'_, 'tcx, D>,
+    ) -> &'r mut Vec<BoundRegionScope<'tcx>>
+    where
+        D: TypeRelatingDelegate<'tcx>,
+    {
+        &mut relate.b_scopes
+    }
+
+    fn relate_generalized_ty<D>(
+        &self,
+        relate: &mut TypeRelating<'_, 'tcx, D>,
+        generalized_ty: Ty<'tcx>,
+    ) -> RelateResult<'tcx, Ty<'tcx>>
+    where
+        D: TypeRelatingDelegate<'tcx>,
+    {
+        relate.relate(self.value_ty(), generalized_ty)
+    }
+}
+
+impl<'tcx, D> TypeRelation<'tcx> for TypeRelating<'_, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.delegate.param_env()
+    }
+
+    fn tag(&self) -> &'static str {
+        "nll::subtype"
+    }
+
+    fn a_is_expected(&self) -> bool {
+        true
+    }
+
+    #[instrument(skip(self, info), level = "trace")]
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        variance: ty::Variance,
+        info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        let old_ambient_variance = self.ambient_variance;
+        self.ambient_variance = self.ambient_variance.xform(variance);
+        self.ambient_variance_info = self.ambient_variance_info.xform(info);
+
+        debug!(?self.ambient_variance);
+
+        let r = self.relate(a, b)?;
+
+        self.ambient_variance = old_ambient_variance;
+
+        debug!(?r);
+
+        Ok(r)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn tys(&mut self, a: Ty<'tcx>, mut b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        let infcx = self.infcx;
+
+        let a = self.infcx.shallow_resolve(a);
+
+        if !D::forbid_inference_vars() {
+            b = self.infcx.shallow_resolve(b);
+        }
+
+        if a == b {
+            // Subtle: if a or b has a bound variable that we are lazily
+            // substituting, then even if a == b, it could be that the values we
+            // will substitute for those bound variables are *not* the same, and
+            // hence returning `Ok(a)` is incorrect.
+            if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
+                return Ok(a);
+            }
+        }
+
+        match (a.kind(), b.kind()) {
+            (_, &ty::Infer(ty::TyVar(vid))) => {
+                if D::forbid_inference_vars() {
+                    // Forbid inference variables in the RHS.
+                    bug!("unexpected inference var {:?}", b)
+                } else {
+                    self.relate_ty_var((a, vid))
+                }
+            }
+
+            (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
+
+            (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+                self.infcx.super_combine_tys(self, a, b)
+            }
+            (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..)) if did.is_local() => {
+                let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
+                let mut generalize = |ty, ty_is_expected| {
+                    let var = infcx.next_ty_var_id_in_universe(
+                        TypeVariableOrigin {
+                            kind: TypeVariableOriginKind::MiscVariable,
+                            span: self.delegate.span(),
+                        },
+                        ty::UniverseIndex::ROOT,
+                    );
+                    if ty_is_expected {
+                        self.relate_ty_var((ty, var))
+                    } else {
+                        self.relate_ty_var((var, ty))
+                    }
+                };
+                let (a, b) = match (a.kind(), b.kind()) {
+                    (&ty::Opaque(..), _) => (a, generalize(b, false)?),
+                    (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
+                    _ => unreachable!(),
+                };
+                self.delegate.register_opaque_type(a, b, true)?;
+                trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
+                Ok(a)
+            }
+
+            (&ty::Projection(projection_ty), _)
+                if D::normalization() == NormalizationStrategy::Lazy =>
+            {
+                Ok(self.relate_projection_ty(projection_ty, b))
+            }
+
+            (_, &ty::Projection(projection_ty))
+                if D::normalization() == NormalizationStrategy::Lazy =>
+            {
+                Ok(self.relate_projection_ty(projection_ty, a))
+            }
+
+            _ => {
+                debug!(?a, ?b, ?self.ambient_variance);
+
+                // Will also handle unification of `IntVar` and `FloatVar`.
+                self.infcx.super_combine_tys(self, a, b)
+            }
+        }
+    }
+
+    #[instrument(skip(self), level = "trace")]
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        debug!(?self.ambient_variance);
+
+        let v_a = self.replace_bound_region(a, ty::INNERMOST, &self.a_scopes);
+        let v_b = self.replace_bound_region(b, ty::INNERMOST, &self.b_scopes);
+
+        debug!(?v_a);
+        debug!(?v_b);
+
+        if self.ambient_covariance() {
+            // Covariance: a <= b. Hence, `b: a`.
