Bad spans from type error within code expanded from proc macro attribute

[euclio]

I tried this code:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=7d0f526db1b816572e927bbaaad2750e

#[tokio::main]
async fn main() {
    Ok(())
}

I expected to see this happen: Type error reported on Ok(())

Instead, this happened: Got this diagnostic:

error[E0308]: mismatched types
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^- help: try adding a semicolon: `;`
  | |
  | expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<(), _>`
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (1836e3b42 2020-05-06)
binary: rustc
commit-hash: 1836e3b42a5b2f37fd79104eedbe8f48a5afdee6
commit-date: 2020-05-06
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 9.0

[emmanueltouzery]

Maybe this should be linked to from #43081

[Aaron1011]

This doesn't appear to be a bug in rustc. The #[tokio::main] attribute produces code like this:

fn main() {
    tokio::runtime::Builder::new_multi_thread()
        .enable_all()
        .build()
        .unwrap()
        .block_on(async {
            {
                Ok(())
            }
        })
}

which gives the error message:

error[E0308]: mismatched types
  --> src/main.rs:2:5
   |
1  |   fn main() {
   |             - expected `()` because of default return type
2  | /     tokio::runtime::Builder::new_multi_thread()
3  | |         .enable_all()
4  | |         .build()
5  | |         .unwrap()
...  |
9  | |             }
10 | |         })
   | |          ^- help: try adding a semicolon: `;`
   | |__________|
   |            expected `()`, found enum `std::result::Result`
   |
   = note: expected unit type `()`
                   found enum `std::result::Result<(), _>`

The error message points to the overall expression, which Tokio spans with Span::call_site(). To improve the diagnostics, Tokio could try to change where the error is emitted (e.g. asserting explicitly asserting the type of the main function), or re-span some of the generated code to point to something user-defined.

[Aaron1011]

Opened https://github.com/tokio-rs/tokio/pull/3039/files

[taiki-e]

This has been fixed in tokio-rs/tokio#3766.

[estebank]

Current output:

error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
2 | async fn main() {
  |                 - expected `()` because of default return type
3 |     Ok(())
  |     ^^^^^^ expected `()`, found `Result<(), _>`
  |
  = note: expected unit type `()`
                  found enum `Result<(), _>`