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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Returning an owned "dyn" object without heap allocations

Hello.

Is it possible to return an owned dyn Trait from a function without using heap-allocations, like Box or Arc?

In my case, the types implementing Trait are expected to be small, no more than N bytes, so I thought maybe it would be possible to return a type that reserves the needed N bytes, and stores a pointer to the dyn vtable, like so (just a high-level sketch):

trait MyTrait {}
struct MyType; // this is no more than 16 bytes
impl MyTrait for MyType {}

struct OnStack<T, const N: usize> {
    vtable: *mut T::Vtable,
    data: [u8; N],
}

fn my_function() -> OnStack<dyn MyTrait, 16> {
    OnStack::new(MyType::new()).expect("MyType is larger than 16 bytes")
}

Rust already uses space reservation for enums: the size of an enum is determined by the size of its largest variant. So maybe it’s also possible to do for other types with variable size, like `dyn Trait`?

Thanks!

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