When `#![no_main]` is set, `#[start]` is silently ignored

[RalfJung]

This came up in rust-lang/miri#3498: a user wrote code roughly like this

#![no_main]
#![feature(start)]

#[start]
fn main() {}

and then was surprised that the resulting program didn't have a start function.

The start attribute seems to be entirely ignored, because usually there would be an error that the signature is wrong.

The combination of no_main and the start attribute should probably emit a suitable diagnostic, instead of silently ignoring the attribute.

[Noratrieb]

The #[start] attribute should be deleted from the compiler with the tracking issue closed, its existence does not make any sense. So I don't think anyone should put effort into trying to fix this. Deleting #[start] has been something I've wanted to do for a bit now, but haven't gotten to it.

[RalfJung]

Ah, I see. In that case Miri probably needs to gain some other mechanism to be able to run no_std binaries. Something like what I sketched here.

[RalfJung]

Cc #29633