Auto merge of rust-lang#120544 - BoxyUwU:enter_forall, r=lcnr

Introduce `enter_forall` to supercede `instantiate_binder_with_placeholders`

r? `@lcnr`

Long term we'd like to experiment with decrementing the universe count after "exiting" binders so that we do not end up creating infer vars in non-root universes even when they logically reside in the root universe. The fact that we dont do this currently results in a number of issues in the new trait solver where we consider goals to be ambiguous because otherwise it would require lowering the universe of an infer var. i.e. the goal  `?x.0 eq <T as Trait<?y.1>>::Assoc` where the alias is rigid would not be able to instantiate `?x` with the alias as there would be a universe error.

This PR is the first-ish sort of step towards being able to implement this as eventually we would want to decrement the universe in `enter_forall`. Unfortunately its Difficult to actually implement decrementing universes nicely so this is a separate step which moves us closer to the long term goal ✨

compiler/rustc_infer/src/infer/mod.rs

@@ -1032,21 +1032,22 @@ impl<'tcx> InferCtxt<'tcx> {
             _ => {}
         }
 
-        let ty::SubtypePredicate { a_is_expected, a, b } =
-            self.instantiate_binder_with_placeholders(predicate);
-
-        Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
+        self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
+            Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
+        })
     }
 
     pub fn region_outlives_predicate(
         &self,
         cause: &traits::ObligationCause<'tcx>,
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) {
-        let ty::OutlivesPredicate(r_a, r_b) = self.instantiate_binder_with_placeholders(predicate);
-        let origin =
-            SubregionOrigin::from_obligation_cause(cause, || RelateRegionParamBound(cause.span));
-        self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
+        self.enter_forall(predicate, |ty::OutlivesPredicate(r_a, r_b)| {
+            let origin = SubregionOrigin::from_obligation_cause(cause, || {
+                RelateRegionParamBound(cause.span)
+            });
+            self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
+        })
     }
 
     /// Number of type variables created so far.
@@ -1455,7 +1456,7 @@ impl<'tcx> InferCtxt<'tcx> {
     // Use this method if you'd like to find some substitution of the binder's
     // variables (e.g. during a method call). If there isn't a [`BoundRegionConversionTime`]
     // that corresponds to your use case, consider whether or not you should
-    // use [`InferCtxt::instantiate_binder_with_placeholders`] instead.
+    // use [`InferCtxt::enter_forall`] instead.
     pub fn instantiate_binder_with_fresh_vars<T>(
         &self,
         span: Span,

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

@@ -38,24 +38,25 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
         // First, we instantiate each bound region in the supertype with a
         // fresh placeholder region. Note that this automatically creates
         // a new universe if needed.
-        let sup_prime = self.infcx.instantiate_binder_with_placeholders(sup);
+        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);
 
-        // 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.
-        let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?;
-
-        debug!("OK result={result:?}");
-        // NOTE: returning the result here would be dangerous as it contains
-        // placeholders which **must not** be named afterwards.
-        Ok(())
+            // Compare types now that bound regions have been replaced.
+            let result = self.sub(sub_is_expected).relate(sub_prime, sup_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(|_| ())
+        })
     }
 }
 
@@ -68,9 +69,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// This is the first step of checking subtyping when higher-ranked things are involved.
     /// For more details visit the relevant sections of the [rustc dev guide].
     ///
+    /// `fn enter_forall` should be preferred over this method.
+    ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn instantiate_binder_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
+    pub fn enter_forall_and_leak_universe<T>(&self, binder: ty::Binder<'tcx, T>) -> T
     where
         T: TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
@@ -106,6 +109,31 @@ impl<'tcx> InferCtxt<'tcx> {
         self.tcx.replace_bound_vars_uncached(binder, delegate)
     }
 
+    /// Replaces all bound variables (lifetimes, types, and constants) bound by
+    /// `binder` with placeholder variables in a new universe and then calls the
+    /// closure `f` with the instantiated value. The new placeholders can only be
+    /// named by inference variables created inside of the closure `f` or afterwards.
+    ///
+    /// This is the first step of checking subtyping when higher-ranked things are involved.
+    /// For more details visit the relevant sections of the [rustc dev guide].
+    ///
+    /// This method should be preferred over `fn enter_forall_and_leak_universe`.
+    ///
+    /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
+    #[instrument(level = "debug", skip(self, f))]
+    pub fn enter_forall<T, U>(&self, forall: ty::Binder<'tcx, T>, f: impl FnOnce(T) -> U) -> U
+    where
+        T: TypeFoldable<TyCtxt<'tcx>> + Copy,
+    {
+        // FIXME: currently we do nothing to prevent placeholders with the new universe being
+        // used after exiting `f`. For example region subtyping can result in outlives constraints
+        // that name placeholders created in this function. Nested goals from type relations can
+        // also contain placeholders created by this function.
+        let value = self.enter_forall_and_leak_universe(forall);
+        debug!("?value");
+        f(value)
+    }
+
     /// See [RegionConstraintCollector::leak_check][1]. We only check placeholder
     /// leaking into `outer_universe`, i.e. placeholders which cannot be named by that
     /// universe.

