rustc-dev-guide: document that //@ add-core-stubs imply -Cforce-unwind-tables=yes

Author: jieyouxu

src/doc/rustc-dev-guide/src/tests/minicore.md

@@ -6,25 +6,37 @@
 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.
 
+<div class="warning">
+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.
+</div>
+
 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.
 
+## Implied compiler flags
+
 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.
+**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-core-stubs` 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.
 
-<div class="warning">
-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.
-</div>
-
 ## Example codegen test that uses `minicore`
 
 ```rust,no_run