error: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
  --> $DIR/issue-109148.rs:6:9
   |
LL |         extern crate core as std;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
...
LL | m!();
   | ---- in this macro invocation
   |
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error