`#[expect(unused)]` false-positive `unfulfilled_lint_expectations` warning on implemented but unused traits

[yuriks]

Code

#[expect(unused)]
trait UnusedTrait {}

struct UsedStruct(u32);

impl UnusedTrait for UsedStruct {}

fn main() {
    let x = UsedStruct(12);
    println!("Hello World! {}", x.0);
}

Current output

warning: this lint expectation is unfulfilled
 --> src/main.rs:1:10
  |
1 | #[expect(unused)]
  |          ^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

warning: `playground` (bin "playground") generated 1 warning

Desired output

Compilation with no warnings

Rationale and extra context

The code above throws an unfulfilled_lint_expectations lint even though, if it were removed, the compiler would emit a dead_code lint for the trait.
This only happens when the trait is implemented for some type but ultimately still ends up unused. If there are no implementations at all then expect suppresses the lint correctly without a warning.

Other cases

// If `#[expect(unused)]` is commented out:
warning: trait `UnusedTrait` is never used
 --> src/main.rs:2:7
  |
2 | trait UnusedTrait {}
  |       ^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `playground` (bin "playground") generated 1 warning

Rust Version

rustc 1.94.0-beta.2 (23a44d3c7 2026-01-25)
binary: rustc
commit-hash: 23a44d3c70448c08dc6a2fc13c1afceab49f2bb9
commit-date: 2026-01-25
host: x86_64-pc-windows-msvc
release: 1.94.0-beta.2
LLVM version: 21.1.8

Anything else?

Playground link: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=1d6bd45080b177104331d8968d8812b8

[eggyal]

@rustbot claim

[mu001999]

In fact we also have:

#[expect(dead_code)]
fn foo() {
    if true {
        foo();
    }
}

This will lead to similar false-positive warnings.

[mu001999]

The problem is that allow(dead_code)/expect(dead_code) will affect the dead code analysis, and then change then final result.

[AMDmi3]

Even simpler repro:

#[derive(Default)]
#[expect(unused)]
struct Foo{}
fn main(){}

warning: this lint expectation is unfulfilled
 --> src/main.rs:2:10
  |
2 | #[expect(unused)]
  |          ^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

[mu001999]

I think a more complete solution would be to first perform a round of dead code analysis before items with allow/expect dead_code added into the worklist.

Then, among dead items, take those whose lint level is allow or expect (recorded as set A) and add them to the worklist for a new round of analysis.

Finally, emit the dead code warning based on the union of the remaining dead items and set A.

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-medium