cargo: Suggest splitting a crate to lib.rs and main.rs if an integration test can't find the crate because it's a binary

Describe the problem you are trying to solve It might be a very frustrating experience for a newbie to read about integration tests, placing tests under tests/integration.rs, trying to import the main crate, only to find a cryptic message that the “crate is not found”.

Here’s an example:

error[E0463]: can't find crate for `example_binary`
 --> tests/integration.rs:1:1
  |
1 | extern crate example_binary;
  | ^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

Describe the solution you’d like In the spirit of Rust’s helpful error messages, it would be good to have something in the vein of “help: actually, the crate example_binary exists, but it’s a binary crate, so it can’t be imported. If you want to test its functionality by calling it from Rust code, try to separate the APIs you want to test into a separate library, (a link to documentation that explains main.rs and lib.rs), with the relevant functions and types exported with pub.”

Notes Of course, even more desirable thing here would be if it Simply Worked. For example, I kind of expected that you could call functions with pub even from a binary main.rs.