error[E0259]: the name `derive_a` is defined multiple times
  --> $DIR/shadow.rs:6:1
   |
LL | extern crate derive_a;
   | ---------------------- previous import of the extern crate `derive_a` here
LL | #[macro_use]
LL | extern crate derive_a;
   | ^^^^^^^^^^^^^^^^^^^^^^ `derive_a` reimported here
   |
   = note: `derive_a` must be defined only once in the type namespace of this module

error: aborting due to previous error

For more information about this error, try `rustc --explain E0259`.