Still clippy warning after macros: fix wrong error messages #4067

[ChristianBeilschmidt]

Version
1.13.0

Platform

• Rust Playground
• Linux 5.11.0-40-generic #44~20.04.2-Ubuntu SMP Tue Oct 26 18:07:44 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Description
In #4067, there was a discussion about a falsely Clippy warning clippy::semicolon_if_nothing_returned in tokio's main and test.
Most problems were solved in #4176.
However, I encountered some leftovers that still pose the same problem.

I could reproduce it in Rust Playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=39638811ab983f5bea782a42d5d184b9

#![warn(clippy::semicolon_if_nothing_returned)]

use tokio; // 1.13.0

async fn foo() -> &'static str {
    "foo"
}

#[tokio::main]
async fn main() {
    let _foo = foo().await;
}

The output of Clippy is

Checking playground v0.0.1 (/playground)
warning: this import is redundant
--> src/main.rs:3:1
|
3 | use tokio; // 1.13.0
| ^^^^^^^^^^ help: remove it entirely
|
= note: #[warn(clippy::single_component_path_imports)] on by default
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_component_path_imports

warning: consider adding a ; to the last statement for consistent formatting
--> src/main.rs:11:5
|
11 | let _foo = foo().await;
| ^^^^^^^^^^^^^^^^^^^^^^^ help: add a ; here: let _foo = foo().await;;
|
note: the lint level is defined here
--> src/main.rs:1:9
|
1 | #![warn(clippy::semicolon_if_nothing_returned)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#semicolon_if_nothing_returned

warning: playground (bin "playground") generated 2 warnings
Finished dev [unoptimized + debuginfo] target(s) in 2.09s

My current state of investigation is, that this problem arises when ….async; is the last command in the function that is rewritten by tokio's main or test.

An expansion brings the following output:

#![feature(prelude_import)]
#![warn(clippy::semicolon_if_nothing_returned)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use tokio;
async fn foo() -> &'static str {
    "foo"
}
fn main() {
    let body = async {
        let _foo = foo().await;
    };
    #[allow(clippy::expect_used)]
    tokio::runtime::Builder::new_multi_thread()
        .enable_all()
        .build()
        .expect("Failed building the Runtime")
        .block_on(body)
}

So, the semicolon is missing here. But if ….async; isn't the last expression, then there is a ;.

[taiki-e]

we need to handle syn::Stmt::Local the same as non-return syn::Stmt::Semi.

tokio/tokio-macros/src/entry.rs

Lines 341 to 342 in b1afd95

	let (tail_return, tail_semicolon) = match body.stmts.last() {
	Some(syn::Stmt::Semi(expr, _)) => match expr {