Add new --test-codegen-backend bootstrap option

[GuillaumeGomez]

This new bootstrap command line flag allows to do:

./x.py test tests/ui/intrinsics/panic-uninitialized-zeroed.rs --stage 1 -j8 --test-codegen-backend gcc

This is the last step before running it into the CI.

Supersedes #144687.

r? @Kobzol

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[GuillaumeGomez]

Sadly I still need to change this env variable to be able to build the compiler with the GCC backend. However, no need anymore for the tests. \o/

[Kobzol]

Why isn't it enough to set rust.codegen-backends = ["gcc"] and then run x test? It should build stage 1 rustc configured to use the GCC backend by default, so when it is used in tests, it should compile everything using the GCC codegen backend.

[GuillaumeGomez]

Because as a first step we want to keep rustc built with LLVM and run tests with the GCC backend to limit the potential impact.

[Kobzol]

If stage 1 tests are enough for you, then using the default codegen backend would be enough for that. We would build rustc using LLVM, then build libstd using GCC, and then run tests using GCC. Or do you think we require stage 2 at this moment?

[GuillaumeGomez]

I have two use cases for this:

1. It's super easy to test another codegen backend without having to edit a config file without (potentially) having to recompile.
2. I'd like to be able to "cross test" rustc built with one backend and using another one to ensure no weird corner cases come up.

[GuillaumeGomez]

Forgot but we're supposed to have a command-line argument as described here.

[Kobzol]

I have two use cases for this:

1. It's super easy to test another codegen backend without having to edit a config file without (potentially) having to recompile.
2. I'd like to be able to "cross test" rustc built with one backend and using another one to ensure no weird corner cases come up.

You can do e.g. x test --set 'rust.codegen-backends=["gcc"]' to run tests with the GCC backend without modifying the config file.

I'm fine with having a flag, but I don't think that what the PR currently does is what you want 🤔It only tells compiletest that you're using a rustc that builds stuff using the GCC backend, but you don't make sure that is indeed what happens. In other words, the flag as currently implemented is a no-op, and the actual functionality that gets you GCC used during tests is setting "gcc" as the first member of "rust.codegen-backends". That's why I was confused about what the flag should do and how does it differ from setting the "gcc" field 😅

I would suggest either:

• Not add a flag for now and just set "gcc" as the first member of rust.codegen-backends. This allows you to use a LLVM-compiled rustc and GCC-compiled stdlib to run (stage 1) tests using the GCC backend.
• Add a flag that will instruct compiletest to append something like -Zcodegen-backend=gcc to the compile flags of every executed test. This allows you to use a LLVM-compiled rustc + LLVM-compiled stdlib and run (even stage 2) tests using the GCC backkend.