change equate for binders to not rely on subtyping

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -483,61 +483,59 @@ impl<'bccx, 'tcx> TypeRelation<'tcx> for NllTypeRelating<'_, 'bccx, 'tcx> {
             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).
-
-            // 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);
-
-            // Note: the order here is important. Create the placeholders first, otherwise
-            // we assign the wrong universe to the existential!
-            self.enter_forall(b, |this, b| {
-                let a = this.instantiate_binder_with_existentials(a);
-                this.relate(a, b)
-            })?;
-
-            self.ambient_variance = variance;
-        }
+        match self.ambient_variance {
+            ty::Variance::Covariant => {
+                // 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).
+
+                // Note: the order here is important. Create the placeholders first, otherwise
+                // we assign the wrong universe to the existential!
+                self.enter_forall(b, |this, b| {
+                    let a = this.instantiate_binder_with_existentials(a);
+                    this.relate(a, b)
+                })?;
+            }
 
-        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.
-
-            // 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.enter_forall(a, |this, a| {
-                let b = this.instantiate_binder_with_existentials(b);
-                this.relate(a, b)
-            })?;
-
-            self.ambient_variance = variance;
+            ty::Variance::Contravariant => {
+                // 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.
+
+                // Note: the order here is important. Create the placeholders first, otherwise
+                // we assign the wrong universe to the existential!
+                self.enter_forall(a, |this, a| {
+                    let b = this.instantiate_binder_with_existentials(b);
+                    this.relate(a, b)
+                })?;
+            }
+
+            ty::Variance::Invariant => {
+                // Invariant, so we want `for<..> A == for<..> B` --
+                // therefore we want `exists<..> A == for<..> B` and
+                // `exists<..> B == for<..> A`.
+                //
+                // See the comment in `fn Equate::binders` for more details.
+
+                // Note: the order here is important. Create the placeholders first, otherwise
+                // we assign the wrong universe to the existential!
+                self.enter_forall(b, |this, b| {
+                    let a = this.instantiate_binder_with_existentials(a);
+                    this.relate(a, b)
+                })?;
+                // Note: the order here is important. Create the placeholders first, otherwise
+                // we assign the wrong universe to the existential!
+                self.enter_forall(a, |this, a| {
+                    let b = this.instantiate_binder_with_existentials(b);
+                    this.relate(a, b)
+                })?;
+            }
+
+            ty::Variance::Bivariant => {}
         }
 
         Ok(a)

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -168,6 +168,7 @@ pub fn check_intrinsic_type(
     let name_str = intrinsic_name.as_str();
 
     let bound_vars = tcx.mk_bound_variable_kinds(&[
+        ty::BoundVariableKind::Region(ty::BrAnon),
         ty::BoundVariableKind::Region(ty::BrAnon),
         ty::BoundVariableKind::Region(ty::BrEnv),
     ]);
@@ -181,7 +182,7 @@ pub fn check_intrinsic_type(
             let env_region = ty::Region::new_bound(
                 tcx,
                 ty::INNERMOST,
-                ty::BoundRegion { var: ty::BoundVar::from_u32(1), kind: ty::BrEnv },
+                ty::BoundRegion { var: ty::BoundVar::from_u32(2), kind: ty::BrEnv },
             );
             let va_list_ty = tcx.type_of(did).instantiate(tcx, &[region.into()]);
             (Ty::new_ref(tcx, env_region, ty::TypeAndMut { ty: va_list_ty, mutbl }), va_list_ty)
@@ -493,9 +494,12 @@ pub fn check_intrinsic_type(
 
             sym::raw_eq => {
                 let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon };
-                let param_ty =
+                let param_ty_lhs =
+                    Ty::new_imm_ref(tcx, ty::Region::new_bound(tcx, ty::INNERMOST, br), param(0));
+                let br = ty::BoundRegion { var: ty::BoundVar::from_u32(1), kind: ty::BrAnon };
+                let param_ty_rhs =
                     Ty::new_imm_ref(tcx, ty::Region::new_bound(tcx, ty::INNERMOST, br), param(0));
-                (1, 0, vec![param_ty; 2], tcx.types.bool)
+                (1, 0, vec![param_ty_lhs, param_ty_rhs], tcx.types.bool)
             }
 
             sym::black_box => (1, 0, vec![param(0)], param(0)),

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

@@ -1,12 +1,13 @@
 use super::combine::{CombineFields, ObligationEmittingRelation};
 use super::StructurallyRelateAliases;
+use crate::infer::BoundRegionConversionTime::HigherRankedType;
 use crate::infer::{DefineOpaqueTypes, SubregionOrigin};
 use crate::traits::PredicateObligations;
 
