`minicore` test auxiliary: using `core` stubs

tests/auxiliary/minicore.rs is a test auxiliary for ui/codegen/assembly test suites. It provides core stubs for tests that need to build for cross-compiled targets but do not need/want to run.

A test can use minicore by specifying the //@ add-core-stubs directive. Then, mark the test with #![feature(no_core)] + #![no_std] + #![no_core]. Due to Edition 2015 extern prelude rules, you will probably need to declare minicore as an extern crate.

Due to the no_std + no_core nature of these tests, //@ add-core-stubs implies and requires that the test will be built with -C panic=abort. Unwinding panics are not supported.

If you find a core item to be missing from the minicore stub, consider adding it to the test auxiliary if it's likely to be used or is already needed by more than one test.

Please note that [`minicore`] is only intended for `core` items, and explicitly **not** `std` or `alloc` items because `core` items are applicable to a wider range of tests.

Example codegen test that uses `minicore`

#![allow(unused)]
fn main() {
//@ add-core-stubs
//@ revisions: meow bark
//@[meow] compile-flags: --target=x86_64-unknown-linux-gnu
//@[meow] needs-llvm-components: x86
//@[bark] compile-flags: --target=wasm32-unknown-unknown
//@[bark] needs-llvm-components: webassembly

#![crate_type = "lib"]
#![feature(no_core)]
#![no_std]
#![no_core]

extern crate minicore;
use minicore::*;

struct Meow;
impl Copy for Meow {} // `Copy` here is provided by `minicore`

// CHECK-LABEL: meow
#[unsafe(no_mangle)]
fn meow() {}
}