Fix E0667: Async Not Allowed in Extern Declarations

Mon, 10 Aug 2026 22:20:04 +0800

1. Symptoms

The Rust compiler emits error E0667 when attempting to declare an async function within an extern block or alongside the #[no_mangle] attribute for foreign function interface definitions. This error manifests during compilation and prevents the binary from being built.

When triggered, the compiler produces output similar to the following:

error[E0667]: `async` is not allowed in extern declarations
 --> src/main.rs:3:1
  |
3 | extern "C" {
  | ^^^^^^^^^^^^^^ invalid decoration
4 |     pub async fn foreign_async_function();
  |              ^^^^^ invalid here

The error message specifically highlights the async keyword as invalid, pointing to its location within the extern block. In some variations of the error, the compiler may also indicate that this restriction applies to both extern blocks and FFI function declarations with #[no_mangle].

Interop on ErrorVault — Developer Error Code Dictionary

Fix E0667: Async Not Allowed in Extern Declarations

1. Symptoms