Fix E0391: Type Size Cannot Be Known at Compilation Time

Fri, 10 Jul 2026 03:08:04 +0800

1. Symptoms

When the Rust compiler encounters error E0391, you will see a message similar to the following:

error[E0391]: the size for values of type `{type}` cannot be known at compilation time
  --> src/main.rs:5:5
   |
5  |     dyn MyTrait,
   |     ^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `MyTrait` cannot be made into an object without a `?Sized` bound

The error typically appears when attempting to use a trait object in a context requiring a sized type. Common indicators include attempting to store dyn Trait directly in a struct field, using unsized types as function parameters without explicit bounds, or creating collections of trait objects without appropriate indirection.

E0391 on ErrorVault — Developer Error Code Dictionary

Fix E0391: Type Size Cannot Be Known at Compilation Time

1. Symptoms