method `diagnostic` has an incompatible type for trait

[xhyrom]

When i'm trying to implement diagnostic or workspace_diagnostic method, I'm receiving this error:

error[E0053]: method `diagnostic` has an incompatible type for trait
  --> lsp/src/main.rs:72:1
   |
72 | #[tower_lsp::async_trait]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | expected `Result<DocumentDiagnosticReportResult, ...>`, found `()`
   | help: change the output type to match the trait: `Pin<Box<(dyn Future<Output = Result<DocumentDiagnosticReportResult, tower_lsp::jsonrpc::Error>> + Send + 'async_trait)>>`
   |
   = note: expected signature `fn(&'life0 Backend, tower_lsp::lsp_types::DocumentDiagnosticParams) -> Pin<Box<(dyn Future<Output = Result<DocumentDiagnosticReportResult, tower_lsp::jsonrpc::Error>> + Send + 'async_trait)>>`
              found signature `fn(&'life0 Backend, tower_lsp::lsp_types::DocumentDiagnosticParams) -> Pin<Box<(dyn Future<Output = ()> + Send + 'async_trait)>>`
   = note: this error originates in the attribute macro `tower_lsp::async_trait` (in Nightly builds, run with -Z macro-backtrace for more info)

#[tower_lsp::async_trait]
impl LanguageServer for Backend {
    async fn initialize(&self, params: InitializeParams) -> Result<InitializeResult> {
        Ok(InitializeResult::default())
    }

    async fn initialized(&self, _: InitializedParams) {
        self.client
            .log_message(
                MessageType::INFO,
                "server initialized!",
            )
            .await;
    }

    async fn shutdown(&self) -> Result<()> {
        Ok(())
    }

    async fn diagnostic(&self, params: DocumentDiagnosticParams) {}
}

[xhyrom]

there's return type, sorry