tokio 1.43.0: Tokio Runtime::Builder disables time and IO drivers by default, time::pause requires current_thread runtime, and TOKIO_WORKER_THREADS=0 panics multi-thread builds

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[dependencies]
tokio = { version = "=1.43.0", features = ["rt", "rt-multi-thread", "macros", "time", "net", "io-util", "sync", "test-util"] }

### Search Result
`search_known_solution` answered with a HIT (`MATCH: COMPATIBLE`, sample `sha256:9b9720fef342edf7ef51c61e4781caeafe87bc3863b3fdfe9efd67fdb0a2caf8`), which proved `LocalSet` non-`Send` future driving and unpolled task starvation. To avoid duplicate coverage, this sample targets Tokio's runtime configuration, driver enablement requirements (`enable_time`/`enable_io`), `time::pause()` runtime-flavor restrictions (`current_thread` vs `multi_thread`), and `TOKIO_WORKER_THREADS` environment variable handling.

### What a model would have written instead
Models construct custom runtimes using `tokio::runtime::Builder::new_current_thread().build()` or `new_multi_thread().build()` expecting timer and network drivers to be enabled by default, and call `tokio::time::pause()` in multi-threaded test setups expecting it to pause the global timer.

### How the wrong version fails
Fails loudly at runtime with panic `"A Tokio 1.x context was found, but timers are disabled. Call enable_time on the runtime builder to enable timers"`, `"A Tokio 1.x context was found, but IO is disabled. Call enable_io on the runtime builder to enable IO"`, `"time::pause() requires the current_thread Tokio runtime"`, or `"TOKIO_WORKER_THREADS cannot be set to 0"`.

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use std::panic::{catch_unwind, AssertUnwindSafe};
use std::time::{Duration, Instant};

fn extract_panic_message(err: Box<dyn std::any::Any + Send>) -> String {
    if let Some(s) = err.downcast_ref::<&str>() {
        s.to_string()
    } else if let Some(s) = err.downcast_ref::<String>() {
        s.clone()
    } else {
        "Unknown panic".to_string()
    }
}

/// 1. Calling time APIs without `enable_time()` or `enable_all()` panics with timer driver error
pub fn time_without_timer_driver_panics() {
    let rt = tokio::runtime::Builder::new_current_thread()
        .build()
        .expect("builder succeeds without drivers");

    // sleep panics when polled without timer driver
    let sleep_result = catch_unwind(AssertUnwindSafe(|| {
        rt.block_on(async {
            tokio::time::sleep(Duration::from_millis(10)).await;
        });
    }));
    assert!(sleep_result.is_err());
    let msg = extract_panic_message(sleep_result.unwrap_err());
    assert!(
        msg.contains("timers are disabled") && msg.contains("enable_time"),
        "Unexpected panic message: {}",
        msg
    );

    // timeout panics immediately even if inner future is ready
    let timeout_result = catch_unwind(AssertUnwindSafe(|| {
        let _ = rt.block_on(async {
            tokio::time::timeout(Duration::from_secs(1), async { 42 }).await
        });
    }));
    assert!(timeout_result.is_err());
    let msg = extract_panic_message(timeout_result.unwrap_err());
    assert!(
        msg.contains("timers are disabled") && msg.contains("enable_time"),
        "Unexpected panic message: {}",
        msg
    );
}

/// 2. Calling net/IO APIs without `enable_io()` or `enable_all()` panics with IO driver error
pub fn net_without_io_driver_panics() {
    let rt = tokio::runtime::Builder::new_current_thread()
        .build()
        .expect("builder succeeds without drivers");

    let bind_result = catch_unwind(AssertUnwindSafe(|| {
        let _ = rt.block_on(async {
            let _ = tokio::net::TcpListener::bind("127.0.0.1:0").await;
        });
    }));
    assert!(bind_result.is_err());
    let msg = extract_panic_message(bind_result.unwrap_err());
    assert!(
        msg.contains("IO is disabled") && msg.contains("enable_io"),
        "Unexpected panic message: {}",
        msg
    );
}

/// 3. Calling `tokio::time::pause()` on a multi-thread runtime panics
pub fn pause_time_on_multi_thread_runtime_panics() {
    let rt = tokio::runtime::Builder::new_multi_thread()
        .worker_threads(2)
        .enable_time()
        .build()
        .expect("multi_thread runtime built with enable_time");

    let pause_result = catch_unwind(AssertUnwindSafe(|| {
        rt.block_on(async {
            tokio::time::pause();
        });
    }));
    assert!(pause_result.is_err());
    let msg = extract_panic_message(pause_result.unwrap_err());
    assert!(
        msg.contains("`time::pause()` requires the `current_thread` Tokio runtime"),
        "Unexpected panic message: {}",
        msg
    );
}

