`pub` in executable modules

What it does

Warn of uses of pub in the top-level module of an executable-only target.

Specifically, pub outside of a mod block in any of these files would trigger the lint:

• src/main.rs
• src/bin/foo.rs
• src/bin/foo/main.rs

Advantage

• pub suppresses "unused" lints, so inadvertently marking something pub may mask that the user forgot to use it.
• pub has no other effect (that I'm aware of) in top-level modules of executable-only targets.

I actually ran into this recently: I work on a project that has several executable targets, and I discovered that two of them were not actually using all of their declared arguments.

Drawbacks

There may be a use for pub inside of an executable-only module that I'm not aware of.

Example

use clap::Parser;

#[derive(Parser, Debug)]
struct Args {
    #[arg(short, long)]
    pub name: String,
}

Could be written as:

use clap::Parser;

#[derive(Parser, Debug)]
struct Args {
    #[arg(short, long)]
    name: String,
}

The issue identifies executable entry points as src/main.rs, src/bin/foo.rs, and src/bin/foo/main.rs; start by checking how these module forms are represented. Define the lint behavior for pub outside mod blocks and verify it against the Args example, including that pub inside a mod is excluded. No test file or implementation entry point is named, so locate the relevant Clippy lint tests before confirming warning and non-warning cases.