About

This crate provides a way to erase the type of a reference or box, retaining the lifetime. These erased references can then be unsafely converted back to their original type. This can be used to store heterogeneous references in a Vec or another datastructure. This is often useful with references into an arena.

The following types are provided:

Example

use erased::Erased;

let mut vec: Vec<Erased<'_>> = Vec::new();
vec.push((&5u64).into());
vec.push((&"Hello World").into());

// SAFETY: Above we insered a `u64` into an empty vec, therefore converting the element back to a `u64` is sound.
assert_eq!(unsafe { *vec[0].get::<u64>() }, 5);
// SAFETY: Above we insered a `&'static str` into a vec containing one element, therefore converting the element back to a `&'static str` is sound.
assert_eq!(unsafe { *vec[1].get::<&'static str>() }, "Hello World");

Comparison with dyn Any

This crate provides similar functionality to downcast_ref and downcast_mut on a dyn Any. The differences are:

• Downcast is checked, it stores the type id of the type that was stored and checks whether the generic type provided matches upon retrieval. The types defined in this crate are unchecked, and have an unsafe precondition that the type matches.
• Since no type information is stored, a Erased<'a> is only one pointer wide, half the size of &'a dyn Any
• dyn Any can currently only be used on types with a static lifetime. Erased can be used on any type.