Suggest changing `main`s return type on E0277 (`?` / `from_residual`)

[epage]

Code

use std::fs::File;
use std::io::prelude::*;

fn main() {
    let mut file = File::create("foo.txt")?;
    file.write_all(b"Hello, world!")?;
}

Current output

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that
implements `FromResidual`)
 --> cargo-14007.rs:5:43
  |
4 | fn main() {
  | --------- this function should return `Result` or `Option` to accept `?`
5 |     let mut file = File::create("foo.txt")?;
  |                                           ^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that
implements `FromResidual`)
 --> cargo-14007.rs:6:37
  |
4 | fn main() {
  | --------- this function should return `Result` or `Option` to accept `?`
5 |     let mut file = File::create("foo.txt")?;
6 |     file.write_all(b"Hello, world!")?;
  |                                     ^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `cargo-14007` (bin "cargo-14007") due to 2 previous errors

Desired output

To suggest changing

fn main() {
    // ...
}

to

fn main() -> Result<(), Box<dyn Error>> {
    // ...
    Ok(())
}

Rationale and extra context

In rust-lang/cargo#14007, concern was raised over people generating a trivial fn main() {} with cargo new and copy/pasting example code that uses ? (which is encouraged in API guidelines). Regardless of the decision made for cargo new, this applies just as well to users hand writing a trivial fn main() {}. These are likely new users who would be intimidated by the current message and wouldn't know what next steps to take

Other cases

No response

Rust Version

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

Anything else?

No response

[surechen]

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