`allow` response header missing for `405 Method Not Allowed` generated by router

[TimWolla]

Expected Behavior

I am expecting to see an allow header for 405 Method Not Allowed responses generated by the router.

As per RFC7231#6.5.5:

[…] The origin server MUST generate an Allow header field in a 405 response containing a list of the target resource's currently supported methods.

Current Behavior

There is no allow header to be seen.

Possible Solution

When the router is sending a 405 in response to a request to an existing route, but with an unknown HTTP method it should add a allow header with the known methods.

Steps to Reproduce (for bugs)

main.rs

use actix_web::{web, App, HttpServer, Responder};

async fn index() -> impl Responder {
    "Index"
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| {
        App::new()
            .service(web::resource("/").route(web::get().to(index)))
    })
    .bind("[::]:8080")?
    .run()
    .await
}

Cargo.toml

[package]
name = "actix_test"
version = "0.1.0"
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
actix-web = "3.3.2"

Allowed method:

$ http GET 'localhost:8080/'
HTTP/1.1 200 OK
content-length: 5
content-type: text/plain; charset=utf-8
date: Mon, 09 Aug 2021 19:15:05 GMT

Index

Disallowed method:

$ http PUT 'localhost:8080/'
HTTP/1.1 405 Method Not Allowed
content-length: 0
date: Mon, 09 Aug 2021 19:15:00 GMT

Context

This bug report was created purely from a standard correctness perspective with actix violating a MUST.

This was previously reported in #885, but I believe the remark about RFC correctness was missed in the issue.

Your Environment

• Rust Version (I.e, output of rustc -V): rustc 1.53.0 (53cb7b09b 2021-06-17)
• Actix Web Version: 3.3.2