error[E0432]: unresolved import `my_core`
  --> $DIR/extern-prelude-from-opaque-fail.rs:20:9
   |
LL |     use my_core;
   |         ^^^^^^^
   |         |
   |         no `my_core` in the root
   |         help: a similar name exists in the module: `my_core`

error[E0432]: unresolved import `my_core`
  --> $DIR/extern-prelude-from-opaque-fail.rs:7:13
   |
LL |         use my_core;
   |             ^^^^^^^ no `my_core` in the root
...
LL | a!();
   | ---- in this macro invocation
   |
   = note: this error originates in the macro `a` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: use of undeclared crate or module `my_core`
  --> $DIR/extern-prelude-from-opaque-fail.rs:11:18
   |
LL |         fn f() { my_core::mem::drop(0); }
   |                  ^^^^^^^ use of undeclared crate or module `my_core`
...
LL | a!();
   | ---- in this macro invocation
   |
   = note: this error originates in the macro `a` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: use of undeclared crate or module `my_core`
  --> $DIR/extern-prelude-from-opaque-fail.rs:24:14
   |
LL |     fn f() { my_core::mem::drop(0); }
   |              ^^^^^^^ use of undeclared crate or module `my_core`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.