Rust: extraction succeeds with missing proc-macro output when the project's `rust-version` exceeds the forced toolchain

Description of the issue

Since the Rust extractor began forcing FIXED_RUST_TOOLCHAIN (d9417a34, first shipped in CodeQL 2.27.1), a project whose Cargo.toml declares a rust-version newer than that toolchain loses its build-script outputs and procedural-macro expansions during extraction.

get_extra_env() in rust/extractor/src/config.rs sets RUSTUP_TOOLCHAIN to the fixed toolchain after merging cargo_extra_env, so the build-script cargo check that rust-analyzer runs uses that toolchain. Cargo refuses the package because its declared rust-version is newer than the active compiler. load_workspace_at in ra_ap_load_cargo logs the error at debug level and continues loading the workspace.

The analysis reports success. Macro-expansion warnings appear in the extraction output, but the underlying Cargo refusal is hidden by the default logging filter, which does not include rust-analyzer's targets.

Because the fixed toolchain trails the latest stable release, a project can be affected whenever its declared minimum exceeds the extractor's selected compiler. Cargo's documentation describes tracking the latest Rust version as one valid rust-version policy, and projects following it will be affected after a stable release until the extractor's toolchain catches up. The open rust-analyzer updates (#22741, #22776) move the fixed toolchain to 1.99.0, which would cover this project's current declaration but not the next stable release.

Evidence

Environment: CodeQL CLI 2.27.1 (FIXED_RUST_TOOLCHAIN = "1.97.0"), github/codeql-action v4.38.2, build-mode: none, ubuntu-latest. The project pins 1.99.0 in rust-toolchain.toml and declares rust-version = "1.99.0".

With RUST_LOG enabling ra_ap_load_cargo=debug, rust-analyzer logs the refusal:

Errors occurred while running build scripts for .../backend/Cargo.toml: error: rustc 1.97.0 is not supported by the following packages:
  [email protected] requires rustc 1.99.0

To confirm the cause, two runs were made on the same revision, differing only in whether CODEQL_EXTRACTOR_RUST_OPTION_CARGO_EXTRA_ARGS=--ignore-rust-version was set. Each queried the finished database for first-party macro expansions:

Job logs, including the database query output:

• Without the flag: https://github.com/unkos-dev/reverie/actions/runs/37893162409/job/113698447396
• With the flag: https://github.com/unkos-dev/reverie/actions/runs/37894117181/job/113701453702

The flag only skips Cargo's version check. It would not help if a build script or procedural-macro dependency genuinely required a newer compiler than the forced toolchain, and cargo_extra_args is not part of the published option schema in rust/codeql-extractor.yml.

Related: #19982 (macro expansion warnings), #22493 (pinning the toolchain).