Conversion Bewteen &impl Trait and &dyn Trait

convert_impl_to_from_dyn.rs

trait Fmt {}

// This allows conversion from ?Sized like `&dyn Fmt` back to `&impl Fmt`
impl Fmt for &dyn Fmt {}

struct FmtStruct();

impl Fmt for FmtStruct {}

fn expects_dyn(fmt: &dyn Fmt) -> &dyn Fmt {
    fmt
}

fn expects_impl(fmt: &impl Fmt) -> &impl Fmt {
    fmt
}

fn test_conversions<F: Fmt>(fmt: &F) {
    // &impl Fmt easily becomes &dyn Fmt.
    let ref_dyn_fmt: &dyn Fmt = expects_dyn(fmt);

    // &dyn Fmt can only become &impl Fmt here because we implemented `Fmt for &impl Fmt + ?Sized`,
    // which is usually &dyn Fmt. We could have also implemnted `Fmt for &dyn Fmt` but that is not
    // general to all ?Sized.
    let _ref_impl_fmt = expects_impl(&ref_dyn_fmt); // Has type &impl Fmt.
}

fn main() {
    let fmt_struct = FmtStruct();
    test_conversions(&fmt_struct);
}