question idea: breaking of dyn-compatible / object-safe

[tkr-sh]

It might be too easy, but something like

struct Cat(Box<dyn Into<String>>)

[WaffleLapkin]

It might be slightly more interesting if you combine it with "functions that take self don't make in dyn-incompatible":

trait A {
    fn f(self);
}

fn __(_: &dyn A) {} //~ ok

trait B {
    fn f(self) -> Self;
}

fn ___(_: &dyn B) {} //~ error

[tkr-sh]

Really well developed!