Mention `RUSTC_BOOTSTRAP` for misc testing (#2136)

src/SUMMARY.md

@@ -32,6 +32,7 @@
         - [Rust for Linux](./tests/rust-for-linux.md)
     - [Performance testing](./tests/perf.md)
     - [Suggest tests tool](./tests/suggest-tests.md)
+    - [Misc info](./tests/misc.md)
 - [Debugging the compiler](./compiler-debugging.md)
     - [Using the tracing/logging instrumentation](./tracing.md)
 - [Profiling the compiler](./profiling.md)

src/tests/intro.md

@@ -155,6 +155,10 @@ chapter](ecosystem.md) for more details.
 A separate infrastructure is used for testing and tracking performance of the
 compiler. See the [Performance testing chapter](perf.md) for more details.
 
+## Miscellaneous information
+
+There are some other useful testing-related info at [Misc info](misc.md).
+
 ## Further reading
 
 The following blog posts may also be of interest:

src/tests/misc.md

@@ -0,0 +1,40 @@
+# Miscellaneous testing-related info
+
+## `RUSTC_BOOTSTRAP` and stability
+
+<!-- date-check: Nov 2024 -->
+
+This is a bootstrap/compiler implementation detail, but it can also be useful
+for testing:
+
+- `RUSTC_BOOTSTRAP=1` will "cheat" and bypass usual stability checking, allowing
+  you to use unstable features and cli flags on a stable `rustc`.
+- `RUSTC_BOOTSTRAP=-1` will force a given `rustc` to pretend that is a stable
+  compiler, even if it's actually a nightly `rustc`. This is useful because some
+  behaviors of the compiler (e.g. diagnostics) can differ depending on whether
+  the compiler is nightly or not.
+
+In `ui` tests and other test suites that support `//@ rustc-env`, you can specify
+
+```rust,ignore
+// Force unstable features to be usable on stable rustc
+//@ rustc-env:RUSTC_BOOTSTRAP=1
+
+// Or force nightly rustc to pretend it is a stable rustc
+//@ rustc-env:RUSTC_BOOTSTRAP=-1
+```
+
+For `run-make` tests, `//@ rustc-env` is not supported. You can do something
+like the following for individual `rustc` invocations.
+
+```rust,ignore
+use run_make_support::rustc;
+
+fn main() {
+    rustc()
+        // Pretend that I am very stable
+        .env("RUSTC_BOOTSTRAP", "-1")
+        //...
+        .run();
+}
+```