Normalize opaques before defining them in the new solver

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -22,6 +22,7 @@ use rustc_middle::traits::solve::{
  CanonicalResponse, Certainty, ExternalConstraintsData, Goal, IsNormalizesToHack, QueryResult,
  Response,
 };
+use rustc_middle::traits::Reveal;
 use rustc_middle::ty::{self, Ty, TyCtxt, UniverseIndex};
 use rustc_middle::ty::{
  CoercePredicate, RegionOutlivesPredicate, SubtypePredicate, TypeOutlivesPredicate,
@@ -305,10 +306,21 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
  mut ty: Ty<'tcx>,
  ) -> Result<Option<Ty<'tcx>>, NoSolution> {
  for _ in 0..self.local_overflow_limit() {
- let ty::Alias(_, projection_ty) = *ty.kind() else {
+ let ty::Alias(kind, projection_ty) = *ty.kind() else {
  return Ok(Some(ty));
  };
 
+ // Don't try normalizing an opaque that is not in the defining scope
+ if kind == ty::Opaque
+ && param_env.reveal() == Reveal::UserFacing
+ && !projection_ty
+ .def_id
+ .as_local()
+ .is_some_and(|def_id| self.can_define_opaque_ty(def_id))
+ {
+ return Ok(Some(ty));
+ }
+
  let normalized_ty = self.next_ty_infer();
  let normalizes_to_goal = Goal::new(
  self.tcx(),

compiler/rustc_trait_selection/src/solve/opaques.rs

@@ -23,12 +23,14 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
  let Some(opaque_ty_def_id) = opaque_ty.def_id.as_local() else {
  return Err(NoSolution);
  };
+
  // FIXME: at some point we should call queries without defining
  // new opaque types but having the existing opaque type definitions.
  // This will require moving this below "Prefer opaques registered already".
  if !self.can_define_opaque_ty(opaque_ty_def_id) {
  return Err(NoSolution);
  }
+
  // FIXME: This may have issues when the args contain aliases...
  match self.tcx().uses_unique_placeholders_ignoring_regions(opaque_ty.args) {
  Err(NotUniqueParam::NotParam(param)) if param.is_non_region_infer() => {
@@ -41,6 +43,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
  }
  Ok(()) => {}
  }
+
  // Prefer opaques registered already.
  let opaque_type_key =
  ty::OpaqueTypeKey { def_id: opaque_ty_def_id, args: opaque_ty.args };
@@ -53,6 +56,22 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
  return self.flounder(&matches);
  }
  }
+
+ // Try normalizing the opaque's hidden type. If it's a ty var,
+ // then refuse to define the opaque type yet. This allows us to
+ // avoid inferring opaque cycles so eagerly.
+ let expected = match self.try_normalize_ty(goal.param_env, expected) {
+ Ok(Some(ty)) if !ty.is_ty_var() => ty,
+ Ok(_) => {
+ return self.evaluate_added_goals_and_make_canonical_response(
+ Certainty::AMBIGUOUS,
+ );
+ }
+ Err(_) => {
+ return Err(NoSolution);
+ }
+ };
+
  // Otherwise, define a new opaque type
  self.insert_hidden_type(opaque_type_key, goal.param_env, expected)?;
  self.add_item_bounds_for_hidden_type(

tests/ui/impl-trait/issues/issue-83919.stderr → tests/ui/impl-trait/issues/issue-83919.current.stderr

@@ -1,5 +1,5 @@
 error[E0277]: `{integer}` is not a future
- --> $DIR/issue-83919.rs:21:26
+ --> $DIR/issue-83919.rs:23:26
  |
 LL | fn get_fut(&self) -> Self::Fut {
  | ^^^^^^^^^ `{integer}` is not a future

tests/ui/impl-trait/issues/issue-83919.next.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+ --> $DIR/issue-83919.rs:25:9
+ |
+LL | fn get_fut(&self) -> Self::Fut {
+ | --------- expected `<Implementor as Foo>::Fut` because of return type
+LL |
+LL | / async move {
+LL | |
+LL | | 42
+LL | | // 42 does not impl Future and rustc does actually point out the error,
+LL | | // but rustc used to panic.
+LL | | // Putting a valid Future here always worked fine.
+LL | | }
+ | |_________^ types differ
+ |
+ = note: expected associated type `<Implementor as Foo>::Fut`
+ found `async` block `{async block@$DIR/issue-83919.rs:25:9: 31:10}`
+ = help: consider constraining the associated type `<Implementor as Foo>::Fut` to `{async block@$DIR/issue-83919.rs:25:9: 31:10}` or calling a method that returns `<Implementor as Foo>::Fut`
+ = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/impl-trait/issues/issue-83919.rs

