Fix E0604: expected class, struct, or trait

Mon, 20 Jul 2026 07:56:04 +0800

1. Symptoms

When the Rust compiler encounters error E0604, you will see output similar to the following:

error[E0604]: expected class, struct, or trait, found `i32`
  --> src/main.rs:3:19
   |
3  | impl SomeTrait for 42 {
   |                 ^^ expected class, struct, or trait

error[E0604]: expected class, struct, or trait, found `module`
  --> src/lib.rs:10:1
   |
10 | impl Debug for my_module {
   |                      ^^^^^^^^^ expected class, struct, or trait

The error message explicitly tells you that the compiler expected a type definition (class, struct, or trait) but encountered something else entirely. This typically manifests when working with trait implementation blocks where the target type has been incorrectly specified.

E0604 on ErrorVault — Developer Error Code Dictionary

Fix E0604: expected class, struct, or trait

1. Symptoms