Tokio: [doc] could not find `main` in `tokio`

[ultrasaurus]

I've attempted to create the smallest, interesting "hello world" app, and it works fine with features = ["full"] and fails when selecting specific features

Version

$ cargo tree | grep tokio
tokio-hello-world v0.1.0 (/Users/sallen/src/rust/tokio-hello-world)
└── tokio v0.2.8
    └── tokio-macros v0.2.3

Platform

$ uname -a
Darwin newair.local 18.7.0 Darwin Kernel Version 18.7.0: Sun Dec  1 18:59:03 PST 2019; root:xnu-4903.278.19~1/RELEASE_X86_64 x86_64 i386 MacBookAir8,1 Darwin

Subcrates

[dependencies]
tokio = { version = "0.2", features = ["macros", "tcp"] }

Description

use tokio::net::TcpStream;
use tokio::prelude::*;

#[tokio::main]
async fn main() {
    // Open a TCP stream to the socket address.
    //
    // Note that this is the Tokio TcpStream, which is fully async.
    let mut stream = TcpStream::connect("127.0.0.1:6142").await.unwrap();
    println!("created stream");

    let result = stream.write(b"hello world\n").await;
    println!("wrote to stream; success={:?}", result.is_ok());
}

I must be missing something very basic. Here are the errors:

   Compiling tokio-hello-world v0.1.0 (/Users/sallen/src/rust/tokio-hello-world)
error[E0433]: failed to resolve: could not find `main` in `tokio`
 --> src/main.rs:6:10
  |
6 | #[tokio::main]
  |          ^^^^ could not find `main` in `tokio`

warning: unused import: `tokio::prelude::*`
 --> src/main.rs:4:5
  |
4 | use tokio::prelude::*;
  |     ^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0277]: the trait bound `str: tokio::net::addr::ToSocketAddrs` is not satisfied
  --> src/main.rs:11:41
   |
11 |     let mut stream = TcpStream::connect("127.0.0.1:6142").await.unwrap();
   |                                         ^^^^^^^^^^^^^^^^ the trait `tokio::net::addr::ToSocketAddrs` is not implemented for `str`
   | 
  ::: /Users/sallen/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.8/src/net/tcp/stream.rs:79:29
   |
79 |     pub async fn connect<A: ToSocketAddrs>(addr: A) -> io::Result<TcpStream> {
   |                             ------------- required by this bound in `tokio::net::tcp::stream::TcpStream::connect`
   |
   = note: required because of the requirements on the impl of `tokio::net::addr::ToSocketAddrs` for `&str`

error[E0599]: no method named `write` found for type `tokio::net::tcp::stream::TcpStream` in the current scope
  --> src/main.rs:14:25
   |
14 |     let result = stream.write(b"hello world\n").await;
   |                         ^^^^^ method not found in `tokio::net::tcp::stream::TcpStream`

error[E0277]: `main` has invalid return type `impl std::future::Future`
 --> src/main.rs:7:17
  |
7 | async fn main() {
  |                 ^ `main` can only return types that implement `std::process::Termination`
  |
  = help: consider using `()`, or a `Result`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0277, E0433, E0599.
For more information about an error, try `rustc --explain E0277`.

[ultrasaurus]

I think this is the simplest set of features needed:
tokio = { version = "0.2", features = ["macros", "tcp", "dns", "io-util"] }

[carllerche]

Off the top of my head I believe tokio::main required both macros and rt-core. The idea is that main starts a runtime so it needs both. In theory, we may add macros that do not require rt-core so we did not want to couple the macros feature flag with rt-core.

[carllerche]

I will try to write a proposal for macros / runtime for 0.3 as there is a lot of weirdness that needs to be smoothed out (this was our first attempt)

[jebrosen]

tokio::main in particular is... well, weird: https://docs.rs/tokio/0.2.8/src/tokio/lib.rs.html#266

It definitely needs macros, it looks like it needs rt-core, and which macro is used depends on whether or not rt-threaded is used. Somewhere in the layers of macros it gets documented as only needing rt-threaded.

[icewind1991]

tokio::main requiring the rt-core feature seems to be a breaking change in 0.2.7, with older versions only macros was needed

[carllerche]

That is unfortunate. I will look into that.

[dekellum]

To confirm, the current docs for these macros only show the need for the _rt-threaded_ feature, which is clearly wrong:

https://docs.rs/tokio/0.2.11/tokio/index.html#attributes

The current approach to using nested macros to apply doc(cfg(feature = …))) attributes, seems to have limitations:

• Only one set of features is chosen at cargo doc time (--all-features for docs.rs), so I don't see how the macro approach could fully express the requirements (e.g. feature1 _or_ feature2) in cases like this?

• AFAICT, the macros fail to apply all nested required features in doc(cfg), in complex cases such as this.

[dekellum]

2225 was merged. Can this be closed as well, or is there something remaining?