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::GenericArgsRef;
 use rustc_middle::ty::TyVar;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 
 use rustc_hir::def_id::DefId;
 use rustc_span::Span;
@@ -167,12 +168,34 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
             return Ok(a);
         }
 
-        if a.skip_binder().has_escaping_bound_vars() || b.skip_binder().has_escaping_bound_vars() {
-            self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
-            self.fields.higher_ranked_sub(b, a, self.a_is_expected)?;
-        } else {
+        if let (Some(a), Some(b)) = (a.no_bound_vars(), b.no_bound_vars()) {
             // Fast path for the common case.
-            self.relate(a.skip_binder(), b.skip_binder())?;
+            self.relate(a, b)?;
+        } else {
+            // 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`.
+
+            let span = self.fields.trace.cause.span;
+            let infcx = self.fields.infcx;
+
+            // Check if `exists<..> A == for<..> B`
+            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)
+            })?;
         }
         Ok(a)
     }

tests/ui/associated-inherent-types/issue-111404-1.rs

@@ -8,7 +8,9 @@ impl<'a> Foo<fn(&'a ())> {
 }
 
 fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
-//~^ ERROR higher-ranked subtype error
+//~^ ERROR mismatched types [E0308]
+//~| ERROR mismatched types [E0308]
+//~| ERROR higher-ranked subtype error
 //~| ERROR higher-ranked subtype error
 
 fn main() {}

tests/ui/associated-inherent-types/issue-111404-1.stderr

@@ -1,3 +1,22 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-111404-1.rs:10:11
+   |
+LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected struct `Foo<fn(&())>`
+              found struct `Foo<for<'b> fn(&'b ())>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-111404-1.rs:10:11
+   |
+LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected struct `Foo<fn(&())>`
+              found struct `Foo<for<'b> fn(&'b ())>`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error: higher-ranked subtype error
   --> $DIR/issue-111404-1.rs:10:1
    |
@@ -12,5 +31,6 @@ LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0308`.

tests/ui/closure-expected-type/expect-fn-supply-fn.rs

@@ -30,14 +30,16 @@ fn expect_free_supply_bound() {
     // Here, we are given a function whose region is bound at closure level,
     // but we expect one bound in the argument. Error results.
     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
-    //~^ ERROR mismatched types
+    //~^ ERROR mismatched types [E0308]
+    //~| ERROR lifetime may not live long enough
 }
 
 fn expect_bound_supply_free_from_fn<'x>(x: &'x u32) {
     // Here, we are given a `fn(&u32)` but we expect a `fn(&'x
     // u32)`. In principle, this could be ok, but we demand equality.
     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
-    //~^ ERROR mismatched types
+    //~^ ERROR mismatched types [E0308]
+    //~| ERROR lifetime may not live long enough
 }
 
 fn expect_bound_supply_free_from_closure() {

tests/ui/closure-expected-type/expect-fn-supply-fn.stderr

@@ -19,6 +19,15 @@ LL | fn expect_free_supply_free_from_fn<'x>(x: &'x u32) {
 LL |     with_closure_expecting_fn_with_free_region(|x: fn(&'x u32), y| {});
    |                                                 ^ requires that `'x` must outlive `'static`
 
+error: lifetime may not live long enough
+  --> $DIR/expect-fn-supply-fn.rs:32:49
+   |
+LL |     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
+   |                                                 ^
+   |                                                 |
+   |                                                 has type `fn(&'1 u32)`
+   |                                                 requires that `'1` must outlive `'static`
+
 error[E0308]: mismatched types
   --> $DIR/expect-fn-supply-fn.rs:32:49
    |
@@ -29,23 +38,32 @@ LL |     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
               found fn pointer `for<'a> fn(&'a _)`
 
 error[E0308]: mismatched types
-  --> $DIR/expect-fn-supply-fn.rs:39:50
+  --> $DIR/expect-fn-supply-fn.rs:40:50
    |
 LL |     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
    |                                                  ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a> fn(&'a _)`
               found fn pointer `fn(&_)`
 
