`main` function definition not on its own line is not detected

[MonliH]

After entering the following code:

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

and clicking the "Build" button, I get the following message:

No main function was detected, so your code was compiled
but not run. If you’d like to execute your code, please
add a main function.

No code in the main function is run.

After a bit more testing, I've found that if the main function's signature is not on its own line, the playground fails to detect the function:

// Also doesn't detect main
const foo: i32 = 10; fn main(){}

// Also doesn't detect main
use std; fn main(){
}

// Does detect main
use std;
fn main() {}

[sollyucko]

Detection code, AFAICT: https://github.com/integer32llc/rust-playground/blob/master/ui/frontend/selectors/index.ts#L13

[shepmaster]

I don’t really consider this to be a problem. Why would you choose to write nonstandard Rust? You can run rustfmt to fix it. If you don’t, you can use the drop down and choose run manually.

[MonliH]

I came across this because I was playing around with code golfing in rust 😄. I needed to store the code somewhere so other people could run it and see my solution, so that's why.

I know its sort of a niche use case, although fixing small things like this makes for a more polished project. That being said, I can see why this wouldn't be high priority. I might be able to patch this as well.

[sollyucko]

I think removing the ^ at the start that requires the RegExp to match at the start of the line should probably work. The starting \s* (matches optional whitespace) and the ending m (^ and $ can match the start/end of the line instead of the whole string) can optionally be removed.

[shepmaster]

removing the ^ at the start

// fn main() {}

[MonliH]

Hmm, yeah. Seems like there would be a a few more edge cases you need to deal with, if you remove that:

/* fn main() {} */

const FOO: &'static str = r#"fn main() {}
"#;

Maybe look for a main function on the server side instead? i.e. is there a way to detect if a main function exists after compiling it?

[shepmaster]

after compiling it?

The compilation command and even the file name itself changes based on how it's being compiled.

[sollyucko]

I guess you could build it, then analyze the binary to detect the presence of a main function (Run) or a test runner (Test), then determine what to do with it.

Also, multi-line comments can still trick the selector:

/*
fn main() {}
*/

Playground URL: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=5c587547e3f69172f6d75909badb8f78

[shepmaster]

To be clear, any and every regex solution is basically "best effort", thus why I don't consider this a priority. If the code does the right thing for the usual and expected cases (i.e. properly formatted code), that's what's important. If it also does the right thing for less-common cases (e.g. const fn main()) without breaking usual cases and without introducing extremely slow performance, that's ok.

[shepmaster]

And this is one reason why the dropdown exists — for when we get it wrong

[MonliH]

Just to note: that wasn't immediately obvious to me when I ran into this. I didn't even notice that playground changed the mode based on whether it detected a main function or not.

Maybe you could (somehow) make this slightly more clear?

[shepmaster]

Maybe you could (somehow) make this slightly more clear?

How do you suggest?

The text changes from run to build and there’s output added that says no main was detected.

[MonliH]

Not sure how much it will help, but maybe add something to that message, along the lines of:

If you think this was a mistake, change the mode to "Run" and try again.

This could add a bit of noise, though.