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

4 files changed, 147 insertions, 0 deletions

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 00000000000..766bd147431
--- /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 00000000000..350f8ea8507
--- /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 00000000000..793d556d08c
--- /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 00000000000..46c083f9305
--- /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);
+}