Dead code lint should include unused default trait methods in the unused chain

[shepmaster]

Code

enum Category {
    Dead,
    Used,
}

trait Demo {
    fn this_is_unused(&self) -> Category {
        Category::Dead
    }
}

fn main() {
    let _c = Category::Used;
}

Current output

warning: variant `Dead` is never constructed
 --> src/main.rs:2:5
  |
1 | enum Category {
  |      -------- variant in this enum
2 |     Dead,
  |     ^^^^
  |
  = note: `#[warn(dead_code)]` on by default

Desired output

(in addition to the existing output)

warning: method `Demo::this_is_unused` is never used
 --> src/lib.rs:LL:CC
  |
L |     fn this_is_unused(&self) -> Category {
  |        ^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

Rationale and extra context

When I encountered this lint, I deleted the enum variant, but then got a compiler error as the variant was used within the trait's method. Nowhere did the compiler let me know that it's because the trait's method was unused.

Other cases

No response

Anything else?

It appears that trait methods are never reported as dead code — this example has no warnings:

trait Demo {
    fn unused_1(&self);
    fn unused_2(&self) {}
}

However, I assume something is analyzing to see that the function is unused because otherwise the enum variant couldn't be transitively marked as unused.

[Urgau]

Apparently, not warning on trait items is known and explicit.

rust/compiler/rustc_passes/src/dead.rs

Line 1044 in 2f8d81f

// We do not warn trait items.

[shepmaster]

not warning on trait items is [...] explicit.

I half expected that to be the case. I have two parallel thoughts:

1. Well, why not? I want to know about (non-public) traits and trait methods that I'm not using.
2. If there is some good reason to not warn about dead trait methods, could we enhance the current message to mention them? For example, "variant Dead is never constructed. It's used in which is also unused"

[mu001999]

@rustbot claim

[shepmaster]

#41883 is the issue about making traits / trait methods detected by the dead code lint.