Introduction

Rust needs to know the size of every type at compile time. A dyn Trait is a dynamically-sized type (DST) because it can represent any concrete type that implements the trait. The compiler rejects attempts to use dyn Trait directly as a value, requiring you to use a reference (&dyn Trait) or a pointer (Box<dyn Trait>) instead.

Symptoms

  • the size for values of type 'dyn Trait' cannot be known at compilation time
  • doesn't have a size known at compile-time
  • the trait 'Sized' is not implemented for 'dyn Trait'
  • Error occurs when returning dyn Trait from a function
  • Error occurs when storing dyn Trait in a struct field

Example error: ```rust trait Processor { fn process(&self, input: &str) -> String; }

fn create_processor() -> dyn Processor { // error[E0277]: the size for values of type dyn Processor // cannot be known at compilation time TextProcessor::new() }

Common Causes

  • Returning dyn Trait instead of Box<dyn Trait> from a function
  • Storing dyn Trait directly in a struct field
  • Using dyn Trait as a function parameter without reference
  • Attempting to create an array of dyn Trait values
  • Generic function accidentally constrained to dyn Trait

Step-by-Step Fix

  1. 1.Box the trait object for owned return values:
  2. 2.```rust
  3. 3.// Before
  4. 4.fn create_processor() -> dyn Processor {
  5. 5.TextProcessor::new()
  6. 6.}

// After: Box<dyn Trait> has known size (pointer size) fn create_processor() -> Box<dyn Processor> { Box::new(TextProcessor::new()) } ```

  1. 1.Use references for borrowed trait objects:
  2. 2.```rust
  3. 3.// For parameters, use references
  4. 4.fn run_processing(processor: &dyn Processor, input: &str) -> String {
  5. 5.processor.process(input)
  6. 6.}

// For return, return a reference with explicit lifetime fn get_default_processor() -> &'static dyn Processor { &DefaultProcessor } ```

  1. 1.Store trait objects in struct fields with Box:
  2. 2.```rust
  3. 3.// Before
  4. 4.struct AppConfig {
  5. 5.processor: dyn Processor, // Error: size not known
  6. 6.}

// After struct AppConfig { processor: Box<dyn Processor>, }

impl AppConfig { fn new(processor: Box<dyn Processor>) -> Self { AppConfig { processor } } } ```

  1. 1.Use generics instead of trait objects when possible:
  2. 2.```rust
  3. 3.// Runtime dispatch (trait object):
  4. 4.fn process_all(processors: &[Box<dyn Processor>], input: &str) -> Vec<String> {
  5. 5.processors.iter().map(|p| p.process(input)).collect()
  6. 6.}

// Compile-time dispatch (generics): faster, no boxing needed fn process_with<P: Processor>(processor: &P, input: &str) -> String { processor.process(input) } ```

  1. 1.**Use impl Trait for return type when concrete type matters**:
  2. 2.```rust
  3. 3.// Instead of boxing a single return type:
  4. 4.fn create_processor() -> Box<dyn Processor> {
  5. 5.Box::new(TextProcessor::new())
  6. 6.}

// Use impl Trait (caller gets the concrete type): fn create_processor() -> impl Processor { TextProcessor::new() } ```

Prevention

  • Default to generics (impl Trait) when a single concrete type is returned
  • Use Box<dyn Trait> only when you need heterogeneous collections
  • Consider enum instead of trait objects for a known set of types (faster dispatch)
  • Use dyn Trait + Send or dyn Trait + Send + Sync for cross-thread usage
  • Understand that Box<dyn Trait> has a vtable pointer overhead (2 pointers total)
  • Prefer &dyn Trait over Box<dyn Trait> when borrowing is sufficient