RUSTC_BOOTSTRAP Environment Variable for Testing

src/tests/misc.md

6d5e8d82af6466362277f796956e15f3aabac0c8815cad9d00000003000004c1

# 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();
}
```

Chunks

0d5ff111 (1st chunk of `src/tests/misc.md`)

Title: RUSTC_BOOTSTRAP Environment Variable for Testing

Summary

The `RUSTC_BOOTSTRAP` environment variable is useful for testing the Rust compiler. Setting it to `1` bypasses stability checks, allowing the use of unstable features on a stable compiler. Setting it to `-1` forces a nightly compiler to behave like a stable one. This can be configured in `ui` tests using `//@ rustc-env:RUSTC_BOOTSTRAP=<value>` or in `run-make` tests via the `rustc().env("RUSTC_BOOTSTRAP", "<value>")` method.