Clippy redundant allocation lint

Question

So I have this trait and a struct that implements it:

trait Trait {
    fn foo(&self) -> u64;
}

/// No Copy trait supported because it's expensive
struct Expensive {
    id: u64,
}

impl Trait for Expensive {
    fn foo(&self) -> u64 {
        self.id
    }
}

I have another structure that I wish to be global, that contains the trait:

struct Cheap {
    pub item: Box<dyn Trait>,
}

thread_local! {
    static CHEAP: Cheap = Cheap {
        item: Box::new(Expensive {
            id: 4
        })
    }
}

fn trait_item() -> Box<dyn Trait> {
    CHEAP.with(|c| c.item)
}

This fails because

error[E0507]: cannot move out of `c.item` which is behind a shared reference
  --> src/main.rs:35:20
   |
35 |     CHEAP.with(|c| c.item)
   |                    ^^^^^^ move occurs because `c.item` has type `std::boxed::Box<dyn Trait>`, which does not implement the `Copy` trait

It's not really feasible to always do the processing within the .with(...) part, because some functions that take in the Trait don't care where it comes from. So I try to return a reference to it instead:

fn trait_item<'a>() -> &'a Box<dyn Trait> {
    CHEAP.with(|c| &c.item)
}

This also fails because I cannot send a reference outside of the with:

error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
  --> src/main.rs:33:20
   |
33 |     CHEAP.with(|c| &c.item)
   |                    ^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 33:16...
  --> src/main.rs:33:16
   |
33 |     CHEAP.with(|c| &c.item)
   |                ^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:33:20
   |
33 |     CHEAP.with(|c| &c.item)
   |                    ^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the function body at 32:15...
  --> src/main.rs:32:15
   |
32 | fn trait_item<'a>() -> &'a Box<dyn Trait> {
   |               ^^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:33:5
   |
33 |     CHEAP.with(|c| &c.item)
   |     ^^^^^^^^^^^^^^^^^^^^^^^

So instead I wrap the whole thing in an Rc like so:

struct Cheap {
    pub item: Rc<Box<dyn Trait>>,
}

thread_local! {
    static CHEAP: Cheap = Cheap {
        item: Rc::new(Box::new(Expensive {
            id: 4
        }))
    }
}

fn trait_item() -> Rc<Box<dyn Trait>> {
    CHEAP.with(|c| c.item.clone())
}

But now clippy complains:

warning: usage of `Rc<Box<T>>`
  --> src/main.rs:41:15
   |
41 |     pub item: Rc<Box<dyn Trait>>,
   |               ^^^^^^^^^^^^^^^^^^ help: try: `Box<dyn Trait>`
   |
   = note: `#[warn(clippy::redundant_allocation)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_allocation

warning: usage of `Rc<Box<T>>`
  --> src/main.rs:53:20
   |
53 | fn trait_item() -> Rc<Box<dyn Trait>> {
   |                    ^^^^^^^^^^^^^^^^^^ help: try: `Box<dyn Trait>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_allocation

warning: 2 warnings emitted

Am I missing something, or is it not actually possible to do what clippy recommends here?

Rust playground | Relevant Clippy page

Answer 1

You can wrap a trait object in a Rc as well, since Rc is also a pointer.

Therefore if you have Rc<Box<T>>, you have two allocations: One for the T and one for the Box (another pointer, that now is on the heap). Instead, use Rc<dyn MyTrait> to only have one allocation.