+            self.push_outlives(v_b, v_a, self.ambient_variance_info);
+        }
+
+        if self.ambient_contravariance() {
+            // Contravariant: b <= a. Hence, `a: b`.
+            self.push_outlives(v_a, v_b, self.ambient_variance_info);
+        }
+
+        Ok(a)
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        mut b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        let a = self.infcx.shallow_resolve(a);
+
+        if !D::forbid_inference_vars() {
+            b = self.infcx.shallow_resolve(b);
+        }
+
+        match b.kind() {
+            ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
+                // Forbid inference variables in the RHS.
+                self.infcx.tcx.sess.delay_span_bug(
+                    self.delegate.span(),
+                    format!("unexpected inference var {:?}", b,),
+                );
+                Ok(a)
+            }
+            // FIXME(invariance): see the related FIXME above.
+            _ => self.infcx.super_combine_consts(self, a, b),
+        }
+    }
+
+    #[instrument(skip(self), level = "trace")]
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        // We want that
+        //
+        // ```
+        // for<'a> fn(&'a u32) -> &'a u32 <:
+        //   fn(&'b u32) -> &'b u32
+        // ```
+        //
+        // but not
+        //
+        // ```
+        // fn(&'a u32) -> &'a u32 <:
+        //   for<'b> fn(&'b u32) -> &'b u32
+        // ```
+        //
+        // We therefore proceed as follows:
+        //
+        // - Instantiate binders on `b` universally, yielding a universe U1.
+        // - Instantiate binders on `a` existentially in U1.
+
+        debug!(?self.ambient_variance);
+
+        if let (Some(a), Some(b)) = (a.no_bound_vars(), b.no_bound_vars()) {
+            // Fast path for the common case.
+            self.relate(a, b)?;
+            return Ok(ty::Binder::dummy(a));
+        }
+
+        if self.ambient_covariance() {
+            // Covariance, so we want `for<..> A <: for<..> B` --
+            // therefore we compare any instantiation of A (i.e., A
+            // instantiated with existentials) against every
+            // instantiation of B (i.e., B instantiated with
+            // universals).
+
+            let b_scope = self.create_scope(b, UniversallyQuantified(true));
+            let a_scope = self.create_scope(a, UniversallyQuantified(false));
+
+            debug!(?a_scope, "(existential)");
+            debug!(?b_scope, "(universal)");
+
+            self.b_scopes.push(b_scope);
+            self.a_scopes.push(a_scope);
+
+            // Reset the ambient variance to covariant. This is needed
+            // to correctly handle cases like
+            //
+            //     for<'a> fn(&'a u32, &'a u32) == for<'b, 'c> fn(&'b u32, &'c u32)
+            //
+            // Somewhat surprisingly, these two types are actually
+            // **equal**, even though the one on the right looks more
+            // polymorphic. The reason is due to subtyping. To see it,
+            // consider that each function can call the other:
+            //
+            // - The left function can call the right with `'b` and
+            //   `'c` both equal to `'a`
+            //
+            // - The right function can call the left with `'a` set to
+            //   `{P}`, where P is the point in the CFG where the call
+            //   itself occurs. Note that `'b` and `'c` must both
+            //   include P. At the point, the call works because of
+            //   subtyping (i.e., `&'b u32 <: &{P} u32`).
+            let variance = std::mem::replace(&mut self.ambient_variance, ty::Variance::Covariant);
+
+            self.relate(a.skip_binder(), b.skip_binder())?;
+
+            self.ambient_variance = variance;
+
+            self.b_scopes.pop().unwrap();
+            self.a_scopes.pop().unwrap();
+        }
+
+        if self.ambient_contravariance() {
+            // Contravariance, so we want `for<..> A :> for<..> B`
+            // -- therefore we compare every instantiation of A (i.e.,
+            // A instantiated with universals) against any
+            // instantiation of B (i.e., B instantiated with
+            // existentials). Opposite of above.
+
+            let a_scope = self.create_scope(a, UniversallyQuantified(true));
+            let b_scope = self.create_scope(b, UniversallyQuantified(false));
+
+            debug!(?a_scope, "(universal)");
+            debug!(?b_scope, "(existential)");
+
+            self.a_scopes.push(a_scope);
+            self.b_scopes.push(b_scope);
+
+            // Reset ambient variance to contravariance. See the
+            // covariant case above for an explanation.
