error[E0038]: the trait `Foo` cannot be made into an object --> $DIR/object-safety.rs:17:33 | LL | let i = Box::new(42_u32) as Box; | ^^^^^^^^^^^^ `Foo` 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-safety.rs:7:8 | LL | trait Foo { | --- this trait cannot be made into an object... LL | fn baz(&self) -> impl Debug; | ^^^ ...because method `baz` references an `impl Trait` type in its return type = help: consider moving `baz` to another trait error[E0038]: the trait `Foo` cannot be made into an object --> $DIR/object-safety.rs:20:13 | LL | let s = i.baz(); | ^^^^^^^ `Foo` 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-safety.rs:7:8 | LL | trait Foo { | --- this trait cannot be made into an object... LL | fn baz(&self) -> impl Debug; | ^^^ ...because method `baz` references an `impl Trait` type in its return type = help: consider moving `baz` to another trait error[E0038]: the trait `Foo` cannot be made into an object --> $DIR/object-safety.rs:17:13 | LL | let i = Box::new(42_u32) as Box; | ^^^^^^^^^^^^^^^^ `Foo` 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-safety.rs:7:8 | LL | trait Foo { | --- this trait cannot be made into an object... LL | fn baz(&self) -> impl Debug; | ^^^ ...because method `baz` references an `impl Trait` type in its return type = help: consider moving `baz` to another trait = note: required for `Box` to implement `CoerceUnsized>` = note: required by cast to type `Box` error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0038`.