Auto merge of #56863 - arielb1:supertrait-self-4, r=nikomatsakis
fix trait objects with a Self-containing projection values
Fixes #56288.
This follows ALT2 in the issue.
beta-nominating since this is a regression.
r? @nikomatsakis
diff --git a/src/librustc/traits/object_safety.rs b/src/librustc/traits/object_safety.rs
index 4b2f817..fe40141 100644
--- a/src/librustc/traits/object_safety.rs
+++ b/src/librustc/traits/object_safety.rs
@@ -190,7 +190,26 @@
// In the case of a trait predicate, we can skip the "self" type.
data.skip_binder().input_types().skip(1).any(|t| t.has_self_ty())
}
- ty::Predicate::Projection(..) |
+ ty::Predicate::Projection(ref data) => {
+ // And similarly for projections. This should be redundant with
+ // the previous check because any projection should have a
+ // matching `Trait` predicate with the same inputs, but we do
+ // the check to be safe.
+ //
+ // Note that we *do* allow projection *outputs* to contain
+ // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
+ // we just require the user to specify *both* outputs
+ // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
+ //
+ // This is ALT2 in issue #56288, see that for discussion of the
+ // possible alternatives.
+ data.skip_binder()
+ .projection_ty
+ .trait_ref(self)
+ .input_types()
+ .skip(1)
+ .any(|t| t.has_self_ty())
+ }
ty::Predicate::WellFormed(..) |
ty::Predicate::ObjectSafe(..) |
ty::Predicate::TypeOutlives(..) |
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 43e7aee..ab1eeff 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -1013,6 +1013,7 @@
let mut associated_types = BTreeSet::default();
for tr in traits::elaborate_trait_ref(tcx, principal) {
+ debug!("conv_object_ty_poly_trait_ref: observing object predicate `{:?}`", tr);
match tr {
ty::Predicate::Trait(pred) => {
associated_types.extend(tcx.associated_items(pred.def_id())
@@ -1020,8 +1021,31 @@
.map(|item| item.def_id));
}
ty::Predicate::Projection(pred) => {
- // Include projections defined on supertraits.
- projection_bounds.push((pred, DUMMY_SP))
+ // A `Self` within the original bound will be substituted with a
+ // `TRAIT_OBJECT_DUMMY_SELF`, so check for that.
+ let references_self =
+ pred.skip_binder().ty.walk().any(|t| t == dummy_self);
+
+ // If the projection output contains `Self`, force the user to
+ // elaborate it explicitly to avoid a bunch of complexity.
+ //
+ // The "classicaly useful" case is the following:
+ // ```
+ // trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
+ // type MyOutput;
+ // }
+ // ```
+ //
+ // Here, the user could theoretically write `dyn MyTrait<Output=X>`,
+ // but actually supporting that would "expand" to an infinitely-long type
+ // `fix $ τ → dyn MyTrait<MyOutput=X, Output=<τ as MyTrait>::MyOutput`.
+ //
+ // Instead, we force the user to write `dyn MyTrait<MyOutput=X, Output=X>`,
+ // which is uglier but works. See the discussion in #56288 for alternatives.
+ if !references_self {
+ // Include projections defined on supertraits,
+ projection_bounds.push((pred, DUMMY_SP))
+ }
}
_ => ()
}
diff --git a/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.rs b/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.rs
new file mode 100644
index 0000000..766bd14
--- /dev/null
+++ b/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.rs
@@ -0,0 +1,50 @@
+// Regression test for #56288. Checks that if a supertrait defines an associated type
+// projection that references `Self`, then that associated type must still be explicitly
+// specified in the `dyn Trait` variant, since we don't know what `Self` is anymore.
+
+trait Base {
+ type Output;
+}
+
+trait Helper: Base<Output=<Self as Helper>::Target> {
+ type Target;
+}
+
+impl Base for u32
+{
+ type Output = i32;
+}
+
+impl Helper for u32
+{
+ type Target = i32;
+}
+
+trait ConstI32 {
+ type Out;
+}
+
+impl<T: ?Sized> ConstI32 for T {
+ type Out = i32;
+}
+
+// Test that you still need to manually give a projection type if the Output type
+// is normalizable.
