Auto merge of #118247 - spastorino:type-equality-subtyping, r=<try>

Fix for TypeId exposes equality-by-subtyping vs normal-form-syntactic-equality unsoundness

Fixes #97156

This PR revives #97427 idea, it sits on top of #118118 because the idea uncovered some problems with IATs.

r? `@lcnr`

This is ICEing yet for `tests/ui/traits/new-solver/escaping-bound-vars-in-writeback-normalization.rs` using the new trait solver.
After #118118 and this ICE is fixed, we would need a rebase and a crater run.

Opening as a WIP for now.

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -138,6 +138,7 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
     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),
     ]);
@@ -151,7 +152,7 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
             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)
@@ -446,9 +447,12 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
 
             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, vec![param_ty; 2], tcx.types.bool)
+                (1, vec![param_ty_lhs, param_ty_rhs], tcx.types.bool)
             }
 
             sym::black_box => (1, vec![param(0)], param(0)),

compiler/rustc_infer/src/infer/equate.rs

@@ -165,8 +165,8 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         }
 
         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)?;
+            self.fields.higher_ranked_equate(a, b, self.a_is_expected)?;
+            self.fields.higher_ranked_equate(b, a, self.a_is_expected)?;
         } else {
             // Fast path for the common case.
             self.relate(a.skip_binder(), b.skip_binder())?;

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

@@ -57,6 +57,40 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
         // placeholders which **must not** be named afterwards.
         Ok(())
     }
+
+    #[instrument(skip(self), level = "debug")]
+    pub fn higher_ranked_equate<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.
+        let sup_prime = self.infcx.instantiate_binder_with_placeholders(sup);
+
+        // 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.equate(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(())
+    }
 }
 
 impl<'tcx> InferCtxt<'tcx> {

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

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

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

@@ -1,16 +1,22 @@
-error: higher-ranked subtype error
-  --> $DIR/issue-111404-1.rs:10:1
+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: higher-ranked subtype error
-  --> $DIR/issue-111404-1.rs:10:1
+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: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.

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

@@ -1,3 +1,5 @@
+// check-pass
+
 // Test that impls for these two types are considered ovelapping:
 //
 // * `for<'r> fn(fn(&'r u32))`
@@ -15,7 +17,8 @@ trait Trait {}
 
 impl Trait for for<'r> fn(fn(&'r ())) {}
 impl<'a> Trait for fn(fn(&'a ())) {}
-//~^ ERROR conflicting implementations
+//~^ WARN conflicting implementations of trait `Trait` for type `for<'r> fn(fn(&'r ()))` [coherence_leak_check]
+//~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error 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

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:19:1
    |
 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: this was previously accepted by the compiler but is being phased out; it will become a hard error 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

@@ -1,3 +1,5 @@
+// check-pass
+
 // Test that we consider these two types completely equal:
 //
 // * `for<'a, 'b> fn(&'a u32, &'b u32)`
@@ -13,7 +15,8 @@
 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 this was previously accepted by the compiler but is being phased out; it will become a hard error 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

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

@@ -1,13 +1,15 @@
-error[E0119]: conflicting implementations of trait `Trait` for type `for<'a, 'b> fn(&'a u32, &'b u32)`
-  --> $DIR/coherence-fn-inputs.rs:15:1
+warning: conflicting implementations of trait `Trait` for type `for<'a, 'b> fn(&'a u32, &'b u32)`
+  --> $DIR/coherence-fn-inputs.rs:17:1
    |
 LL | impl Trait for for<'a, 'b> fn(&'a u32, &'b u32) {}
    | ----------------------------------------------- first implementation here
 LL | impl Trait for for<'c> fn(&'c u32, &'c u32) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a, 'b> fn(&'a u32, &'b u32)`
    |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error 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/const-generics/generic_const_exprs/typeid-equality-by-subtyping.stderr

@@ -7,5 +7,12 @@ LL |         WHAT_A_TYPE => 0,
    = note: the traits must be derived, manual `impl`s are not sufficient
    = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `for<'a, 'b> fn(&'a (), &'b ()): WithAssoc<T>` is not satisfied
+  --> $DIR/typeid-equality-by-subtyping.rs:44:51
+   |
+LL | fn unsound<T>(x: <One as WithAssoc<T>>::Assoc) -> <Two as WithAssoc<T>>::Assoc
+   |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `WithAssoc<T>` is not implemented for `for<'a, 'b> fn(&'a (), &'b ())`
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0277`.

tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs

@@ -2,8 +2,6 @@
 //
 // In particular, we test this pattern in trait solving, where it is not connected
 // to any part of the source code.
-//
-// check-pass
 
 trait Trait<T> {}
 
@@ -34,4 +32,5 @@ fn main() {
     //   This is because we can use `'static`.
 
     foo::<()>();
+    //~^ ERROR implementation of `Trait` is not general enough
 }

tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.stderr

@@ -0,0 +1,11 @@
+error: implementation of `Trait` is not general enough
+  --> $DIR/hrtb-exists-forall-trait-covariant.rs:34:5
+   |
+LL |     foo::<()>();
+   |     ^^^^^^^^^^^ implementation of `Trait` is not general enough
+   |
+   = note: `()` must implement `Trait<for<'b> fn(fn(&'b u32))>`
+   = note: ...but it actually implements `Trait<fn(fn(&'0 u32))>`, for some specific lifetime `'0`
+
+error: aborting due to 1 previous error
+

tests/ui/lub-glb/old-lub-glb-hr-eq.rs

@@ -3,8 +3,6 @@
 // error. However, now that we handle subtyping correctly, we no
 // longer get an error, because we recognize these two types as
 // equivalent!
-//
-// check-pass
 
 fn foo(x: fn(&u8, &u8), y: for<'a> fn(&'a u8, &'a u8)) {
     // The two types above are actually equivalent. With the older
@@ -13,6 +11,7 @@ fn foo(x: fn(&u8, &u8), y: for<'a> fn(&'a u8, &'a u8)) {
     let z = match 22 {
         0 => x,
         _ => y,
+        //~^ ERROR `match` arms have incompatible types [E0308]
     };
 }
 

tests/ui/lub-glb/old-lub-glb-hr-eq.stderr

@@ -0,0 +1,19 @@
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/old-lub-glb-hr-eq.rs:13:14
+   |
+LL |       let z = match 22 {
+   |  _____________-
+LL | |         0 => x,
+   | |              - this is found to be of type `for<'a, 'b> fn(&'a u8, &'b u8)`
+LL | |         _ => y,
+   | |              ^ one type is more general than the other
+LL | |
+LL | |     };
+   | |_____- `match` arms have incompatible types
+   |
+   = note: expected fn pointer `for<'a, 'b> fn(&'a u8, &'b u8)`
+              found fn pointer `for<'a> fn(&'a u8, &'a u8)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/new-solver/member-constraints-in-root-universe.rs

@@ -1,5 +1,4 @@
 // compile-flags: -Ztrait-solver=next
-// check-pass
 
 trait Trait {
     type Ty;
@@ -11,6 +10,8 @@ impl Trait for for<'a> fn(&'a u8, &'a u8) {
 
 // argument is necessary to create universes before registering the hidden type.
 fn test<'a>(_: <fn(&u8, &u8) as Trait>::Ty) -> impl Sized {
+    //~^ ERROR the type `<for<'a, 'b> fn(&'a u8, &'b u8) as Trait>::Ty` is not well-formed
+    //~| ERROR the size for values of type `<for<'a, 'b> fn(&'a u8, &'b u8) as Trait>::Ty` cannot be known at compilation time [E0277]
     "hidden type is `&'?0 str` with '?0 member of ['static,]"
 }
 

tests/ui/traits/new-solver/member-constraints-in-root-universe.stderr

@@ -0,0 +1,22 @@
+error: the type `<for<'a, 'b> fn(&'a u8, &'b u8) as Trait>::Ty` is not well-formed
+  --> $DIR/member-constraints-in-root-universe.rs:12:16
+   |
+LL | fn test<'a>(_: <fn(&u8, &u8) as Trait>::Ty) -> impl Sized {
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the size for values of type `<for<'a, 'b> fn(&'a u8, &'b u8) as Trait>::Ty` cannot be known at compilation time
+  --> $DIR/member-constraints-in-root-universe.rs:12:13
+   |
+LL | fn test<'a>(_: <fn(&u8, &u8) as Trait>::Ty) -> impl Sized {
+   |             ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `<for<'a, 'b> fn(&'a u8, &'b u8) as Trait>::Ty`
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn test<'a>(_: &<fn(&u8, &u8) as Trait>::Ty) -> impl Sized {
+   |                +
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.