Rust: remove pre-1.94 format macro fallback

[paldepind]

Context

1. When we began the update to rust-analyzer 0.0.347 we noted an incompatibility with pre-1.94 Rust toolchain versions.
2. The commit 7faf81f put a workaround in place for this incompatibility: we we manually expand certain macros.
3. With Rust: Update rust-analyzer to 0.0.347 and pin toolchain #22493 we moved in a different direction, where we always do extraction with a 1.97.0 Rust toolchain.
4. The < 1.94 is now dead code.

This PR

The PR essentially just reverts 7faf81f, but it's not an exact revert, since some later commits tweaked things a bit.

Spelled out the PR:

• Deletes the synthetic reconstruction of format-family macro expansions
• Remove the dedicated pre-1.94 QL fixture and toolchain installation

[Copilot]

Copilot review overview

🟢 Approval recommended

The cleanup is internally consistent, preserves required environment handling, and includes appropriate validation.

Review tier: Balanced
Findings: None

What changed in this PR

Removes obsolete Rust toolchain overrides now that toolchain pinning is managed independently.

Changes:

• Removes fixed-toolchain selection, logging, and preinstallation.
• Uses standard cargo while preserving configured environment variables.
• Removes the direct ra_ap_toolchain dependency and regenerates Bazel metadata.

File	Description
rust/​ql/​test/​setup.sh	Stops preinstalling Rust 1.97.
rust/​ql/​integration-tests/​conftest.py	Removes integration-test toolchain installation.
rust/​extractor/​src/​toolchain.rs	Deletes fixed-toolchain logic.
rust/​extractor/​src/​qltest.rs	Runs standard cargo with configured environment.
rust/​extractor/​src/​main.rs	Removes toolchain logging and module registration.
rust/​extractor/​src/​config.rs	Stops overriding RUSTUP_TOOLCHAIN.
rust/​extractor/​Cargo.toml	Removes the direct toolchain dependency.
MODULE.bazel	Removes the obsolete direct Bazel repository import.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​ra_ap_toolchain/​BUILD.bazel	Deletes obsolete generated alias.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​ra_ap_toolchain-0.0.347/​BUILD.bazel	Deletes versioned generated alias.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​crates.bzl	Regenerates direct dependency metadata.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​BUILD.bazel	Removes top-level toolchain aliases.
Cargo.lock	Removes the extractor’s direct dependency entry.

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[hvitved]

The DCA run was lacking -X use-database-cache=false -X change-ql-submodule-in-semmle-code=true, so I have started a new run. Otherwise LGTM.

[paldepind]

Thanks for starting DCA correctly. There are some extraction errors. I'll investigate.

[paldepind]

Extraction errors are for DCA projects on an old nightly toolchain. For these projects the format-macro fallback is in fact not dead code.

I'm putting back in draft to re-consider. We could go ahead as-is, but it might be better just to implement #22493 (comment) to avoid noise in QA.

[paldepind]

The last DCA is much better.

• The problems on projects that use an old nightly toolchain are gone.
• One project, mist-os, still shows a significant increase in extraction errors. However, the relative change is not that great. I think there are some unrelated extraction issues on that project, and the manual macro expansion patched those over for the format! related macros. If we care about that we should probably solve the root issue, but I think the slight increase in extraction errors is perfectly fine for now.

[hvitved]

🎉