Fix E0770: Async Fn Return Type Cannot Contain Impl Trait

Mon, 15 Jun 2026 10:20:04 +0800

Fix E0770: Async Fn Return Type Cannot Contain `impl Trait`

Rust’s async fn syntax provides a convenient way to write asynchronous code, but it comes with strict limitations on what return types are allowed. Error E0770 is a compile-time error that occurs when developers attempt to use impl Trait in the return type of an async fn. Understanding why this restriction exists and how to work around it is essential for writing idiomatic async Rust code.

Fix E0575: Rust Async Closure Type Mismatch Error

Tue, 19 May 2026 15:08:04 +0800

1. Symptoms

When you encounter Rust error E0575, the compiler produces output similar to the following:

error[E0575]: expected an `async` closure, found a non-`async` closure
   --> src/main.rs:10:23
    |
10  |     let closure = |x: i32| { x + 1 };
    |                   ^^^^^^^^^^^^^^ expected async closure
    |
    = note: closures that are `async` must be declared with the `async` keyword
    = note: expected closure that is `async`, found closure that is not `async`

The error manifests in several common scenarios:

Callback registration: When passing closures to async-compatible APIs that require async closures
Higher-order async functions: Functions expecting async closures as parameters receive regular closures
Async trait methods: Trait method signatures requiring async closures receive synchronous closures
Executor APIs: Async runtime methods that expect async closures for spawn or timeout operations

The compiler explicitly states the mismatch between the expected closure type (async) and the provided closure type (non-async). This error prevents code from compiling because async and non-async closures have fundamentally different type signatures at the function signature level.

Futures on ErrorVault — Developer Error Code Dictionary

Fix E0770: Async Fn Return Type Cannot Contain Impl Trait

Fix E0770: Async Fn Return Type Cannot Contain impl Trait

Fix E0575: Rust Async Closure Type Mismatch Error

1. Symptoms

Fix E0770: Async Fn Return Type Cannot Contain `impl Trait`