How to return an empty peekable iterator in Rust?

Question

I have the following Rust function

fn query_list_or_empty<'a, P, R>(
    conn: &'a mut SimpleConnection,
    sql: &'a str,
    params: P,
) -> Peekable<Box<dyn Iterator<Item = Result<R, FbError>> + 'a>>
where
    P: rsfbclient::IntoParams,
    R: FromRow + 'static,
{
    match conn.query_iter(sql, params) {
        Ok(iter) => iter.peekable(),
        Err(e) => {
            error!("Failed to execute query: {}", e);

            Box::new(std::iter::empty::<Result<R, FbError>>()).peekable()
        }
    }
}

In case of an error I just want to log the error and return an empty peekable iterator, however I can not find how to initialize an empty list of the correct type - Peekable<Box<dyn Iterator<Item = Result<R, FbError>> + 'a>>

I found how to return an empty NONE PEEKABLE iterator like this:

fn query_list_or_empty<'a, P, R>(
    conn: &'a mut SimpleConnection,
    sql: &'a str,
    params: P,
) -> Box<dyn Iterator<Item = Result<R, FbError>> + 'a>
where
    P: rsfbclient::IntoParams,
    R: FromRow + 'static,
{
    match conn.query_iter(sql, params) {
        Ok(iter) => iter,
        Err(e) => {
            error!("Failed to execute query: {}", e);

            Box::new(std::iter::empty::<Result<R, FbError>>())
        }
    }
}

It works fine and I can call it and add the "packability" later like this:

            let mut rows_iter = query_list_or_empty(
                &mut conn,
                &config.select_candidates,
                (&config.step_id, &config.block_period),
            )
            .peekable(); // <- make it peekeable

Although it is a solution, I do want to make it work - this is challenging Rust I want to master....

Cerberus · Accepted Answer · 2025-08-13 05:52:34Z

Minimized example, analogous to the first code snippet from the question (playground):

use std::iter::Peekable;

// Stub for rsfbclient's method
fn query_iter() -> Result<Box<dyn Iterator<Item = ()>>, ()> {
    todo!()
}

fn query_list_or_empty() -> Peekable<Box<dyn Iterator<Item = ()>>> {
    match query_iter() {
        Ok(iter) => iter.peekable(),
        Err(e) => Box::new(std::iter::empty()).peekable()
    }
}

This errors, because Rust can't coerce Box<T> to Box<dyn Trait> inside some other type - in this case, inside Peekable; it must be coerced while it is still a Box.

One possible way to fix this is to add coercion before calling .peekable(); however, it becomes somewhat cumbersome, because most of the type must be spelled explicitly, only an associated type can be inferred:

match query_iter() {
    Ok(iter) => iter.peekable(),
    Err(e) => (Box::new(std::iter::empty()) as Box<dyn Iterator<Item = _>>).peekable()
}

However, you can also just place .peekable() in the same function, but out of the match:

match query_iter() {
    Ok(iter) => iter,
    Err(e) => Box::new(std::iter::empty()),
}.peekable()

...in which case, as you've already noticed, Box can be coerced automatically; first match arm here is guiding type inference.

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