Suggest changing mains return type on E0277 (? / from_residual)

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

@rustbot

@rustbot claim

For E0277 suggest adding `Result` return type for function which usin…

Rollup merge of rust-lang#126187 - surechen:fix_125997, r=oli-obk

Unrolled build for rust-lang#126187

Suggest adding Result return type for associated method in E0277.

Rollup merge of rust-lang#128084 - surechen:fix_125997_v1, r=cjgillot

Unrolled build for rust-lang#128084

I believe #126187 addressed my primary concern (fn main) and #128084 went further than that. I'm fine with this being closed.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b681e44892295838385e98410ed31197

   Compiling playground v0.0.1 (/playground)
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/main.rs:2:53
  |
1 | fn main() {
  | --------- this function should return `Result` or `Option` to accept `?`
2 |     let mut _file = std::fs::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 `()`
help: consider adding return type
  |
1 ~ fn main() -> Result<(), Box<dyn std::error::Error>> {
2 |     let mut _file = std::fs::File::create("foo.txt")?;
3 +     Ok(())
  |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

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Suggest changing `main`s return type on E0277 (`?` / `from_residual`) #125997

Code

Current output

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Rationale and extra context

Other cases

Rust Version

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Suggest changing mains return type on E0277 (? / from_residual) #125997

Description

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

Activity

surechen commented on Jun 7, 2024

epage commented on Apr 29, 2025

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Suggest changing `main`s return type on E0277 (`?` / `from_residual`) #125997