Understanding Rust Error E0782: Missing dyn Keyword

Introduction to Rust Error E0782

Rust Error E0782 occurs when a trait object is missing the 'dyn' keyword. Trait objects allow you to call methods on types not known until runtime, as long as they conform to a specific trait. To create a trait object, you should use the syntax 'Box<dyn Trait>'. This article aims to provide an in-depth understanding of Rust Error E0782 and its solution, illustrated through code examples.

Erroneous Code Example

Consider the following Rust code:

fn main() {
trait Foo {}
fn test(arg: Box<Foo>) {} // error!

Here, the 'test' function tries to take a trait object of type 'Box<Foo>' as an argument. However, this results in Rust Error E0782 because 'dyn' is not used.

Reason Behind Rust Error E0782

The absence of the 'dyn' keyword makes it difficult to distinguish whether 'arg' is a trait object or simply a heap-allocated type named 'Foo'. The 'dyn' keyword is essential for explicitly expressing that the argument is a trait object. As a result, its absence leads to Rust Error E0782.

How to Fix Rust Error E0782

To resolve Rust Error E0782, you need to add the 'dyn' keyword before the trait name, making the correct syntax 'Box<dyn Trait>'.

Corrected Code Example

Here's the corrected Rust code with the added 'dyn' keyword:

fn main() {
trait Foo {}
fn test(arg: Box<dyn Foo>) {} // ok!

The above code is now free from Rust Error E0782 since the 'dyn' keyword is correctly used with the trait object.

Edition Changes Impacting Rust Error E0782

It's important to note that before Rust edition 2021, missing the 'dyn' keyword was allowed. However, it now results in Rust Error E0782, enforcing the use of 'dyn' for clarifying trait objects.


In summary, Rust Error E0782 arises when the 'dyn' keyword is missing in a trait object declaration. To avoid this error, make sure you include 'dyn' before the trait name as in 'Box<dyn Trait>'. By doing so, you maintain code clarity and avoid the pitfalls associated with undeclared trait objects.