error[E0463]: can't find crate for `core`
  --> $DIR/missing-std.rs:6:1
   |
LL | extern crate core;
   | ^^^^^^^^^^^^^^^^^^ can't find crate
   |
   = note: the `x86_64-unknown-uefi` target may not be installed
   = help: consider downloading the target with `rustup target add x86_64-unknown-uefi`
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0463`.