Only walk the identity substituted version of struct fields

compiler/rustc_ty_utils/src/opaque_types.rs

@@ -82,10 +82,12 @@ fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<!> {
                         // start seeing the error below.
 
                         // Collect opaque types nested within the associated type bounds of this opaque type.
+                        // We use identity substs here, because we already know that the opaque type uses
+                        // only generic parameters, and thus substituting would not give us more information.
                         for (pred, span) in self
                             .tcx
                             .explicit_item_bounds(alias_ty.def_id)
-                            .subst_iter_copied(self.tcx, alias_ty.substs)
+                            .subst_identity_iter_copied()
                         {
                             trace!(?pred);
                             self.visit_spanned(span, pred);
@@ -158,6 +160,25 @@ fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<!> {
                     }
                 }
             }
+            ty::Adt(def, _) if def.did().is_local() => {
+                if !self.seen.insert(def.did().expect_local()) {
+                    return ControlFlow::Continue(());
+                }
+                for variant in def.variants().iter() {
+                    for field in variant.fields.iter() {
+                        // Don't use the `ty::Adt` substs, we either
+                        // * found the opaque in the substs
+                        // * will find the opaque in the unsubstituted fields
+                        // The only other situation that can occur is that after substituting,
+                        // some projection resolves to an opaque that we would have otherwise
+                        // not found. While we could substitute and walk those, that would mean we
+                        // would have to walk all substitutions of an Adt, which can quickly
+                        // degenerate into looking at an exponential number of types.
+                        let ty = self.tcx.type_of(field.did).subst_identity();
+                        self.visit_spanned(self.tcx.def_span(field.did), ty);
+                    }
+                }
+            }
             _ => trace!(kind=?t.kind()),
         }
         ControlFlow::Continue(())

tests/ui/type-alias-impl-trait/hidden_behind_projection_behind_struct_field.rs

@@ -0,0 +1,28 @@
+//! This test shows that a field type that is a projection that resolves to an opaque,
+//! is not a defining use. While we could substitute the struct generics, that would
+//! mean we would have to walk all substitutions of an `Foo`, which can quickly
+//! degenerate into looking at an exponential number of types depending on the complexity
+//! of a program.
+
+#![feature(impl_trait_in_assoc_type)]
+
+struct Bar;
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo() -> Foo<Self>;
+}
+
+impl Trait for Bar {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() -> Foo<Bar> {
+        Foo { field: () }
+        //~^ ERROR: mismatched types
+    }
+}
+
+struct Foo<T: Trait> {
+    field: <T as Trait>::Assoc,
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/hidden_behind_projection_behind_struct_field.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/hidden_behind_projection_behind_struct_field.rs:19:22
+   |
+LL |     type Assoc = impl std::fmt::Debug;
+   |                  -------------------- the expected opaque type
+LL |     fn foo() -> Foo<Bar> {
+LL |         Foo { field: () }
+   |                      ^^ expected opaque type, found `()`
+   |
+   = note: expected opaque type `<Bar as Trait>::Assoc`
+                found unit type `()`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/hidden_behind_projection_behind_struct_field.rs:18:8
+   |
+LL |     fn foo() -> Foo<Bar> {
+   |        ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/hidden_behind_struct_field.rs

@@ -0,0 +1,30 @@
+//! This test shows that the appearance of an opaque type
+//! in the substs of a struct are enough to make it count
+//! for making the function a defining use. It doesn't matter
+//! if the opaque type is actually used in the field.
+
+#![feature(impl_trait_in_assoc_type)]
+// check-pass
+
+use std::marker::PhantomData;
+
+struct Bar;
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo() -> Foo<Self::Assoc>;
+}
+
+impl Trait for Bar {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() -> Foo<Self::Assoc> {
+        let foo: Foo<()> = Foo { field: PhantomData };
+        foo
+    }
+}
+
+struct Foo<T> {
+    field: PhantomData<T>,
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/hidden_behind_struct_field2.rs

@@ -0,0 +1,26 @@
+//! This test shows that we can even follow projections
+//! into associated types of the same impl if they are
+//! indirectly mentioned in a struct field.
+
+#![feature(impl_trait_in_assoc_type)]
+// check-pass
+
+struct Bar;
+
+trait Trait: Sized {
+    type Assoc;
+    fn foo() -> Foo;
+}
+
+impl Trait for Bar {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() -> Foo {
+        Foo { field: () }
+    }
+}
+
+struct Foo {
+    field: <Bar as Trait>::Assoc,
+}
+
+fn main() {}