Rebase fallout from TypeRelating::binders, inline higher_ranked_sub

compiler/rustc_infer/src/infer/relate/higher_ranked.rs

@@ -1,70 +1,11 @@
 //! Helper routines for higher-ranked things. See the `doc` module at
 //! the end of the file for details.
 
-use super::combine::CombineFields;
 use crate::infer::CombinedSnapshot;
-use crate::infer::{HigherRankedType, InferCtxt};
+use crate::infer::InferCtxt;
 use rustc_middle::ty::fold::FnMutDelegate;
-use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
-use rustc_middle::ty::{self, Binder, Ty, TyCtxt, TypeFoldable};
-
-impl<'a, 'tcx> CombineFields<'a, 'tcx> {
-    /// Checks whether `for<..> sub <: for<..> sup` holds.
-    ///
-    /// For this to hold, **all** instantiations of the super type
-    /// have to be a super type of **at least one** instantiation of
-    /// the subtype.
-    ///
-    /// This is implemented by first entering a new universe.
-    /// We then replace all bound variables in `sup` with placeholders,
-    /// and all bound variables in `sub` with inference vars.
-    /// We can then just relate the two resulting types as normal.
-    ///
-    /// Note: this is a subtle algorithm. For a full explanation, please see
-    /// the [rustc dev guide][rd]
-    ///
-    /// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html
-    #[instrument(skip(self), level = "debug")]
-    pub fn higher_ranked_sub<T>(
-        &mut self,
-        sub: Binder<'tcx, T>,
-        sup: Binder<'tcx, T>,
-        sub_is_expected: bool,
-    ) -> RelateResult<'tcx, ()>
-    where
-        T: Relate<'tcx>,
-    {
-        let span = self.trace.cause.span;
-        // First, we instantiate each bound region in the supertype with a
-        // fresh placeholder region. Note that this automatically creates
-        // a new universe if needed.
-        self.infcx.enter_forall(sup, |sup_prime| {
-            // Next, we instantiate each bound region in the subtype
-            // with a fresh region variable. These region variables --
-            // but no other preexisting region variables -- can name
-            // the placeholders.
-            let sub_prime =
-                self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub);
-            debug!("a_prime={:?}", sub_prime);
-            debug!("b_prime={:?}", sup_prime);
-
-            // Compare types now that bound regions have been replaced.
-            // Reorder the inputs so that the expected is passed first.
-            let result = if sub_is_expected {
-                self.sub().relate(sub_prime, sup_prime)
-            } else {
-                self.sup().relate(sup_prime, sub_prime)
-            };
-
-            if result.is_ok() {
-                debug!("OK result={result:?}");
-            }
-            // NOTE: returning the result here would be dangerous as it contains
-            // placeholders which **must not** be named afterwards.
-            result.map(|_| ())
-        })
-    }
-}
+use rustc_middle::ty::relate::RelateResult;
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
 
 impl<'tcx> InferCtxt<'tcx> {
     /// Replaces all bound variables (lifetimes, types, and constants) bound by

compiler/rustc_infer/src/infer/relate/type_relating.rs

@@ -1,4 +1,5 @@
 use super::combine::CombineFields;
+use crate::infer::BoundRegionConversionTime::HigherRankedType;
 use crate::infer::{
     DefineOpaqueTypes, ObligationEmittingRelation, StructurallyRelateAliases, SubregionOrigin,
 };
@@ -214,16 +215,59 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
         {
             self.relate(a, b)?;
         } else {
+            let span = self.fields.trace.cause.span;
+            let infcx = self.fields.infcx;
+
             match self.ambient_variance {
+                // Checks whether `for<..> sub <: for<..> sup` holds.
+                //
+                // For this to hold, **all** instantiations of the super type
+                // have to be a super type of **at least one** instantiation of
+                // the subtype.
+                //
+                // This is implemented by first entering a new universe.
+                // We then replace all bound variables in `sup` with placeholders,
+                // and all bound variables in `sub` with inference vars.
+                // We can then just relate the two resulting types as normal.
+                //
+                // Note: this is a subtle algorithm. For a full explanation, please see
+                // the [rustc dev guide][rd]
+                //
+                // [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html
                 ty::Covariant => {
-                    self.fields.higher_ranked_sub(a, b, true)?;
+                    infcx.enter_forall(b, |b| {
+                        let a = infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, a);
+                        self.relate(a, b)
+                    })?;
                 }
                 ty::Contravariant => {
-                    self.fields.higher_ranked_sub(b, a, false)?;
+                    infcx.enter_forall(a, |a| {
+                        let b = infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, b);
+                        self.relate(a, b)
+                    })?;
                 }
+
+                // When **equating** binders, we check that there is a 1-to-1
+                // correspondence between the bound vars in both types.
+                //
+                // We do so by separately instantiating one of the binders with
+                // placeholders and the other with inference variables and then
+                // equating the instantiated types.
+                //
+                // We want `for<..> A == for<..> B` -- therefore we want
+                // `exists<..> A == for<..> B` and `exists<..> B == for<..> A`.
+                // Check if `exists<..> A == for<..> B`
                 ty::Invariant => {
-                    self.fields.higher_ranked_sub(a, b, true)?;
-                    self.fields.higher_ranked_sub(b, a, false)?;
+                    infcx.enter_forall(b, |b| {
+                        let a = infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, a);
+                        self.relate(a, b)
+                    })?;
+
+                    // Check if `exists<..> B == for<..> A`.
+                    infcx.enter_forall(a, |a| {
+                        let b = infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, b);
+                        self.relate(a, b)
+                    })?;
                 }
                 ty::Bivariant => {
                     unreachable!("Expected bivariance to be handled in relate_with_variance")