+            let variance =
+                std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);
+
+            self.relate(a.skip_binder(), b.skip_binder())?;
+
+            self.ambient_variance = variance;
+
+            self.b_scopes.pop().unwrap();
+            self.a_scopes.pop().unwrap();
+        }
+
+        Ok(a)
+    }
+}
+
+impl<'tcx, D> ConstEquateRelation<'tcx> for TypeRelating<'_, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
+        self.delegate.const_equate(a, b);
+    }
+}
+
+/// When we encounter a binder like `for<..> fn(..)`, we actually have
+/// to walk the `fn` value to find all the values bound by the `for`
+/// (these are not explicitly present in the ty representation right
+/// now). This visitor handles that: it descends the type, tracking
+/// binder depth, and finds late-bound regions targeting the
+/// `for<..`>.  For each of those, it creates an entry in
+/// `bound_region_scope`.
+struct ScopeInstantiator<'me, 'tcx> {
+    next_region: &'me mut dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
+    // The debruijn index of the scope we are instantiating.
+    target_index: ty::DebruijnIndex,
+    bound_region_scope: &'me mut BoundRegionScope<'tcx>,
+}
+
+impl<'me, 'tcx> TypeVisitor<'tcx> for ScopeInstantiator<'me, 'tcx> {
+    fn visit_binder<T: TypeVisitable<'tcx>>(
+        &mut self,
+        t: &ty::Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.target_index.shift_in(1);
+        t.super_visit_with(self);
+        self.target_index.shift_out(1);
+
+        ControlFlow::CONTINUE
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let ScopeInstantiator { bound_region_scope, next_region, .. } = self;
+
+        match *r {
+            ty::ReLateBound(debruijn, br) if debruijn == self.target_index => {
+                bound_region_scope.map.entry(br).or_insert_with(|| next_region(br));
+            }
+
+            _ => {}
+        }
+
+        ControlFlow::CONTINUE
+    }
+}
+
+/// The "type generalizer" is used when handling inference variables.
+///
+/// The basic strategy for handling a constraint like `?A <: B` is to
+/// apply a "generalization strategy" to the type `B` -- this replaces
+/// all the lifetimes in the type `B` with fresh inference
+/// variables. (You can read more about the strategy in this [blog
+/// post].)
+///
+/// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
+/// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
+/// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
+/// establishes `'0: 'x` as a constraint.
+///
+/// As a side-effect of this generalization procedure, we also replace
+/// all the bound regions that we have traversed with concrete values,
+/// so that the resulting generalized type is independent from the
+/// scopes.
+///
+/// [blog post]: https://is.gd/0hKvIr
+struct TypeGeneralizer<'me, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    infcx: &'me InferCtxt<'me, 'tcx>,
+
+    delegate: &'me mut D,
+
+    /// After we generalize this type, we are going to relate it to
+    /// some other type. What will be the variance at this point?
+    ambient_variance: ty::Variance,
+
+    first_free_index: ty::DebruijnIndex,
+
+    /// The vid of the type variable that is in the process of being
+    /// instantiated. If we find this within the value we are folding,
+    /// that means we would have created a cyclic value.
+    for_vid_sub_root: ty::TyVid,
+
+    /// The universe of the type variable that is in the process of being
+    /// instantiated. If we find anything that this universe cannot name,
+    /// we reject the relation.
+    universe: ty::UniverseIndex,
+}
+
+impl<'tcx, D> TypeRelation<'tcx> for TypeGeneralizer<'_, 'tcx, D>
+where
+    D: TypeRelatingDelegate<'tcx>,
+{
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.delegate.param_env()
+    }
+
+    fn tag(&self) -> &'static str {
+        "nll::generalizer"
+    }
+
+    fn a_is_expected(&self) -> bool {
+        true
+    }
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        variance: ty::Variance,
+        _info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        debug!(
+            "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
+            variance, a, b
+        );
+
+        let old_ambient_variance = self.ambient_variance;
+        self.ambient_variance = self.ambient_variance.xform(variance);
+
+        debug!(
+            "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
+            self.ambient_variance
+        );
+
+        let r = self.relate(a, b)?;
+
+        self.ambient_variance = old_ambient_variance;
+
+        debug!("TypeGeneralizer::relate_with_variance: r={:?}", r);
+
+        Ok(r)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, _: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        use crate::infer::type_variable::TypeVariableValue;
+
+        debug!("TypeGeneralizer::tys(a={:?})", a);
+
+        match *a.kind() {
+            ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
+                if D::forbid_inference_vars() =>
+            {
+                bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
+            }
+
+            ty::Infer(ty::TyVar(vid)) => {
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variables = &mut inner.type_variables();
+                let vid = variables.root_var(vid);
+                let sub_vid = variables.sub_root_var(vid);
+                if sub_vid == self.for_vid_sub_root {
+                    // If sub-roots are equal, then `for_vid` and
+                    // `vid` are related via subtyping.
