error[E0259]: the name `empty` is defined multiple times
  --> $DIR/multiple-extern-by-macro-for-custom.rs:10:9
   |
LL | extern crate empty;
   | ------------------- previous import of the extern crate `empty` here
...
LL |         extern crate std as empty;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ `empty` reimported here
...
LL | m!();
   | ---- in this macro invocation
   |
   = note: `empty` must be defined only once in the type namespace of this module
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
help: you can use `as` to change the binding name of the import
   |
LL |         extern crate std as other_empty;
   |

error: aborting due to 1 previous error

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