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.

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.

A test can use minicore by specifying the //@ add-minicore directive. Then, mark the test with #![feature(no_core)] + #![no_std] + #![no_core], and import the crate into the test with extern crate minicore (edition 2015) or use minicore (edition 2018+).

Implied compiler flags

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

Tests will also be built with -C force-unwind-tables=yes to preserve CFI directives in assembly tests.

TL;DR: //@ add-minicore implies two compiler flags:

  1. -C panic=abort
  2. -C force-unwind-tables=yes

Adding more core stubs

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.

Staying in sync with core

The minicore items must be kept up to date with core. For consistent diagnostic output between using core and minicore, any diagnostic attributes (e.g. on_unimplemented) should be replicated exactly in minicore.

Example codegen test that uses minicore

//@ add-minicore
//@ 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() {}