feat: add Rust (rust2cpg) frontend support

[Lekssays]

Summary

Joern 4.0.581 ships rust2cpg. This wires Rust into codebadger's language registry and satisfies the two frontend-specific runtime requirements needed for a real (non-empty) Rust CPG. Verified end-to-end: rust2cpg on a minimal crate produces a CPG with the expected user methods (add, greet, main).

Registry wiring (src/defaults.py, src/utils/validators.py, config.example.yaml)

• SUPPORTED_LANGUAGES += rust (both the defaults.py and validators.py copies, and config.example.yaml)
• LANGUAGE_COMMANDS: rust → /opt/joern/joern-cli/rust2cpg
• FRONTEND_CAPABILITIES: rust2cpg → {exclude_regex} (its --help exposes only --exclude/--exclude-regex)
• LANGUAGE_EXTENSIONS / SCOPE_SOURCE_EXTENSIONS: rust → rs
• Snippet language-inference signals for Rust (fn, let mut, macros, use std::, impl, #[...], ::)

Image changes (Dockerfile)

Both were required to get a non-empty CPG, and both are verified in the rebuilt image:

1. Base image eclipse-temurin:21-jdk-jammy → :21-jdk-noble. rust2cpg's native astgen (rust_ast_gen-linux) is linked against GLIBC_2.39; jammy (Ubuntu 22.04) ships glibc 2.35, so it fails with version `GLIBC_2.39' not found and silently yields an empty CPG. noble (24.04) ships glibc 2.39. glibc is backward-compatible, so every other frontend's native astgen keeps working.
2. Install the Rust toolchain (rustup, minimal profile → rustc + cargo). rust2cpg's astgen loads the crate by shelling out to cargo/rustc and errors (Are cargo and rustc on your PATH?) without them. docker exec inherits the image ENV PATH, so the frontend finds the toolchain at build time.

Notable decision: Rust is omitted from languages_with_exclusions

Exactly like Go already is. rust2cpg matches --exclude-regex against cargo-resolved paths, and the default exclusion_patterns were observed to collapse a Rust CPG to 0 methods. Rust stays in supported_languages and builds fine — just without exclusion filtering.

ABAP (abap2cpg) — evaluated, left out

abap2cpg is the other frontend new to 4.0.581. Its native abapgen expects a specific serialized ABAP format and rejected ordinary hand-written ABAP (No ABAP files were parsed), so support couldn't be verified. Happy to add it in a follow-up given real ABAP sources to test against.

Verification

• New image: glibc 2.39, cargo 1.97.0, rustc 1.97.0, JOERN_VERSION=4.0.581.
• rust2cpg on a demo crate (loaded into Joern) → <global>, add, greet, main.
• tests/test_validators.py (123) and tests/test_cpg_generator.py + tests/test_build_scoping.py (29) pass. (The unrelated pre-existing RichHandler collection errors in other test modules are an environment/rich-version issue, not from this change.)

Note: config.yaml (gitignored runtime config) was updated locally the same way; only tracked files are in this PR.

🤖 Generated with Claude Code