+trait NormalizableHelper:
+ Base<Output=<Self as ConstI32>::Out>
+{
+ type Target;
+}
+
+impl NormalizableHelper for u32
+{
+ type Target = i32;
+}
+
+fn main() {
+ let _x: Box<dyn Helper<Target=i32>> = Box::new(2u32);
+ //~^ ERROR the value of the associated type `Output` (from the trait `Base`) must be specified
+
+ let _y: Box<dyn NormalizableHelper<Target=i32>> = Box::new(2u32);
+ //~^ ERROR the value of the associated type `Output` (from the trait `Base`) must be specified
+}
diff --git a/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.stderr b/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.stderr
new file mode 100644
index 0000000..350f8ea
--- /dev/null
+++ b/src/test/ui/traits/trait-object-with-self-in-projection-output-bad.stderr
@@ -0,0 +1,21 @@
+error[E0191]: the value of the associated type `Output` (from the trait `Base`) must be specified
+ --> $DIR/trait-object-with-self-in-projection-output-bad.rs:45:17
+ |
+LL | type Output;
+ | ------------ `Output` defined here
+...
+LL | let _x: Box<dyn Helper<Target=i32>> = Box::new(2u32);
+ | ^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` must be specified
+
+error[E0191]: the value of the associated type `Output` (from the trait `Base`) must be specified
+ --> $DIR/trait-object-with-self-in-projection-output-bad.rs:48:17
+ |
+LL | type Output;
+ | ------------ `Output` defined here
+...
+LL | let _y: Box<dyn NormalizableHelper<Target=i32>> = Box::new(2u32);
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` must be specified
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0191`.
diff --git a/src/test/ui/traits/trait-object-with-self-in-projection-output-good.rs b/src/test/ui/traits/trait-object-with-self-in-projection-output-good.rs
new file mode 100644
index 0000000..793d556
--- /dev/null
+++ b/src/test/ui/traits/trait-object-with-self-in-projection-output-good.rs
@@ -0,0 +1,28 @@
+// compile-pass
+
+// Regression test related to #56288. Check that a supertrait projection (of
+// `Output`) that references `Self` can be ok if it is referencing a projection (of
+// `Self::Target`, in this case). Note that we still require the user to manually
+// specify both `Target` and `Output` for now.
+
+trait Base {
+ type Output;
+}
+
+trait Helper: Base<Output=<Self as Helper>::Target> {
+ type Target;
+}
+
+impl Base for u32
+{
+ type Output = i32;
+}
+
+impl Helper for u32
+{
+ type Target = i32;
+}
+
+fn main() {
+ let _x: Box<dyn Helper<Target=i32, Output=i32>> = Box::new(2u32);
+}
diff --git a/src/test/ui/traits/trait-object-with-self-in-projection-output-repeated-supertrait.rs b/src/test/ui/traits/trait-object-with-self-in-projection-output-repeated-supertrait.rs
new file mode 100644
index 0000000..46c083f
--- /dev/null
+++ b/src/test/ui/traits/trait-object-with-self-in-projection-output-repeated-supertrait.rs
@@ -0,0 +1,48 @@
+// compile-pass
+
+// Regression test related to #56288. Check that a supertrait projection (of
+// `Output`) that references `Self` is ok if there is another occurence of
+// the same supertrait that specifies the projection explicitly, even if
+// the projection's associated type is not explicitly specified in the object type.
+//
+// Note that in order for this to compile, we need the `Self`-referencing projection
+// to normalize fairly directly to a concrete type, otherwise the trait resolver
+// will hate us.
+//
+// There is a test in `trait-object-with-self-in-projection-output-bad.rs` that
+// having a normalizing, but `Self`-containing projection does not *by itself*
+// allow you to avoid writing the projected type (`Output`, in this example)
+// explicitly.
+
+trait ConstI32 {
+ type Out;
+}
+
+impl<T: ?Sized> ConstI32 for T {
+ type Out = i32;
+}
+
+trait Base {
+ type Output;
+}
+
+trait NormalizingHelper: Base<Output=<Self as ConstI32>::Out> + Base<Output=i32> {
+ type Target;
+}
+
+impl Base for u32
+{
+ type Output = i32;
+}
+
+impl NormalizingHelper for u32
+{
+ type Target = i32;
+}
+
+fn main() {
+ // Make sure this works both with and without the associated type
+ // being specified.
+ let _x: Box<dyn NormalizingHelper<Target=i32>> = Box::new(2u32);
+ let _y: Box<dyn NormalizingHelper<Target=i32, Output=i32>> = Box::new(2u32);
+}
