Wrong unfulfilled lint expectation for `dead_code` on a specific associated function

[weiznich]

Code

use std::any::Any;

pub trait Foo: Any {}

impl Foo for Box<dyn Foo> {}

impl dyn Foo {
    #[expect(dead_code)]
    fn downcast_ref<T: 'static>(&self) -> Option<&T> {
        let this = self as &dyn Any;
        if let Some(boxed) = this.downcast_ref::<Box<dyn Foo>>() {
            boxed.downcast_ref::<T>()
        } else {
            this.downcast_ref::<T>()
        }     
    }
}

Current output

warning: this lint expectation is unfulfilled
 --> src/main.rs:8:14
  |
8 |     #[expect(dead_code)]
  |              ^^^^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

Desired output

No warning

Rationale and extra context

This function is unused and it is reported as unused without the #[expect] attribute. My expectation would be that adding the #[expect] attribute suppresses the warning

Playground link

Other cases

use std::any::Any;
pub trait Foo: Any {}

impl Foo for Box<dyn Foo> {}

impl dyn Foo {
    fn downcast_ref<T: 'static>(&self) -> Option<&T> {
        let this = self as &dyn Any;
        if let Some(boxed) = this.downcast_ref::<Box<dyn Foo>>() {
            boxed.downcast_ref::<T>()
        } else {
            this.downcast_ref::<T>()
        }     
    }
}

reports

warning: method `downcast_ref` is never used
 --> src/main.rs:9:8
  |
7 | impl dyn Foo {
  | ------------ method in this implementation
8 |     //#[expect(dead_code)]
9 |     fn downcast_ref<T: 'static>(&self) -> Option<&T> {
  |        ^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

Rust Version

1.93.0-nightly (2025-11-11 25d319a0f656ee8faa7a)

Anything else?

No response

[Urgau]

Seems to have something to do with recursive calls.

#[expect(dead_code)]
fn foo() {
    foo()
}

@rustbot label +S-has-mcve

[Urgau]

Okay, I think I know what it going on.

We consider functions with an #[allow(dead_code)]/#[expect(dead_code)] as being kind-of "live" (for reasons explained here) so we mark any such functions for exploration.

In the case of recursive functions we end-up finding the inner call to it-self so we mark it as live, so we don't trigger a warning dead_code lint, which makes the lint expectation unfulfilled.

(This behavior is not a problem in general as we never end-up exploring the such dead functions)

---

It's unclear what the best way to fix this would be, since we also need to consider indirect recursion.

[mu001999]

This has been fixed by #154377