The on_connect closure, provided to HttpServer via the builder pattern, is not being invoked when a new TCP connection is established. This behavior has been confirmed using a minimal, self-contained example where a panic! inside the callback never triggers, and no associated side effects (e.g., printing to stderr, creating a file) occur. The server continues to run and eventually times out the connection as if the callback were never registered.

Cargo.toml

[package]
name = "actix_on_connect_test"
version = "0.1.0"
edition = "2021"
[dependencies]
actix-web = "4"
env_logger = "0.11"
futures-util = "0.3"

src/main.rs

use actix_web::{web, App, HttpRequest, HttpResponse, HttpServer};

// A simple handler that will be called for a GET request
async fn index(_req: HttpRequest) -> HttpResponse {
    println!(">>> Handler 'index' triggered.");
    HttpResponse::Ok().body("Hello from the handler!")
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    env_logger::init_from_env(env_logger::Env::new().default_filter_or("info"));

    println!("Starting server on 0.0.0.0:8080...");

    HttpServer::new(|| App::new().route("/", web::get().to(index)))
        .bind(("0.0.0.0", 8080))?
        .on_connect(|_,_| {
            // This code is expected to execute immediately upon a TCP connection being accepted.
            eprintln!("\n!!! on_connect is about to panic! If you see this, the callback is executing. !!!");
            panic!("Intentional panic from on_connect to prove execution.");
        })
        .run()
        .await
}

Actual Behavior

The telnet client successfully connects (Connected to...) but the server does not crash.
No output from eprintln! is visible in the server's logs.
The connection remains open until a timeout occurs, at which point the client receives a 408 Request Timeout and disconnects.
For the curl request, the server successfully handles the HTTP request via the index handler, but the on_connect callback is never executed beforehand.
The server process continues to run without any sign that the on_connect closure was ever triggered.

Environment Details

Actix-Web Version: 4.x (actix-web = "4")
Rust Version: rustc 1.82.0 (f6e511eec 2024-10-15) (built from a source tarball)
Operating System: nixos-version
24.11.718051.9b5ac7ad4529 (Vicuna), Linux nixos 6.6.90 #1-NixOS SMP PREEMPT_DYNAMIC Fri May 9 07:44:08 UTC 2025 x86_64 GNU/Linux, inside a VM, within VirtManager, on a Linux host.
Client used for testing: telnet, curl, and netcat (with various shell functions).
Server run command: cargo run