How to access the main crate when testing a rust project

When creating a project with a test like so:

cargo init --bin projectname
mkdir projectname/tests
echo "extern crate projectname;" > projectname/tests/test.rs
cd projectname/
cargo build

I get this error when testing:

cargo test
   Compiling projectname v0.1.0 (file:///home/username/Lab/projectname)
error[E0463]: can't find crate for `projectname`
 --> tests/test.rs:1:1
  |
1 | extern crate projectname;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: Could not compile `projectname`.
warning: build failed, waiting for other jobs to finish...
error: build failed

So my question is: How can I access functions in ´projectname/src/main.rs´ from projectname/tests/test.rs?