@@ -1,7 +1,9 @@
-#![feature(impl_trait_in_assoc_type)]
-
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 // edition:2021
 
+#![feature(impl_trait_in_assoc_type)]
+
 use std::future::Future;
 
 trait Foo {
@@ -19,8 +21,9 @@ impl Foo for Implementor {
  type Fut = impl Future<Output = Self::Fut2>;
 
  fn get_fut(&self) -> Self::Fut {
- //~^ ERROR `{integer}` is not a future
+ //[current]~^ ERROR `{integer}` is not a future
  async move {
+ //[next]~^ ERROR mismatched types
  42
  // 42 does not impl Future and rustc does actually point out the error,
  // but rustc used to panic.

tests/ui/impl-trait/recursive-generator.stderr → tests/ui/impl-trait/recursive-generator.current.stderr

@@ -1,5 +1,5 @@
 error[E0720]: cannot resolve opaque type
- --> $DIR/recursive-generator.rs:5:13
+ --> $DIR/recursive-generator.rs:8:13
  |
 LL | fn foo() -> impl Generator<Yield = (), Return = ()> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ recursive opaque type

tests/ui/impl-trait/recursive-generator.next.stderr

@@ -0,0 +1,12 @@
+error[E0720]: cannot resolve opaque type
+ --> $DIR/recursive-generator.rs:8:13
+ |
+LL | fn foo() -> impl Generator<Yield = (), Return = ()> {
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ recursive opaque type
+...
+LL | let mut gen = Box::pin(foo());
+ | ------- generator captures itself here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0720`.

tests/ui/impl-trait/recursive-generator.rs

@@ -1,3 +1,6 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
 #![feature(generators, generator_trait)]
 
 use std::ops::{Generator, GeneratorState};

tests/ui/impl-trait/two_tait_defining_each_other.stderr → tests/ui/impl-trait/two_tait_defining_each_other.current.stderr

@@ -1,16 +1,16 @@
 error: opaque type's hidden type cannot be another opaque type from the same scope
- --> $DIR/two_tait_defining_each_other.rs:12:5
+ --> $DIR/two_tait_defining_each_other.rs:16:5
  |
 LL | x // A's hidden type is `Bar`, because all the hidden types of `B` are compared with each other
  | ^ one of the two opaque types used here has to be outside its defining scope
  |
 note: opaque type whose hidden type is being assigned
- --> $DIR/two_tait_defining_each_other.rs:4:10
+ --> $DIR/two_tait_defining_each_other.rs:8:10
  |
 LL | type B = impl Foo;
  | ^^^^^^^^
 note: opaque type being used as hidden type
- --> $DIR/two_tait_defining_each_other.rs:3:10
+ --> $DIR/two_tait_defining_each_other.rs:7:10
  |
 LL | type A = impl Foo;
  | ^^^^^^^^

tests/ui/impl-trait/two_tait_defining_each_other.rs

@@ -1,3 +1,7 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+//[next] check-pass
+
 #![feature(type_alias_impl_trait)]
 
 type A = impl Foo;
@@ -10,7 +14,7 @@ fn muh(x: A) -> B {
  return Bar; // B's hidden type is Bar
  }
  x // A's hidden type is `Bar`, because all the hidden types of `B` are compared with each other
- //~^ ERROR opaque type's hidden type cannot be another opaque type
+ //[current]~^ ERROR opaque type's hidden type cannot be another opaque type
 }
 
 struct Bar;

tests/ui/impl-trait/two_tait_defining_each_other3.stderr → tests/ui/impl-trait/two_tait_defining_each_other3.current.stderr

