Understanding Rust Error E0476
Rust Error E0476
In Rust, Error E0476 occurs when the coerced type does not outlive the value being coerced to. This error is related to the unstable CoerceUnsized trait and can be encountered when implementing custom coercions of unsized types behind a smart pointer. Let's take a closer look at an example of erroneous code that triggers this error:
#![allow(unused)]
#![feature(coerce_unsized)]
#![feature(unsize)]
fn main() {
use std::marker::Unsize;
use std::ops::CoerceUnsized;
// error: lifetime of the source pointer does not outlive
// lifetime bound of the object type
impl<'a, 'b, T, S> CoerceUnsized<&'a T> for &'b S where S: Unsize<T> {}
}Why does this error occur?
In the example above, the error points out that the lifetime of the source pointer ('b) does not outlive the lifetime bound of the object type ('a). Specifically, 'b is not a subtype of 'a. Rust enforces strict lifetime constraints to ensure memory safety, and in this case, the lifetimes used in the coercion are not compatible.
How to fix Rust Error E0476?
To fix Rust Error E0476, you should adjust the lifetimes involved in the coercion so that they follow Rust's lifetime constraints. One way to do this is to ensure that the source pointer's lifetime is a subtype of the object type's lifetime, as shown in the following example:
#![allow(unused)]
#![feature(coerce_unsized)]
#![feature(unsize)]
fn main() {
use std::marker::Unsize;
use std::ops::CoerceUnsized;
// No error: the lifetime of the source pointer outlives
// the lifetime bound of the object type
impl<'a, T, S> CoerceUnsized<&'a T> for &'a S where S: Unsize<T> {}
}In this fixed example, both the source pointer and the object type share the same lifetime ('a), ensuring that the source pointer outlives the object type's lifetime, thus resolving the error.