error: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:19:9
   |
LL |         extern crate std as core;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
...
LL | define_other_core!();
   | -------------------- in this macro invocation
   |
   = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0659]: `Vec` is ambiguous
  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:13:9
   |
LL |         Vec::panic!();
   |         ^^^ ambiguous name
   |
   = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
note: `Vec` could refer to the crate imported here
  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:5:9
   |
LL |         extern crate std as Vec;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^
...
LL | define_vec!();
   | ------------- in this macro invocation
note: `Vec` could also refer to a struct from prelude
  --> $SRC_DIR/std/src/prelude/mod.rs:LL:COL
   = note: this error originates in the macro `define_vec` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0659`.