error: associated item referring to unboxed trait object for its own trait
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:4:13
   |
LL | trait A: Sized {
   |       - in this trait
LL |     fn f(a: dyn A) -> dyn A;
   |             ^^^^^     ^^^^^
   |
help: you might have meant to use `Self` to refer to the implementing type
   |
LL |     fn f(a: Self) -> Self;
   |             ~~~~     ~~~~

error[E0038]: the trait `A` cannot be made into an object
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:4:13
   |
LL |     fn f(a: dyn A) -> dyn A;
   |             ^^^^^ `A` cannot be made into an object
   |
note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:3:10
   |
LL | trait A: Sized {
   |       -  ^^^^^ ...because it requires `Self: Sized`
   |       |
   |       this trait cannot be made into an object...

error: associated item referring to unboxed trait object for its own trait
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:9:13
   |
LL | trait B {
   |       - in this trait
LL |     fn f(a: dyn B) -> dyn B;
   |             ^^^^^     ^^^^^
   |
help: you might have meant to use `Self` to refer to the implementing type
   |
LL |     fn f(a: Self) -> Self;
   |             ~~~~     ~~~~

error[E0038]: the trait `B` cannot be made into an object
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:9:13
   |
LL |     fn f(a: dyn B) -> dyn B;
   |             ^^^^^ `B` cannot be made into an object
   |
note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $DIR/dyn-incompatible-trait-should-use-self-2021.rs:9:8
   |
LL | trait B {
   |       - this trait cannot be made into an object...
LL |     fn f(a: dyn B) -> dyn B;
   |        ^ ...because associated function `f` has no `self` parameter
help: consider turning `f` into a method by giving it a `&self` argument
   |
LL |     fn f(&self, a: dyn B) -> dyn B;
   |          ++++++
help: alternatively, consider constraining `f` so it does not apply to trait objects
   |
LL |     fn f(a: dyn B) -> dyn B where Self: Sized;
   |                             +++++++++++++++++

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0038`.