error[E0259]: the name `core` is defined multiple times
  --> $DIR/multiple-extern-by-macro-for-buitlin.rs:9:9
   |
LL | extern crate core;
   | ------------------ previous import of the extern crate `core` here
...
LL |         extern crate std as core;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ `core` reimported here
...
LL | m!();
   | ---- in this macro invocation
   |
   = note: `core` 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_core;
   |

error: aborting due to 1 previous error

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