Static DEREF_INTO_DYN_SUPERTRAIT 

pub static DEREF_INTO_DYN_SUPERTRAIT: &Lint

The deref_into_dyn_supertrait lint is emitted whenever there is a Deref implementation for dyn SubTrait with a dyn SuperTrait type as the Output type.

These implementations are “shadowed” by trait upcasting (stabilized since 1.86.0). The deref functions is no longer called implicitly, which might change behavior compared to previous rustc versions.

Example

#![deny(deref_into_dyn_supertrait)]
#![allow(dead_code)]

use core::ops::Deref;

trait A {}
trait B: A {}
impl<'a> Deref for dyn 'a + B {
    type Target = dyn A;
    fn deref(&self) -> &Self::Target {
        todo!()
    }
}

fn take_a(_: &dyn A) { }

fn take_b(b: &dyn B) {
    take_a(b);
}

{{produces}}

Explanation

The trait upcasting coercion added a new coercion rule, taking priority over certain other coercion rules, which causes some behavior change compared to older rustc versions.

deref can be still called explicitly, it just isn’t called as part of a deref coercion (since trait upcasting coercion takes priority).