Use alias-eq in structural normalization

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1498,7 +1498,7 @@ pub fn try_structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx>
         let ty = self.resolve_vars_with_obligations(ty);
 
         if self.next_trait_solver()
-            && let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, _) = ty.kind()
+            && let ty::Alias(..) = ty.kind()
         {
             match self
                 .at(&self.misc(sp), self.param_env)

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -3,7 +3,7 @@
 use rustc_infer::traits::{FulfillmentError, TraitEngine};
 use rustc_middle::ty::{self, Ty};
 
-use crate::traits::{query::evaluate_obligation::InferCtxtExt, NormalizeExt, Obligation};
+use crate::traits::{NormalizeExt, Obligation};
 
 pub trait StructurallyNormalizeExt<'tcx> {
     fn structurally_normalize(
@@ -16,42 +16,43 @@ fn structurally_normalize(
 impl<'tcx> StructurallyNormalizeExt<'tcx> for At<'_, 'tcx> {
     fn structurally_normalize(
         &self,
-        mut ty: Ty<'tcx>,
+        ty: Ty<'tcx>,
         fulfill_cx: &mut dyn TraitEngine<'tcx>,
     ) -> Result<Ty<'tcx>, Vec<FulfillmentError<'tcx>>> {
         assert!(!ty.is_ty_var(), "should have resolved vars before calling");
 
         if self.infcx.next_trait_solver() {
-            // FIXME(-Znext-solver): correctly handle
-            // overflow here.
-            for _ in 0..256 {
-                let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, alias) = *ty.kind() else {
-                    break;
+            // FIXME(-Znext-solver): Should we resolve opaques here?
+            let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, _) = *ty.kind() else {
+                return Ok(ty);
             };
 
             let new_infer_ty = self.infcx.next_ty_var(TypeVariableOrigin {
                 kind: TypeVariableOriginKind::NormalizeProjectionType,
                 span: self.cause.span,
             });
+
+            // We simply emit an `alias-eq` goal here, since that will take care of
+            // normalizing the LHS of the projection until it is a rigid projection
+            // (or a not-yet-defined opaque in scope).
             let obligation = Obligation::new(
                 self.infcx.tcx,
                 self.cause.clone(),
                 self.param_env,
-                    ty::NormalizesTo { alias, term: new_infer_ty.into() },
+                ty::PredicateKind::AliasRelate(
+                    ty.into(),
+                    new_infer_ty.into(),
+                    ty::AliasRelationDirection::Equate,
+                ),
             );
-                if self.infcx.predicate_may_hold(&obligation) {
+
             fulfill_cx.register_predicate_obligation(self.infcx, obligation);
             let errors = fulfill_cx.select_where_possible(self.infcx);
             if !errors.is_empty() {
                 return Err(errors);
             }
-                    ty = self.infcx.resolve_vars_if_possible(new_infer_ty);
-                } else {
-                    break;
-                }
-            }
 
-            Ok(ty)
+            Ok(self.infcx.resolve_vars_if_possible(new_infer_ty))
         } else {
             Ok(self.normalize(ty).into_value_registering_obligations(self.infcx, fulfill_cx))
         }

tests/ui/traits/next-solver/alias-bound-unsound.rs

@@ -23,7 +23,7 @@ fn main() {
     let x = String::from("hello, world");
     drop(<() as Foo>::copy_me(&x));
     //~^ ERROR overflow evaluating the requirement `<() as Foo>::Item: Sized`
-    //~| ERROR overflow evaluating the requirement `<() as Foo>::Item normalizes-to _`
+    //~| ERROR overflow evaluating the requirement `<() as Foo>::Item == _`
     //~| ERROR overflow evaluating the requirement `<() as Foo>::Item well-formed`
     //~| ERROR overflow evaluating the requirement `String <: <() as Foo>::Item`
     //~| ERROR overflow evaluating the requirement `&<() as Foo>::Item well-formed`

tests/ui/traits/next-solver/alias-bound-unsound.stderr

@@ -19,7 +19,7 @@ LL |     drop(<() as Foo>::copy_me(&x));
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`alias_bound_unsound`)
 
-error[E0275]: overflow evaluating the requirement `<() as Foo>::Item normalizes-to _`
+error[E0275]: overflow evaluating the requirement `<() as Foo>::Item == _`
   --> $DIR/alias-bound-unsound.rs:24:10
    |
 LL |     drop(<() as Foo>::copy_me(&x));
@@ -59,7 +59,6 @@ LL |     drop(<() as Foo>::copy_me(&x));
    |          ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`alias_bound_unsound`)
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: aborting due to 7 previous errors
 

tests/ui/traits/next-solver/coerce-ambig-alias-to-rigid-alias.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Znext-solver
+// check-pass
+
+trait Trait {
+    type Assoc;
+}
+
+fn call<T: Trait>(_: <T as Trait>::Assoc, _: T) {}
+
+fn foo<T: Trait>(rigid: <T as Trait>::Assoc, t: T) {
+    // Check that we can coerce `<?0 as Trait>::Assoc` to `<T as Trait>::Assoc`.
+    call::<_ /* ?0 */>(rigid, t);
+}
+
+fn main() {}

tests/ui/traits/next-solver/object-unsafety.rs

@@ -13,7 +13,6 @@ pub fn copy_any<T>(t: &T) -> T {
     //~^ ERROR the type `&<dyn Setup<From = T> as Setup>::From` is not well-formed
     //~| ERROR the trait bound `dyn Setup<From = T>: Setup` is not satisfied
     //~| ERROR mismatched types
-    //~| ERROR mismatched types
     //~| ERROR the type `<dyn Setup<From = T> as Setup>::From` is not well-formed
     //~| ERROR the size for values of type `<dyn Setup<From = T> as Setup>::From` cannot be known at compilation time
 

tests/ui/traits/next-solver/object-unsafety.stderr

@@ -36,20 +36,6 @@ note: function defined here
 LL | fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
    |    ^^^^                    --------------
 
-error[E0308]: mismatched types
-  --> $DIR/object-unsafety.rs:12:5
-   |
-LL | pub fn copy_any<T>(t: &T) -> T {
-   |                 -            - expected `T` because of return type
-   |                 |
-   |                 expected this type parameter
-LL |     copy::<dyn Setup<From=T>>(t)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
-   |
-   = note: expected type parameter `T`
-             found associated type `<dyn Setup<From = T> as Setup>::From`
-   = note: you might be missing a type parameter or trait bound
-
 error: the type `<dyn Setup<From = T> as Setup>::From` is not well-formed
   --> $DIR/object-unsafety.rs:12:5
    |
@@ -72,7 +58,7 @@ help: consider further restricting the associated type
 LL | pub fn copy_any<T>(t: &T) -> T where <dyn Setup<From = T> as Setup>::From: Sized {
    |                                +++++++++++++++++++++++++++++++++++++++++++++++++
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.