flake: fix build with slang feature

[Smephite]

The slang feature (enabled by default) requires cmake and network access for FetchContent dependencies (slang, fmt, mimalloc), both unavailable in the nix sandbox. This breaks the flake build.

To support sandboxed builds without patching, build.rs now accepts optional env vars (SLANG_SRC_DIR, FMT_SRC_DIR, MIMALLOC_SRC_DIR). When set, they are forwarded as FETCHCONTENT_SOURCE_DIR_* cmake variables and used to derive include paths. Normal builds are unaffected — cmake still fetches everything when the vars are unset. This also simplifies the nixpkgs packaging in NixOS/nixpkgs#532944, which currently patches build.rs with replaceVars to achieve the same effect.

The flake is updated to use this mechanism: pre-fetched GitHub sources are passed via the env vars, cmake and python3 are added as build inputs, source filters are extended for C++/test files, NO_COLOR=1 is set for owo-colors test compatibility, and run_all.sh is run as an integration test.

[micprog]

Looks OK to me, maybe @haflex wants to comment on the nix side?

[fischeti]

The changes in build.rs looks good to me. I don't have any experience for flake nix, but I trust you there :)

The question is whether this requires a new release of bender-slang. Does nix pull from crates.io? or does it just build from source?

[Smephite]

Thanks guys!

The question is whether this requires a new release of bender-slang. Does nix pull from crates.io? or does it just build from source?

No release needed. bender-slang is a path dependency in Cargo.toml, so nix builds it directly from the source tree. It doesn't pull individual crates from crates.io as pre-built binaries; all dependencies are fetched as source via Cargo.lock and compiled from scratch inside the nix sandbox.