E0308 — Result Type Mismatch

What causes this error

The return type is Result<T, E> but the returned value has a different T or E type. The ? operator may be converting between incompatible error types.

How to fix it

Ensure Ok and Err types match the function signature. Implement From<ErrorType> for your error type to enable ? operator conversion. Use .map_err() to convert error types. Use a crate like anyhow or thiserror for ergonomic error handling.

fn foo() -> Result<(), String> {
  Ok(42) // expected ()
}