Lint when using PhantomData without lifetime

What it does

Explains to the user that PhantomData field doesn't do anything because lifetime isn't available.

What does this lint do?

Informs the user that it has useless PhantomData usage, and shows him the places where lifetime should be added.

Categories (optional)

• Kind: See https://github.com/rust-lang/rust-clippy/blob/master/README.md#clippy for list of lint kinds

I would say clippy::correctness or clippy::suspicious

clippy::correctness

What is the advantage of the recommended code over the original code

• less code
• it is indicated that even though the code compiles fine there is a reason why PhantomData doesn't do anything.

Drawbacks

None.

Example

use std::marker::PhantomData;

pub struct Repository {}

impl Repository {
    fn create_commit(&self) -> Commit {
        Commit { _marker: PhantomData }
    }
}


pub struct Commit {
    _marker: PhantomData<Repository>,
}


#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_useless_phantom_data() {
        let repo = Repository {};
        let commit = repo.create_commit();

        drop(repo);
        drop(commit);
    }
}

Could be written as:

pub struct Commit<'a> {
    _marker: PhantomData<&'a Repository>,
}

Start from the issue's Rust example and compare the two Commit definitions to understand the intended diagnostic. The work is done when Clippy reports useless PhantomData usage, identifies where a lifetime should be added, and covers the provided case with a regression test.