Describe minicore test auxiliary and directive

src/SUMMARY.md

@@ -25,6 +25,7 @@
     - [Compiletest](./tests/compiletest.md)
         - [UI tests](./tests/ui.md)
         - [Test directives](./tests/directives.md)
+        - [Minicore](./tests/minicore.md)
     - [Ecosystem testing](./tests/ecosystem.md)
         - [Crater](./tests/crater.md)
         - [Fuchsia](./tests/fuchsia.md)

src/tests/compiletest.md

@@ -283,6 +283,9 @@ more information.
 
 See also the [assembly tests](#assembly-tests) for a similar set of tests.
 
+If you need to work with `#![no_std]` cross-compiling tests, consult the
+[`minicore` test auxiliary](./minicore.md) chapter.
+
 [`tests/codegen`]: https://github.com/rust-lang/rust/tree/master/tests/codegen
 [FileCheck]: https://llvm.org/docs/CommandGuide/FileCheck.html
 
@@ -303,6 +306,9 @@ information.
 
 See also the [codegen tests](#codegen-tests) for a similar set of tests.
 
+If you need to work with `#![no_std]` cross-compiling tests, consult the
+[`minicore` test auxiliary](./minicore.md) chapter.
+
 [`tests/assembly`]: https://github.com/rust-lang/rust/tree/master/tests/assembly
 
 

src/tests/minicore.md

@@ -0,0 +1,45 @@
+# `minicore` test auxiliary
+
+[`tests/auxiliary/minicore.rs`][`minicore`] 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 `//@ use-minicore` 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, `//@ use-minicore`
+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. However, 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`
+
+```rust,no_run
+//@ use-minicore
+//@ revisions: meow
+//@[meow] compile-flags: --target=x86_64-unknown-linux-gnu
+//@[meow] needs-llvm-components: x86
+
+#![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
+#[no_mangle]
+fn meow() {}
+```
+
+[minicore]: https://github.com/rust-lang/rust/tree/master/tests/auxiliary/minicore.rs

src/tests/ui.md

@@ -13,6 +13,9 @@ used for many other purposes. For example, tests can also be configured to [run
 the resulting program](#controlling-passfail-expectations) to verify its
 behavior.
 
+If you need to work with `#![no_std]` cross-compiling tests, consult the
+[`minicore` test auxiliary](./minicore.md) chapter.
+
 [`tests/ui`]: https://github.com/rust-lang/rust/blob/master/tests/ui
 
 ## General structure of a test