Improve compiler help for missing module error (E0583)

[Zerotask]

I created a fresh project with cargo init, a new empty folder user

and my main.rs looks as following:

mod user;

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

If I execute cargo run, the compiler gives me the help:

Since I created a folder user the compiler could also suggest adding a src\user\mod.rs.

My suggested help for this case would be:

= help: to create the module `user`, create file "src\user.rs" or "src\user\mod.rs"

The compiler could always show this help, not only if there is a folder with that name.

[fee1-dead]

@rustbot claim