warn on mod lib; in main.rs, etc

[durka]

When a crate imports a module that cargo is also compiling as its own crate root, things can get hairy. The UX can be pretty terrible as in this case:

// src/lib.rs
lazy_static! { static ref FOO: i32 = 42; }

// src/main.rs
#[macro_use] extern crate lazy_static;

mod lib;

fn main() {}

error: cannot find macro `lazy_static!` in this scope
 --> src/lib.rs:1:1
  |
1 | lazy_static! { static ref FOO: i32 = 42; }
  | ^^^^^^^^^^^

OK, the error says I need to import lazy_static into lib.rs. Let's do that:

// src/lib.rs
#[macro_use] extern crate lazy_static;
lazy_static! { static ref FOO: i32 = 42; }

warning: static item is never used: `FOO`
 --> src/lib.rs:2:1
  |
2 | lazy_static! { static ref FOO: i32 = 42; }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default
  = note: this error originates in a macro outside of the current crate

error[E0468]: an `extern crate` loading macros must be at the crate root
 --> src/lib.rs:1:14
  |
1 | #[macro_use] extern crate lazy_static;
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^

WTF! I left the crate out, got an error, I put it in, got an error saying to take it back out. Rust is such a crazy language, OMG!

Of course the second error comes from a totally separate rustc run, but there's next to no indication of that. But it seems like we should be able to detect this situation and tell the user what's going on.

[matklad]

Off-hand, I don't know how we can warn about this: looks like this needs information from both Cargo and the compiler, and as such lies in the realm of IDEs / RLS (We certainly can add this warning to IntelliJ Rust, because we have all the data).

However, I think this is better be fixed somehow: just the other day, the student I am mentoring faced this same issue (they were a novice rust programmer, so they've written mod lib; instead of extern crate miniz_oxide), and it took me like half an hour to understand what's going on here :(

[dwijnand]

Looks like it's unknown how to fix this and might require cross Cargo and compiler changes, so labelling it "E-hard". (I also accidentally closed it.)