+error: lifetime may not live long enough
+  --> $DIR/expect-fn-supply-fn.rs:40:50
+   |
+LL | fn expect_bound_supply_free_from_fn<'x>(x: &'x u32) {
+   |                                     -- lifetime `'x` defined here
+...
+LL |     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
+   |                                                  ^ requires that `'x` must outlive `'static`
+
 error[E0308]: mismatched types
-  --> $DIR/expect-fn-supply-fn.rs:48:50
+  --> $DIR/expect-fn-supply-fn.rs:50:50
    |
 LL |     with_closure_expecting_fn_with_bound_region(|x: Foo<'_>, y| {
    |                                                  ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a> fn(&'a _)`
               found fn pointer `fn(&_)`
 
-error: aborting due to 5 previous errors
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/coherence/coherence-fn-covariant-bound-vs-static.rs

@@ -1,10 +1,15 @@
-// Test that impls for these two types are considered ovelapping:
+//@ check-pass
+
+// These types were previously considered equal as they are subtypes of each other.
+// This has been changed in #118247 and we now consider them to be disjoint.
+//
+// In our test:
 //
 // * `for<'r> fn(fn(&'r u32))`
 // * `fn(fn(&'a u32)` where `'a` is free
 //
-// This is because, for `'a = 'static`, the two types overlap.
-// Effectively for them to be equal to you get:
+// These were considered equal as for `'a = 'static` subtyping succeeds in both
+// directions:
 //
 // * `for<'r> fn(fn(&'r u32)) <: fn(fn(&'static u32))`
 //   * true if `exists<'r> { 'r: 'static }` (obviously true)
@@ -15,12 +20,7 @@ trait Trait {}
 
 impl Trait for for<'r> fn(fn(&'r ())) {}
 impl<'a> Trait for fn(fn(&'a ())) {}
-//~^ ERROR conflicting implementations
-//
-// Note in particular that we do NOT get a future-compatibility warning
-// here. This is because the new leak-check proposed in [MCP 295] does not
-// "error" when these two types are equated.
-//
-// [MCP 295]: https://github.com/rust-lang/compiler-team/issues/295
+//~^ WARN conflicting implementations of trait `Trait` for type `for<'r> fn(fn(&'r ()))` [coherence_leak_check]
+//~| WARN the behavior may change in a future release
 
 fn main() {}

tests/ui/coherence/coherence-fn-covariant-bound-vs-static.stderr

@@ -1,13 +1,15 @@
-error[E0119]: conflicting implementations of trait `Trait` for type `for<'r> fn(fn(&'r ()))`
-  --> $DIR/coherence-fn-covariant-bound-vs-static.rs:17:1
+warning: conflicting implementations of trait `Trait` for type `for<'r> fn(fn(&'r ()))`
+  --> $DIR/coherence-fn-covariant-bound-vs-static.rs:22:1
    |
 LL | impl Trait for for<'r> fn(fn(&'r ())) {}
    | ------------------------------------- first implementation here
 LL | impl<'a> Trait for fn(fn(&'a ())) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'r> fn(fn(&'r ()))`
    |
+   = warning: the behavior may change in a future release
+   = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
    = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
+   = note: `#[warn(coherence_leak_check)]` on by default
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted
 
-For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/coherence-fn-inputs.rs

@@ -1,19 +1,21 @@
-// Test that we consider these two types completely equal:
+//@ check-pass
+
+// These types were previously considered equal as they are subtypes of each other.
+// This has been changed in #118247 and we now consider them to be disjoint.
 //
 // * `for<'a, 'b> fn(&'a u32, &'b u32)`
 // * `for<'c> fn(&'c u32, &'c u32)`
 //
-// For a long time we considered these to be distinct types. But in fact they
-// are equivalent, if you work through the implications of subtyping -- this is
-// because:
+// These types are subtypes of each other as:
 //
 // * `'c` can be the intersection of `'a` and `'b` (and there is always an intersection)
 // * `'a` and `'b` can both be equal to `'c`
 
 trait Trait {}
 impl Trait for for<'a, 'b> fn(&'a u32, &'b u32) {}
 impl Trait for for<'c> fn(&'c u32, &'c u32) {
-    //~^ ERROR conflicting implementations
+    //~^ WARN conflicting implementations of trait `Trait` for type `for<'a, 'b> fn(&'a u32, &'b u32)` [coherence_leak_check]
+    //~| WARN the behavior may change in a future release
     //
     // Note in particular that we do NOT get a future-compatibility warning
     // here. This is because the new leak-check proposed in [MCP 295] does not