+                    debug!("TypeGeneralizer::tys: occurs check failed");
+                    Err(TypeError::Mismatch)
+                } else {
+                    match variables.probe(vid) {
+                        TypeVariableValue::Known { value: u } => {
+                            drop(variables);
+                            self.relate(u, u)
+                        }
+                        TypeVariableValue::Unknown { universe: _universe } => {
+                            if self.ambient_variance == ty::Bivariant {
+                                // FIXME: we may need a WF predicate (related to #54105).
+                            }
+
+                            let origin = *variables.var_origin(vid);
+
+                            // Replacing with a new variable in the universe `self.universe`,
+                            // it will be unified later with the original type variable in
+                            // the universe `_universe`.
+                            let new_var_id = variables.new_var(self.universe, origin);
+
+                            let u = self.tcx().mk_ty_var(new_var_id);
+                            debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
+                            Ok(u)
+                        }
+                    }
+                }
+            }
+
+            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
+                // No matter what mode we are in,
+                // integer/floating-point types must be equal to be
+                // relatable.
+                Ok(a)
+            }
+
+            ty::Placeholder(placeholder) => {
+                if self.universe.cannot_name(placeholder.universe) {
+                    debug!(
+                        "TypeGeneralizer::tys: root universe {:?} cannot name\
+                         placeholder in universe {:?}",
+                        self.universe, placeholder.universe
+                    );
+                    Err(TypeError::Mismatch)
+                } else {
+                    Ok(a)
+                }
+            }
+
+            _ => relate::super_relate_tys(self, a, a),
+        }
+    }
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        _: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        debug!("TypeGeneralizer::regions(a={:?})", a);
+
+        if let ty::ReLateBound(debruijn, _) = *a && debruijn < self.first_free_index {
+            return Ok(a);
+        }
+
+        // For now, we just always create a fresh region variable to
+        // replace all the regions in the source type. In the main
+        // type checker, we special case the case where the ambient
+        // variance is `Invariant` and try to avoid creating a fresh
+        // region variable, but since this comes up so much less in
+        // NLL (only when users use `_` etc) it is much less
+        // important.
+        //
+        // As an aside, since these new variables are created in
+        // `self.universe` universe, this also serves to enforce the
+        // universe scoping rules.
+        //
+        // FIXME(#54105) -- if the ambient variance is bivariant,
+        // though, we may however need to check well-formedness or
+        // risk a problem like #41677 again.
+
+        let replacement_region_vid = self.delegate.generalize_existential(self.universe);
+
+        Ok(replacement_region_vid)
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        _: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        match a.kind() {
+            ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
+                bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
+            }
+            ty::ConstKind::Infer(InferConst::Var(vid)) => {
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variable_table = &mut inner.const_unification_table();
+                let var_value = variable_table.probe_value(vid);
+                match var_value.val.known() {
+                    Some(u) => self.relate(u, u),
+                    None => {
+                        let new_var_id = variable_table.new_key(ConstVarValue {
+                            origin: var_value.origin,
+                            val: ConstVariableValue::Unknown { universe: self.universe },
+                        });
+                        Ok(self.tcx().mk_const_var(new_var_id, a.ty()))
+                    }
+                }
+            }
+            ty::ConstKind::Unevaluated(..) if self.tcx().lazy_normalization() => Ok(a),
+            _ => relate::super_relate_consts(self, a, a),
+        }
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        _: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        debug!("TypeGeneralizer::binders(a={:?})", a);
+
+        self.first_free_index.shift_in(1);
+        let result = self.relate(a.skip_binder(), a.skip_binder())?;
+        self.first_free_index.shift_out(1);
+        Ok(a.rebind(result))
+    }
+}