error[E0038]: the trait `NotObjectSafe` cannot be made into an object --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:21:13 | LL | fn car() -> dyn NotObjectSafe { | ^^^^^^^^^^^^^^^^^ `NotObjectSafe` cannot be made into an object | note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:3:8 | LL | trait NotObjectSafe { | ------------- this trait cannot be made into an object... LL | fn foo() -> Self; | ^^^ ...because associated function `foo` has no `self` parameter = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead: A B help: consider turning `foo` into a method by giving it a `&self` argument | LL | fn foo(&self) -> Self; | +++++ help: alternatively, consider constraining `foo` so it does not apply to trait objects | LL | fn foo() -> Self where Self: Sized; | +++++++++++++++++ error[E0038]: the trait `NotObjectSafe` cannot be made into an object --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:28:17 | LL | fn cat() -> Box { | ^^^^^^^^^^^^^^^^^ `NotObjectSafe` cannot be made into an object | note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:3:8 | LL | trait NotObjectSafe { | ------------- this trait cannot be made into an object... LL | fn foo() -> Self; | ^^^ ...because associated function `foo` has no `self` parameter = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead: A B help: consider turning `foo` into a method by giving it a `&self` argument | LL | fn foo(&self) -> Self; | +++++ help: alternatively, consider constraining `foo` so it does not apply to trait objects | LL | fn foo() -> Self where Self: Sized; | +++++++++++++++++ error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0038`.