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Thanks for a genuinely excellent PR, @WizzardSK — the write-up, the upstream-gap issue (libretro/libretro-super#2030), and the fact that you read the #91899 post-mortem before touching the toolchain all show. Adopted.

Verified the job against the actual upstream template (ci-templates/rust-linux-x64.yml), and your reasoning holds point-for-point:

• Extending .libretro-rust-linux-x64 (the base) rather than -default is exactly right: the base sets no TARGET_ARCH; only -default pins it to x86_64-unknown-linux-gnu. The template builds cargo build --release --target ${TARGET_ARCH}, so setting TARGET_ARCH: aarch64-unknown-linux-gnu here is what redirects it.
• STRIP is used by the template as $STRIP --strip-unneeded ${CORENAME}_libretro.so, default host strip — so overriding it to aarch64-linux-gnu-strip is necessary and correct.
• The base defines no before_script, so yours is purely additive (no extends-array clobber), and rustup target add ${RUST_TARGET} matches what .rust-target-install provides — the #91899 pin trap is handled identically. mv target/${TARGET_ARCH}/release/librustynes.so → rustynes_libretro.so resolves correctly with CORENAME=rustynes.

Closed the gap you flagged ("built natively, couldn't test the cross-compile from x86_64"): cargo check --release -p rustynes-libretro --target aarch64-unknown-linux-gnu cross from x86_64 is clean across the whole graph (cpu/ppu/apu/mappers/core/rust-libretro, bindgen included). The buildbot remains the authority on the link/package half.

Companion PR #334 (now merged) adds aarch64-unknown-linux-gnu to this repo's libretro-cross early-warning gate, so the cross-compile your job ships is regression-tested on every GitHub PR from now on — not buildbot-only.

One correction to my earlier note, plus a finding worth passing back to you — both from getting #334 green. I'd first assumed the gate needed no header provisioning (a local cargo check passed clean). On the actual Ubuntu runner it failed at bindgen: /usr/include/stdint.h: 'bits/libc-header-start.h' file not found — the Debian-multiarch split, where glibc's stdint.h pulls in an arch-specific bits/ header a stock x86_64 runner ships only for x86_64. (My local check passed only because an Arch host's glibc headers aren't arch-split.) The fix in #334: install gcc-aarch64-linux-gnu for its libc6-dev-arm64-cross headers under /usr/aarch64-linux-gnu/include, and point bindgen there via --sysroot + -isystem. Confirmed green on the runner afterward.

The relevant bit for your job: that apt-get install gcc-aarch64-linux-gnu you already added does double duty — it's not only the link-time cross linker/STRIP provider, it's also what lands the aarch64 glibc headers bindgen needs to parse libretro.h. So the ingredients are present on the buildbot. The one open question I couldn't settle from here is whether libretro's build image finds those headers automatically the way the buildbot expects, or needs the same explicit hint the GitHub runner did. If the buildbot's aarch64 job ever errors at bindgen with that bits/libc-header-start.h message, the one-line fix is to add to the job:

    BINDGEN_EXTRA_CLANG_ARGS_aarch64_unknown_linux_gnu: "-isystem /usr/aarch64-linux-gnu/include"

(additive — harmless if the image already resolves them). Flagging it so it's a known quantity rather than a surprise; this repo treats the buildbot as observe-post-merge-and-fix-fast, so we'll watch that leg.

On the aside: the rust-libretro 0.3.2 retro_set_environment-before-retro_init panic is upstream and matches what we already track about that (unmaintained) release; RetroArch's call order sidesteps it. Good to have it written down.

Merging now — thanks again for a model contribution.