`cargo init`'s boilerplate doesn't work with `?`

[jsha]

cargo init currently produces boilerplate that looks like this:

fn main() {
    println!("Hello, world!");
}

However, if someone starts from this boilerplate and pastes in examples from documentation that use ?, they'll get an error. main doesn't return Result, so ? can't be used. The API guidelines recommend that examples use ? rather than unwrap.

As an example, if someone tries to paste in this small File example:

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

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

They get some rather confusing errors (what's FromResidual?):

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:5:43
    |
4   | / fn main() {
5   | |     let mut file = File::create("foo.txt")?;
    | |                                           ^ cannot use the `?` operator in a function that returns `()`
6   | |     file.write_all(b"Hello, world!")?;
7   | | }
    | |_- this function should return `Result` or `Option` to accept `?`
    |
    = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
note: required by `from_residual`
   --> /home/jsha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/try_trait.rs:339:5
    |
339 |     fn from_residual(residual: R) -> Self;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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:6:37
    |
4   | / fn main() {
5   | |     let mut file = File::create("foo.txt")?;
6   | |     file.write_all(b"Hello, world!")?;
    | |                                     ^ cannot use the `?` operator in a function that returns `()`
7   | | }
    | |_- this function should return `Result` or `Option` to accept `?`
    |
    = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
note: required by `from_residual`
   --> /home/jsha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/try_trait.rs:339:5
    |
339 |     fn from_residual(residual: R) -> Self;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0277`.

This is a problem I remember running into when I was first learning the language. I suspect it may be an issue for other language learners too.

I propose changing the boilerplate to:

use std::error::Error;

fn main() -> Result<(), Box<dyn Error>> {
    println!("Hello, world!");
    Ok(())
}

This compiles cleanly and passes clippy.

[jfrimmel]

I have mixed feelings above this. Personally I use plain fn main() more often than fn main() -> Result<_, _>. Therefore the current cargo template suits my style more than the proposed code. But I'm fine both ways.

But: I think we should improve the current diagnostics to output something like that:

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:6:37
    |
4   | / fn main() {
5   | |     let mut file = File::create("foo.txt")?;
6   | |     file.write_all(b"Hello, world!")?;
    | |                                     ^ cannot use the `?` operator in a function that returns `()`
7   | | }
    | |_- help: add a fallible return type: `fn main() -> Result<(), Box<dyn std::error::Error>`

[camelid]

I also rarely use fn main() -> Result<_, _>. Personally, I think it'd be preferable to just improve the error rather than changing the output of cargo new. I appreciate that the current output of cargo new is quite simple and, at least for binaries, doesn't add extra boilerplate that I don't need.

[jsha]

My experience as a beginning Rust dev was that I often needed to cut-and-paste examples from documents, and I was often frustrated by them not working until I remembered to add a -> Result to my main. I also found that the error messages at the time led me down a path of writing Results with a concrete error type rather than a dyn Error. Looking through my juvenilia I see:

fn main() -> io::Result<()> {
fn main() -> Result<(), Box<dyn error::Error>> {
fn main() -> Result<(), Box<dyn std::error::Error>> {
fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
fn main() -> Result<(), String> {

I think it's really useful for new users if cargo new offers a simple solution that "just works" in almost all cases. Or to put it another way: as experienced devs, we know we can remove the-> Result boilerplate if we don't want it; or we can just write our own boilerplate, since it's so short. The time spent by experienced devs deleting an unwanted -> Result is outweighed by the frustration experienced by new devs when their copy-pasted code doesn't compile. Learning Rust can be a frustrating experience, and if there's a common error that almost all beginners are likely to hit, I think it's really valuable to remove that error from their experience entirely.

[camelid]

I didn't hit this as a beginner Rust programmer. I would manually handle my errors and then call process::exit if there was an error. I also don't think I copied and pasted code that would have caused this error.

So maybe many other people did hit this when they were beginners, but I myself don't have the perspective of hitting this as a beginner.

[camelid]

Also, Box<dyn Error> can add its own confusion in my experience since I sometimes have to cast manually with Box::from or similar to fix inference errors, which even as an experienced user I find confusing.

[weihanglo]

This should be moved to rust-lang/cargo, no?

Anyway, Cargo has this semi-related issue about cargo-template #5151.

[Enselic]

Triage: Yep, this should be moved to cargo. Can someone with sufficient privileges do that please?