Miscellaneous testing-related info

RUSTC_BOOTSTRAP and stability

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

// 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.

use run_make_support::rustc;

fn main() {
    rustc()
        // Pretend that I am very stable
        .env("RUSTC_BOOTSTRAP", "-1")
        //...
        .run();
}