Rust Error E0203: Handling Multiple Relaxed Default Bounds

Understanding Rust Error E0203

Rust error E0203 occurs when there are multiple relaxed default bounds within a single definition in your Rust code. The current implementation of the Rust language does not support multiple relaxed bounds for default traits.

Erroneous code example:

#![allow(unused)]
fn main() {
  struct Bad<T: ?Sized + ?Send> {
      inner: T
  }
}

In the code above, the type T has both ?Sized and ?Send, which represents multiple relaxed bounds in the same definition. This is not permitted and will result in error E0203.

Fixing Rust Error E0203

To fix Rust Error E0203, you must use only one relaxed bound. See the example below:

#![allow(unused)]
fn main() {
  struct Good<T: ?Sized> {
      inner: T
  }
}

In the corrected code, the type T only has the ?Sized relaxed bound. This will prevent error E0203 from occurring.

Using the Sized and Send Traits

To better understand Rust error E0203, it's important to know the purpose and usage of the Sized and Send traits.

Sized Trait

The Sized trait is automatically added to types whose size is known at compile-time. It is used when you need to ensure that the memory footprint of your data types is known.

#![allow(unused)]
fn main() {
  struct Container<T: Sized> {
      item: T
  }
}

The code above enforces that the type T must implement the Sized trait, which means its memory size is known at compile-time.

Send Trait

The Send trait is used to signify that your data can safely be transferred between threads. A type implementing the Send trait is said to be 'thread-safe'.

#![allow(unused)]
fn main() {
  struct ThreadSafe<T: Send> {
      data: T
  }
}

In the code snippet above, the type T must implement the Send trait, ensuring it can be safely transferred across threads.