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 it 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.
• RUSTC_BOOTSTRAP=-allow-unstable-compiler-flags=<flags> will force a given rustc to pretend it is a stable compiler, even if it’s actually a nightly rustc. However, the compiler will still accept the passed unstable compiler flags <flags> (the allowed flag should be separated by commas). For example, if you use RUSTC_BOOTSTRAP=-allow-unstable-compiler-flags=foo,bar, it will be possible to pass -Zfoo and -Zbar to the compiler. This is useful for internal testing of the compiler, where we need to pass unstable flags required by compiletest, but we also want to test the compiler’s behavior on the stable channel.

UI tests

Note that setting RUSTC_BOOTSTRAP=-1 in ui tests (e.g. via //@ rustc-env) is pointless since ui tests inherit the bootstrap env var from bootstrap (so it’s set to 1 by default), and since compiletest itself passes -Z flags (so -1 breaks the test).

To test stable-only behavior, use the act-as-stable directive, which will set RUSTC_BOOTSTRAP=-allow-unstable-compiler-flags with the flags required by compiletest for you. With this directive, the compiler will not allow using unstable language features or unstable flags that are not set by compiletest.

Run-make tests

For run-make/run-make-cargo tests, //@ act-as-stable is not supported. You can use the run_make_support::stable_bare_rustc function to create an instance of the compiler that acts as the stable channel (this internally uses RUSTC_BOOTSTRAP=-1).