Cannot attach PoisonError to thiserror variant

In my code I want to use thiserror + anyhow and I want to create error variant from PoisonError, but my code not compiles and return an error:

error[E0599]: no method named `map_err` found for struct `tokio::sync::MutexGuard<'_, Option<String>>` in the current scope
  --> src/
25 |         .map_err(|source| MutexError::LockAcquired { source })?;
   |          ^^^^^^^ help: there is a method with a similar name: `map_or`

this is my code:

use std::sync::Arc;
use thiserror::Error;
use tokio::sync::{Mutex, mpsc};
use anyhow::{Context, Result};

#[derive(Error, Debug)]
pub enum MutexError<T> {
    #[error("lock is acquired")]
    LockAcquired { source: std::sync::PoisonError<T> },

struct Test {
    item: Arc<Mutex<Option<String>>>,

async fn main() -> Result<()> {
    let test = Test {
        item: Arc::new(Mutex::new(None)),
    let x = test.item
        .map_err(|source| MutexError::LockAcquired { source })?;

This is sandbox to reproduce.

Could somebody explain how to fix this issue and correctly attach custom error to mutex ?

tokio's Mutex::lock doesn't return a Result, so there's no error to map.

Note that in contrast to std::sync::Mutex, this implementation does not poison the mutex when a thread holding the MutexGuard panics. In such a case, the mutex will be unlocked. If the panic is caught, this might leave the data protected by the mutex in an inconsistent state.

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