Sprinkle breadcrumbs around to lead people to the rust-invalid ABI

Author: workingjubilee

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

@@ -59,6 +59,11 @@ The output is normalized to ignore unwanted differences, see the
 [Normalization](#normalization) section. If the file is missing, then
 compiletest expects the corresponding output to be empty.
 
+A common reason to use normalization, revisions, and most of the other following tools,
+is to account for platform differences. Consider alternatives to these tools, like
+e.g. using the `extern "rust-invalid"` ABI that is invalid on every platform
+instead of fixing the test to use cross-compilation and testing every possibly-invalid ABI.
+
 There can be multiple stdout/stderr files. The general form is:
 
 ```text

tests/ui/SUMMARY.md

@@ -8,6 +8,8 @@ For now, only immediate subdirectories under `tests/ui/` are described, but thes
 
 These tests deal with *Application Binary Interfaces* (ABI), mostly relating to function name mangling (and the `#[no_mangle]` attribute), calling conventions, or compiler flags which affect ABI.
 
+Tests for unsupported ABIs can be made cross-platform by using the `extern "rust-invalid"` ABI, which is considered unsupported on every platform.
+
 ## `tests/ui/allocator`
 
 These tests exercise `#![feature(allocator_api)]` and the `#[global_allocator]` attribute.