Getting PhantomData to have a 'static lifetime

[nicrd]

I'm trying to implement a generic "id container" struct that is strongly typed. Basically its a struct with a generic parameter to hold the type and a small payload.

Since the generic parameter is unused within the struct, I need to add a PhantomData<T> member. When this parameter's type does not have a 'static lifetime, this affects the entire struct.

The following code that reproduces this issue:

use std::marker::PhantomData;

struct Tag<T>(usize, PhantomData<T>);
struct WithRef<'a>(&'a i32);

trait SomeTrait {}
impl<T> SomeTrait for Tag<T> {}

fn ok() {
    let obj = Tag::<i32>(1, PhantomData);
    let _dyn_obj: &(dyn SomeTrait + 'static) = &obj;
}

fn bad<'b>() {
    let obj = Tag::<WithRef<'b>>(2, PhantomData);
    let _dyn_obj: &(dyn SomeTrait + 'static) = &obj;
}

outputs this compiler error:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements                                    
  --> src\main.rs:15:15
   |
15 |     let obj = Tag::<WithRef<'b>>(2, PhantomData);
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'b` as defined on the function body at 14:8...
  --> src\main.rs:14:8
   |
14 | fn bad<'b>() {
   |        ^^
note: ...so that the expression is assignable
  --> src\main.rs:15:15
   |
15 |     let obj = Tag::<WithRef<'b>>(2, PhantomData);
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected  `Tag<WithRef<'_>>`
              found  `Tag<WithRef<'b>>`
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the expression is assignable
  --> src\main.rs:16:48
   |
16 |     let _dyn_obj: &(dyn SomeTrait + 'static) = &obj;

   |                                                ^^^^
   = note: expected  `&(dyn SomeTrait + 'static)`
              found  `&dyn SomeTrait`

PhantomData in std::marker - Rust mentions that:

If your struct does not in fact own the data of type T, it is better to use a reference type, like PhantomData<&'a T> (ideally) or PhantomData<*const T> (if no lifetime applies), so as not to indicate ownership.

But neither PhantomData<&'a T>nor PhantomData<*const T> makes this code compile.

Is there any way to keep the generic parameter without loosing the 'static lifetime?

Thanks!

[mjbshaw]

This forum is for the development of Rust itself. Questions about using Rust are better suited for https://users.rust-lang.org/

[nicrd]

My bad!

I reposted the question there.

Thanks for the heads up!