/// 4. `tokio::time::pause()` on `current_thread` enables instant virtual time advancement
pub fn pause_and_advance_time_on_current_thread() {
    let rt = tokio::runtime::Builder::new_current_thread()
        .enable_time()
        .build()
        .expect("current_thread runtime built with enable_time");

    rt.block_on(async {
        tokio::time::pause();

        let real_start = Instant::now();
        let tokio_start = tokio::time::Instant::now();

        // Advance mock time by 1 hour (3600 seconds)
        let sleep_fut = tokio::time::sleep(Duration::from_secs(3600));
        tokio::time::advance(Duration::from_secs(3600)).await;
        sleep_fut.await;

        let real_elapsed = real_start.elapsed();
        let tokio_elapsed = tokio::time::Instant::now().duration_since(tokio_start);

        // Real wall-clock time is near zero (less than 100ms)
        assert!(
            real_elapsed < Duration::from_millis(100),
            "Paused time sleep+advance should complete instantly, took {:?}",
            real_elapsed
        );
        // Tokio virtual time advanced by at least 1 hour
        assert!(
            tokio_elapsed >= Duration::from_secs(3600),
            "Tokio virtual clock must advance by at least 3600s, was {:?}",
            tokio_elapsed
        );
    });
}

/// 5. Driver flags enable normal operation when properly configured
pub fn drivers_enabled_allow_time_and_io() {
    let rt = tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .expect("current_thread runtime with enable_all");

    rt.block_on(async {
        // Time operations work
        let res = tokio::time::timeout(Duration::from_millis(100), async {
            tokio::time::sleep(Duration::from_millis(1)).await;
            42
        })
        .await;
        assert_eq!(res.unwrap(), 42);

        // IO operations work on localhost/127.0.0.1
        let listener = tokio::net::TcpListener::bind("127.0.0.1:0")
            .await
            .expect("bind listener");
        let local_addr = listener.local_addr().expect("local addr");
        assert!(local_addr.port() > 0);
    });
}

/// 6. TOKIO_WORKER_THREADS=0 environment variable causes default multi_thread build to panic, but explicit worker_threads overrides
pub fn worker_threads_zero_env_var_behavior() {
    // Save previous env var if set
    let prev = std::env::var("TOKIO_WORKER_THREADS").ok();

    std::env::set_var("TOKIO_WORKER_THREADS", "0");

    // Unconfigured worker threads reads TOKIO_WORKER_THREADS=0 and panics
    let zero_result = catch_unwind(AssertUnwindSafe(|| {
        let _ = tokio::runtime::Builder::new_multi_thread().build();
    }));
    assert!(zero_result.is_err());
    let msg = extract_panic_message(zero_result.unwrap_err());
    assert!(
        msg.contains("TOKIO_WORKER_THREADS") && msg.contains("0"),
        "Unexpected panic message: {}",
        msg
    );

    // Explicit worker_threads(2) overrides the TOKIO_WORKER_THREADS env var and succeeds
    let override_result = catch_unwind(AssertUnwindSafe(|| {
        let rt = tokio::runtime::Builder::new_multi_thread()
            .worker_threads(2)
            .build()
            .expect("explicit worker_threads overrides env var");
        rt.block_on(async { 100 })
    }));
    assert_eq!(override_result.unwrap(), 100);

    // Restore environment
    match prev {
        Some(v) => std::env::set_var("TOKIO_WORKER_THREADS", v),
        None => std::env::remove_var("TOKIO_WORKER_THREADS"),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_time_without_timer_driver_panics() {
        time_without_timer_driver_panics();
    }

    #[test]
    fn test_net_without_io_driver_panics() {
        net_without_io_driver_panics();
    }

    #[test]
    fn test_pause_time_on_multi_thread_runtime_panics() {
        pause_time_on_multi_thread_runtime_panics();
    }

    #[test]
    fn test_pause_and_advance_time_on_current_thread() {
        pause_and_advance_time_on_current_thread();
    }

    #[test]
    fn test_drivers_enabled_allow_time_and_io() {
        drivers_enabled_allow_time_and_io();
    }

    #[test]
    fn test_worker_threads_zero_env_var_behavior() {
        worker_threads_zero_env_var_behavior();
    }
}