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

@@ -261,52 +261,55 @@ where
         Ok(a)
     }
 
-    #[instrument(skip(self), level = "debug")]
-    fn instantiate_binder_with_placeholders<T>(&mut self, binder: ty::Binder<'tcx, T>) -> T
+    fn enter_forall<T, U>(
+        &mut self,
+        binder: ty::Binder<'tcx, T>,
+        f: impl FnOnce(&mut Self, T) -> U,
+    ) -> U
     where
         T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
     {
-        if let Some(inner) = binder.no_bound_vars() {
-            return inner;
-        }
-
-        let mut next_region = {
-            let nll_delegate = &mut self.delegate;
-            let mut lazy_universe = None;
+        let value = if let Some(inner) = binder.no_bound_vars() {
+            inner
+        } else {
+            let mut next_region = {
+                let nll_delegate = &mut self.delegate;
+                let mut lazy_universe = None;
+
+                move |br: ty::BoundRegion| {
+                    // 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 = nll_delegate.create_next_universe();
+                        lazy_universe = Some(universe);
+                        universe
+                    });
+
+                    let placeholder = ty::PlaceholderRegion { universe, bound: br };
+                    debug!(?placeholder);
+                    let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
+                    debug!(?placeholder_reg);
+
+                    placeholder_reg
+                }
+            };
 
-            move |br: ty::BoundRegion| {
-                // 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 = nll_delegate.create_next_universe();
-                    lazy_universe = Some(universe);
-                    universe
-                });
-
-                let placeholder = ty::PlaceholderRegion { universe, bound: br };
-                debug!(?placeholder);
-                let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
-                debug!(?placeholder_reg);
-
-                placeholder_reg
-            }
-        };
+            let delegate = FnMutDelegate {
+                regions: &mut next_region,
+                types: &mut |_bound_ty: ty::BoundTy| {
+                    unreachable!("we only replace regions in nll_relate, not types")
+                },
+                consts: &mut |_bound_var: ty::BoundVar, _ty| {
+                    unreachable!("we only replace regions in nll_relate, not consts")
+                },
+            };
 
-        let delegate = FnMutDelegate {
-            regions: &mut next_region,
-            types: &mut |_bound_ty: ty::BoundTy| {
-                unreachable!("we only replace regions in nll_relate, not types")
-            },
-            consts: &mut |_bound_var: ty::BoundVar, _ty| {
-                unreachable!("we only replace regions in nll_relate, not consts")
-            },
+            self.infcx.tcx.replace_bound_vars_uncached(binder, delegate)
         };
 
-        let replaced = self.infcx.tcx.replace_bound_vars_uncached(binder, delegate);
-        debug!(?replaced);
-
-        replaced
+        debug!(?value);
+        f(self, value)
     }
 
     #[instrument(skip(self), level = "debug")]
@@ -630,10 +633,10 @@ where
 
             // Note: the order here is important. Create the placeholders first, otherwise
             // we assign the wrong universe to the existential!
-            let b_replaced = self.instantiate_binder_with_placeholders(b);
-            let a_replaced = self.instantiate_binder_with_existentials(a);
-
-            self.relate(a_replaced, b_replaced)?;
+            self.enter_forall(b, |this, b| {
+                let a = this.instantiate_binder_with_existentials(a);
+                this.relate(a, b)
+            })?;
 
             self.ambient_variance = variance;
         }
@@ -650,10 +653,10 @@ where
             let variance =
                 std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);
 
-            let a_replaced = self.instantiate_binder_with_placeholders(a);
-            let b_replaced = self.instantiate_binder_with_existentials(b);
-
-            self.relate(a_replaced, b_replaced)?;
+            self.enter_forall(a, |this, a| {
+                let b = this.instantiate_binder_with_existentials(b);
+                this.relate(a, b)
+            })?;
 
             self.ambient_variance = variance;
         }