You've reached an impossible internal state. If you ever…

Error message

You've reached an impossible internal state. If you ever see this error message please open an issue at https://github.com/diesel-rs/diesel providing example code how to trigger this error.

What it means

An internal `unreachable!` in the MySQL/MariaDB statement iterator (`Iterator::next`). While fetching rows, the iterator's pending binds are expected to be in the `PrivateMysqlRow::Direct` state; any other state means diesel's internal state machine got corrupted. This is a diesel bug, not a user error — the message asks for a bug report.

Solutions

Ensure the previous query's rows are fully consumed or the iterator is dropped before reusing the connection.
Do not share one connection across threads; use a pool (r2d2/deadpool-diesel) or `Pool`-managed connections.
Upgrade diesel to the latest patch release — this is an internal invariant the maintainers may have fixed.
Reproduce with a minimal example and open an issue at https://github.com/diesel-rs/diesel as the message requests.

Defensive patterns

Strategy: try-catch

Try / catch

// panics are not catchable normally; guard by never sharing a connection mid-fetch
let rows = users.load::<User>(&mut conn).expect("query failed"); // fully consume before reusing conn
// if catching panics is essential:
let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| users.load::<User>(&mut conn)));

Prevention

Never share one MysqlConnection across threads; use a pool.
Fully consume or drop result iterators before the next query.
Keep diesel updated and read changelogs for iterator fixes.

When it happens

Trigger: Calling `next()` on a `MysqlConnection` query iterator while internal row buffers were left in a non-`Direct` state — caused by internal logic bugs around `populate_row_buffers`, interleaved statement use, or concurrent use of the same connection mid-fetch.

Common situations: Sharing a `MysqlConnection` across threads without pooling; using the connection for another query while a previous result set was only partially consumed; diesel version regressions on MariaDB/mysqlclient; unsafe misuse via `with_raw_connection`-style APIs.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/fb0cfad8ac659079. Report an issue: GitHub.

Appendix: source

Thrown at diesel/src/mysql_like/connection/stmt/iterator.rs:70

    }
}

impl<DB: MysqlLikeBackend> Iterator for StatementIterator<'_, DB> {
    type Item = QueryResult<MysqlRow<DB>>;

    fn next(&mut self) -> Option<Self::Item> {
        // check if we own the only instance of the bind buffer
        // if that's the case we can reuse the underlying allocations
        // if that's not the case, we need to copy the output bind buffers
        // to somewhere else
        let res = if let Some(binds) = Rc::get_mut(&mut self.last_row) {
            if let PrivateMysqlRow::Direct(binds) = RefCell::get_mut(binds) {
                self.stmt.populate_row_buffers(binds)
            } else {
                // any other state than `PrivateMysqlRow::Direct` is invalid here
                // and should not happen. If this ever happens this is a logic error
                // in the code above
                unreachable!(
                    "You've reached an impossible internal state. \
                     If you ever see this error message please open \
                     an issue at https://github.com/diesel-rs/diesel \
                     providing example code how to trigger this error."
                )
            }
        } else {
            // The shared bind buffer is in use by someone else,
            // this means we copy out the values and replace the used reference
            // by the copied values. After this we can advance the statement
            // another step
            let mut last_row = {
                let mut last_row = match self.last_row.try_borrow_mut() {
                    Ok(o) => o,
                    Err(_e) => {
                        return Some(Err(DeserializationError(
                            "Failed to reborrow row. Try to release any `MysqlField` or `MysqlValue` \
                             that exists at this point"

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