error: implementation of `DeserializeBorrowed` is not general enough
  --> src\main.rs:33:50
   |
33 |     handle_path::<Bar, _, _>(todo!(), |bar: Bar| "()");
   |                                                  ^^^^ implementation of `DeserializeBorrowed` is not general enough
   |
   = note: `Bar<'_>` must implement `DeserializeBorrowed<'_>`
   = note: ...but `DeserializeBorrowed<'_>` is actually implemented for the type `Bar<'_>`

/// Wrapper to use `Send + !Sync` as `Send + Sync`
pub struct SendToSync<T> {
    value: T,
}

/// # Safety
/// This is safe because wrapper never gives access to `&T` from `&self`.
/// This is essentially `Mutex::get_mut` without paying for the mutex.
unsafe impl<T> Sync for SendToSync<T> where T: Send {}

impl<T> SendToSync<T> {
    pub fn new(value: T) -> Self {
        SendToSync { value }
    }
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.value
    }
}