@@ -1,16 +1,16 @@
 error: opaque type's hidden type cannot be another opaque type from the same scope
- --> $DIR/two_tait_defining_each_other3.rs:10:16
+ --> $DIR/two_tait_defining_each_other3.rs:14:16
  |
 LL | return x; // B's hidden type is A (opaquely)
  | ^ one of the two opaque types used here has to be outside its defining scope
  |
 note: opaque type whose hidden type is being assigned
- --> $DIR/two_tait_defining_each_other3.rs:4:10
+ --> $DIR/two_tait_defining_each_other3.rs:8:10
  |
 LL | type B = impl Foo;
  | ^^^^^^^^
 note: opaque type being used as hidden type
- --> $DIR/two_tait_defining_each_other3.rs:3:10
+ --> $DIR/two_tait_defining_each_other3.rs:7:10
  |
 LL | type A = impl Foo;
  | ^^^^^^^^

tests/ui/impl-trait/two_tait_defining_each_other3.rs

@@ -1,3 +1,7 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+//[next] check-pass
+
 #![feature(type_alias_impl_trait)]
 
 type A = impl Foo;
@@ -8,7 +12,7 @@ trait Foo {}
 fn muh(x: A) -> B {
  if false {
  return x; // B's hidden type is A (opaquely)
- //~^ ERROR opaque type's hidden type cannot be another opaque type
+ //[current]~^ ERROR opaque type's hidden type cannot be another opaque type
  }
  Bar // A's hidden type is `Bar`, because all the return types are compared with each other
 }

tests/ui/traits/new-solver/opaque-hidden-ty-is-rigid-projection.rs

@@ -0,0 +1,8 @@
+// known-bug: unknown
+// compile-flags: -Ztrait-solver=next
+
+fn test<T: Iterator>(x: T::Item) -> impl Sized {
+ x
+}
+
+fn main() {}

tests/ui/traits/new-solver/opaque-hidden-ty-is-rigid-projection.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+ --> $DIR/opaque-hidden-ty-is-rigid-projection.rs:5:5
+ |
+LL | fn test<T: Iterator>(x: T::Item) -> impl Sized {
+ | ----------
+ | |
+ | the expected opaque type
+ | expected `impl Sized` because of return type
+LL | x
+ | ^ types differ
+ |
+ = note: expected opaque type `impl Sized`
+ found associated type `<T as Iterator>::Item`
+help: consider constraining the associated type `<T as Iterator>::Item` to `impl Sized`
+ |
+LL | fn test<T: Iterator<Item = impl Sized>>(x: T::Item) -> impl Sized {
+ | +++++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/assoc-type-const.rs

@@ -1,6 +1,9 @@
 // Tests that we properly detect defining usages when using
 // const generics in an associated opaque type
+
 // check-pass
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 
 #![feature(impl_trait_in_assoc_type)]
 

tests/ui/type-alias-impl-trait/assoc-type-lifetime.rs

@@ -1,6 +1,9 @@
 // Tests that we still detect defining usages when
 // lifetimes are used in an associated opaque type
+
 // check-pass
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 
 #![feature(impl_trait_in_assoc_type)]
 

tests/ui/type-alias-impl-trait/issue-78450.rs

@@ -1,4 +1,6 @@
 // check-pass
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 
 #![feature(impl_trait_in_assoc_type)]
 

tests/ui/type-alias-impl-trait/wf-in-associated-type.fail.stderr

@@ -1,5 +1,5 @@
 error[E0309]: the parameter type `T` may not live long enough
- --> $DIR/wf-in-associated-type.rs:36:23
+ --> $DIR/wf-in-associated-type.rs:38:23
  |
 LL | type Opaque = impl Sized + 'a;
  | ^^^^^^^^^^^^^^^ ...so that the type `&'a T` will meet its required lifetime bounds
@@ -10,7 +10,7 @@ LL | impl<'a, T: 'a> Trait<'a, T> for () {
  | ++++
 
 error[E0309]: the parameter type `T` may not live long enough
- --> $DIR/wf-in-associated-type.rs:36:23
+ --> $DIR/wf-in-associated-type.rs:38:23
  |
 LL | type Opaque = impl Sized + 'a;
  | ^^^^^^^^^^^^^^^ ...so that the reference type `&'a T` does not outlive the data it points at

tests/ui/type-alias-impl-trait/wf-in-associated-type.rs

@@ -1,14 +1,16 @@
 // WF check for impl Trait in associated type position.
 //
-// revisions: pass fail
+// revisions: pass pass_next fail
 // [pass] check-pass
+// [pass_next] compile-flags: -Ztrait-solver=next
+// [pass_next] check-pass
 // [fail] check-fail
 
 #![feature(impl_trait_in_assoc_type)]
 
 // The hidden type here (`&'a T`) requires proving `T: 'a`.
 // We know it holds because of implied bounds from the impl header.
-#[cfg(pass)]
+#[cfg(any(pass, pass_next))]
 mod pass {
  trait Trait<Req> {
  type Opaque1;