main.rs

trait A {}
trait B: A {}

struct JustA {}
impl A for JustA {}

struct AandB {}

impl A for AandB {}
impl B for AandB {}

fn build_just_a() -> Box<A> {
    Box::new(JustA {})
}

fn build_a_and_b() -> Box<B> {
    Box::new(AandB {})
}

fn main() {
    let aaa: Vec<Box<A>> = vec![build_just_a(), build_a_and_b()];

    println!("aaa: {}", aaa.len());
}

output.txt

   Compiling trait_inheritance v0.1.0 (/Users/simon/tmp/trait_inheritance)
error[E0308]: mismatched types
  --> src/main.rs:21:44
   |
21 |     let aaa: Vec<Box<A>> = vec![build_a(), build_a_and_b()];
   |                                            ^^^^^^^^^^^^^^^ expected trait `A`, found trait `B`
   |
   = note: expected type `std::boxed::Box<dyn A>`
              found type `std::boxed::Box<(dyn B + 'static)>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `trait_inheritance`.

To learn more, run the command again with --verbose.