Improve fidelity of rustc-crates-on-stable test

[jieyouxu]

Use RUSTC_BOOTSTRAP=-1 to make the underlying rustc think it is a stable rustc, really making sure that these compiler crates can be compiled on the stable channel.

cc @Kobzol who suggested this in #132993 (comment), thanks!

You can tell this works because if I keep .env("RUSTFLAGS", "-Zallow-features="), the underlying rustc thinks it is very stable and thus rejects the unstable -Zallow-features flag.

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `X:\repos\rust\build\x86_64-pc-windows-msvc\stage1\bin\rustc.exe - --crate-name ___ --print=file-names -Zallow-features= --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit code: 1)
  --- stderr
  error: the option `Z` is only accepted on the nightly compiler

r? compiler

[jieyouxu]

Asked cargo team about if cargo has some way of pretending it is a stable cargo in https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Pretend.20to.20be.20a.20stable.20cargo.

[lqd]

While it should be doable for rustc, IIRC cargo features are checked for the workspace and not only the crates you build (you could for example add -Zallow-features='' to reject unstable cargo features if it doesn't support opting into its stable behavior) which would then also prevent other compiler crates from using e.g. edition 2024.

Which is also maybe a reason why stage0 cargo could or could not be used to simulate this stable cargo. But again any behavior here should not impact the crates that r-a doesn't use.

[jieyouxu]

While it should be doable for rustc, IIRC cargo features are checked for the workspace and not only the crates you build (you could for example add -Zallow-features='' to reject unstable cargo features if it doesn't support opting into its stable behavior) which would then also prevent other compiler crates from using e.g. edition 2024.

Which is also maybe a reason why stage0 cargo could or could not be used to simulate this stable cargo. But again any behavior here should not impact the crates that r-a doesn't use.

Good point about the workspace because other compiler crates necessarily require using unstable cargo features, so we can't ask this cargo to pretend that it is a stable cargo. However, what I can do here is:

1. Use -Zallow-features= on a stage1 (or whatever) cargo to ask that no unstable cargo features are allowed for the list of compiler crates.
2. Use RUSTC_BOOTSTRAP=-1 to double-check that the underlying compiler crates can compile on stable.

EDIT: right, cargo -Zallow-features='' propagates -Zallow-features='' to the underlying rustc, which can't work because rustc thinks it's stable 🤣

[jieyouxu]

I thought about this, and I don't think this change is quite correct because it must not affect other compiler crates. Thanks for the reminder @lqd!