How to resolve the error [the method blocking_kind exists for reference &Result<Article, rusqlite::Error>, but its trait bounds were not satisfied]?

[percy507]

Help!!! 😭

use crate::state::AppState;
use anyhow::Result;
use rusqlite::params;
use serde::{Deserialize, Serialize};
use tauri::{command, State};

#[derive(Debug, Serialize, Deserialize)]
pub struct Article {
  id: String,
  content: String,
}

#[command] // error happens here
pub fn db_get_article(state: State<AppState>, id: &str) -> Result<Article> {
  let db = &(*state).db;
  let result: Article = db.conn.query_row(
    "SELECT id,content from article WHERE id=?1",
    params![id],
    |row| Ok(Article { id: row.get(0)?, content: row.get(1)? }),
  )?;
  Ok(result)
}

# error description
error[E0599]: the method `blocking_kind` exists for reference `&Result<Article, anyhow::Error>`, but its trait bounds were not satisfied
   --> src/db/article.rs:13:1
    |
13  | #[command]
    | ^^^^^^^^^^ method cannot be called on `&Result<Article, anyhow::Error>` due to unsatisfied trait bounds
    |
   ::: /Users/mps/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/result.rs:503:1
    |
503 | pub enum Result<T, E> {
    | ---------------------
    | |
    | doesn't satisfy `Result<Article, anyhow::Error>: Serialize`
    | doesn't satisfy `_: tauri::command::private::ResultKind`
    |
   ::: /Users/mps/.cargo/registry/src/github.com-1ecc6299db9ec823/anyhow-1.0.55/src/lib.rs:370:1
    |
370 | pub struct Error {
    | ---------------- doesn't satisfy `anyhow::Error: Into<InvokeError>`
    |
   ::: src/db/mod.rs:32:3
    |
32  |   tauri::generate_handler![article::db_get_article, article::db_add_article]
    |   -------------------------------------------------------------------------- in this macro invocation
    |
    = note: the following trait bounds were not satisfied:
            `anyhow::Error: Into<InvokeError>`
            which is required by `Result<Article, anyhow::Error>: tauri::command::private::ResultKind`
            `Result<Article, anyhow::Error>: Serialize`
            which is required by `&Result<Article, anyhow::Error>: tauri::command::private::SerializeKind`
    = note: this error originates in the macro `article::__cmd__db_get_article` (in Nightly builds, run with -Z macro-backtrace for more info)

[FabianLars]

Everything you return from commands must implement Serialize, this includes Errors and anyhow's error type doesn't implement it.
You basically have 2 choices here, either you convert errors to Strings or implement your own custom Error which implements Serialize and has From implementations for external errors. Here's the relevant section in our wip docs: https://jonaskruckenberg.github.io/tauri-docs-wip/development/inter-process-communication.html#error-handling

[percy507]

@FabianLars Thank you. Your answer salvages me from the mud pond. 👍

[Borber]

May I ask how you finally solved the problem?

[percy507]

For me, I did below steps.

1. Define a custom Error for tauri::command and implement Serialize for it.

use serde::{ser::Serializer, Deserialize, Serialize};

// create the error type that represents all errors possible in our program
#[derive(Debug, thiserror::Error)]
pub enum CommandError {
  #[error(transparent)]
  RusqliteError(#[from] rusqlite::Error),
}

// we must manually implement serde::Serialize
impl Serialize for CommandError {
  fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
  where
    S: Serializer,
  {
    serializer.serialize_str(self.to_string().as_ref())
  }
}

pub type CommandResult<T, E = CommandError> = anyhow::Result<T, E>;

2. Then modifiy return type of the command.

// before
pub fn db_get_article(state: State<AppState>, id: &str) -> Result<Article> {

// after
pub fn db_get_article( state: State<AppState>, id: &str) -> CommandResult<Article> {

That's all.