`unused_phantomdata`

What it does

I just wrote this odd code in a test:

use std::marker::PhantomData;

#[repr(transparent)]
struct H<T> {
    h: Box<Option<T>>,
    _m: (),
    __m: PhantomData<T>,
}

I think if the type parameter is used elsewhere, or the lifetime, there should be a lint suggesting to remove it (the PhantomData).

Not sure if there are any cases where this is necessary, though. This section in the docs is a bit worrying: https://doc.rust-lang.org/nightly/std/marker/struct.PhantomData.html#ownership-and-the-drop-check

Advantage

• Removes an unnecessary PhantomData since it already owns/uses the type parameter/lifetime

Drawbacks

No response

Example

use std::marker::PhantomData;

#[repr(transparent)]
struct H<T> {
    h: Box<Option<T>>,
    _m: (),
    __m: PhantomData<T>,
}

Could be written as:

use std::marker::PhantomData;

#[repr(transparent)]
struct H<T> {
    h: Box<Option<T>>,
    _m: (),
}

The issue names no files, tests, or entry points. Start by studying the example and the linked PhantomData ownership and drop-check documentation; done means establishing when PhantomData is unnecessary and defining a lint that suggests its removal without affecting required semantics.