From 895adea2f6c6213d32c916db934f2069f1c35f34 Mon Sep 17 00:00:00 2001
From: Kris Williams <115474+kriswill@users.noreply.github.com>
Date: Tue, 30 Jun 2026 08:34:24 -0700
Subject: [PATCH] feat(build): add a macOS cross-architecture dev shell

Add a `cross` flake dev shell that supplies the target-arch zlib +
libgit2 (fetched as prebuilt substitutes), so `nix develop .#cross` +
`scripts/build.sh --arch x86_64` links a working x86_64 binary on an
arm64 host (and vice versa). The native clang cross-emits object code
via -arch (see ARCHFLAGS in scripts/env.sh); this shell only provides
the target-arch dependency libraries the link needs. Running the
produced binary on Apple Silicon still requires Rosetta.

Depends on the env.sh / Makefile.cbm arch changes (the build fix for
this issue). Refs #705.

Signed-off-by: Kris Williams <115474+kriswill@users.noreply.github.com>
---
 flake.nix | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/flake.nix b/flake.nix
index e3b7d76dd..92c16c90a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -7,6 +7,52 @@
     let
       systems = [ "aarch64-darwin" "x86_64-darwin" "aarch64-linux" "x86_64-linux" ];
       forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+
+      # Cross dev shell (macOS only): build the *other* darwin arch locally.
+      # The native clang already cross-EMITS object code via -arch (scripts/env.sh
+      # exports ARCHFLAGS); this shell only puts the target-arch zlib (a hard
+      # `-lz` dependency) and libgit2 (optional, found via pkg-config) on the
+      # link path, since the host-arch copies can't satisfy an x86_64/arm64 link.
+      # Building needs no Rosetta; *running* an x86_64 binary on Apple Silicon
+      # does. The target-arch libs are fetched as prebuilt substitutes. Returns
+      # {} on Linux/Windows (no cross target).
+      crossDevShells = pkgs:
+        let
+          inherit (nixpkgs) lib;
+          crossTarget =
+            {
+              "aarch64-darwin" = "x86_64-darwin";
+              "x86_64-darwin" = "aarch64-darwin";
+            }
+            .${pkgs.system} or null;
+          mkCrossShell =
+            targetSystem:
+            let
+              tpkgs = nixpkgs.legacyPackages.${targetSystem};
+              targetArch = if targetSystem == "x86_64-darwin" then "x86_64" else "arm64";
+            in
+            pkgs.mkShell {
+              # Host toolchain only (clang comes from the shell stdenv); do NOT
+              # pull host zlib/libgit2 in — that's the wrong-arch copy we avoid.
+              nativeBuildInputs = [ pkgs.pkg-config pkgs.gnumake ];
+              shellHook = ''
+                # libgit2 headers + libs come through pkg-config (the Makefile
+                # auto-detects it). zlib is a bare `-lz`/`#include <zlib.h>`, so
+                # supply its include (header is arch-independent) and lib dir by
+                # hand. NIX_LDFLAGS is searched before the link's own -L, so the
+                # target-arch slice resolves; the host-arch copy (if any reaches
+                # ld) is skipped as "wrong architecture".
+                export PKG_CONFIG_PATH="${lib.getDev tpkgs.libgit2}/lib/pkgconfig:${lib.getDev tpkgs.zlib}/lib/pkgconfig''${PKG_CONFIG_PATH:+:$PKG_CONFIG_PATH}"
+                export NIX_CFLAGS_COMPILE="-I${lib.getDev tpkgs.zlib}/include ''${NIX_CFLAGS_COMPILE:-}"
+                export NIX_LDFLAGS="-L${lib.getLib tpkgs.zlib}/lib -L${lib.getLib tpkgs.libgit2}/lib ''${NIX_LDFLAGS:-}"
+                echo "[cross] target ${targetSystem} — build with: scripts/build.sh --arch ${targetArch}"
+              '';
+            };
+        in
+        lib.optionalAttrs (crossTarget != null) {
+          # `nix develop .#cross` then `scripts/build.sh --arch <target>`.
+          cross = mkCrossShell crossTarget;
+        };
     in
     {
       packages = forAllSystems (pkgs: {
@@ -49,6 +95,6 @@
           # otherwise the build falls back to shelling out to `git log`.
           packages = [ pkgs.pkg-config pkgs.libgit2 ];
         };
-      });
+      } // crossDevShells pkgs);
     };
 }