flake: cross building on arm64 from x86_64, need to cleanup and improve

flake.nix

@@ -21,17 +21,19 @@
       ...
     }:
     flake-utils.lib.eachDefaultSystem (
-      system:
+      localSystem:
       let
+        crossSystem = "aarch64-linux";
         pkgs = import nixpkgs {
-          inherit system;
+          inherit crossSystem localSystem;
           overlays = [ (import rust-overlay) ];
         };
 
         inherit (pkgs) lib;
 
         tailwindStylesheet = import ./crates/plcom/tailwind.nix {
-          pkgs = import nixpkgs { inherit system; };
+          stdenvNoCC = pkgs.stdenvNoCC;
+          tailwindcss = pkgs.tailwindcss;
           src = ./crates/plcom;
           inputFile = "css/main.css";
         };
@@ -52,12 +54,92 @@
           '';
         };
 
-        # in
-        craneLib = crane.mkLib pkgs;
+        craneLib = (crane.mkLib pkgs).overrideToolchain (p: p.rust-bin.stable.latest.default);
 
         # Get metadata from Cargo.toml
         metadata = craneLib.crateNameFromCargoToml { cargoToml = ./crates/plcom/Cargo.toml; };
 
+        # TODO: move to its own module
+        crateExpression =
+          {
+            openssl,
+            libiconv,
+            lib,
+            pkg-config,
+            qemu,
+            stdenv,
+          }:
+          craneLib.buildPackage (
+            metadata
+            // {
+              src = lib.cleanSourceWith {
+                src = craneLib.path ./.; # The original, unfiltered source
+                filter =
+                  path: type:
+                  # Assets for codegen
+                  (lib.hasSuffix ".json" path)
+                  ||
+                    # Default filter from crane (allow .rs files)
+                    (craneLib.filterCargoSources path type);
+              };
+              cargoExtraArgs = "-p plcom";
+              strictDeps = true;
+
+              # Build-time tools which are target agnostic. build = host = target = your-machine.
+              # Emulators should essentially also go `nativeBuildInputs`. But with some packaging issue,
+              # currently it would cause some rebuild.
+              # We put them here just for a workaround.
+              # See: https://github.com/NixOS/nixpkgs/pull/146583
+              depsBuildBuild = [
+                qemu
+              ];
+
+              # Dependencies which need to be build for the current platform
+              # on which we are doing the cross compilation. In this case,
+              # pkg-config needs to run on the build platform so that the build
+              # script can find the location of openssl. Note that we don't
+              # need to specify the rustToolchain here since it was already
+              # overridden above.
+              nativeBuildInputs =
+                [
+                  pkg-config
+                  stdenv.cc
+                ]
+                ++ lib.optionals stdenv.buildPlatform.isDarwin [
+                  libiconv
+                ];
+
+              # Dependencies which need to be built for the platform on which
+              # the binary will run. In this case, we need to compile openssl
+              # so that it can be linked with our executable.
+              buildInputs = [
+                # Add additional build inputs here
+                openssl
+              ];
+
+              # Tell cargo about the linker and an optional emulater. So they can be used in `cargo build`
+              # and `cargo run`.
+              # Environment variables are in format `CARGO_TARGET_<UPPERCASE_UNDERSCORE_RUST_TRIPLE>_LINKER`.
+              # They are also be set in `.cargo/config.toml` instead.
+              # See: https://doc.rust-lang.org/cargo/reference/config.html#target
+              CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = "${stdenv.cc.targetPrefix}cc";
+              CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUNNER = "qemu-aarch64";
+
+              # Tell cargo which target we want to build (so it doesn't default to the build system).
+              # We can either set a cargo flag explicitly with a flag or with an environment variable.
+              # cargoExtraArgs = "--target aarch64-unknown-linux-gnu";
+              CARGO_BUILD_TARGET = "aarch64-unknown-linux-gnu";
+
+              # These environment variables may be necessary if any of your dependencies use a
+              # build-script which invokes the `cc` crate to build some other code. The `cc` crate
+              # should automatically pick up on our target-specific linker above, but this may be
+              # necessary if the build script needs to compile and run some extra code on the build
+              # system.
+              HOST_CC = "${stdenv.cc.nativePrefix}cc";
+              TARGET_CC = "${stdenv.cc.targetPrefix}cc";
+            }
+          );
+
         # Common derivation arguments used for all builds
         commonArgs = {
           src = lib.cleanSourceWith {
@@ -105,13 +187,10 @@
         );
 
         # Build crate
-        plcomBinary = craneLib.buildPackage (
-          commonArgs
-          // {
-            cargoExtraArgs = "-p plcom";
-            cargoArtifacts = plcomClippy;
-          }
-        );
+        # Assuming the above expression was in a file called myCrate.nix
+        # this would be defined as:
+        # my-crate = pkgs.callPackage ./myCrate.nix { };
+        plcomBinary = pkgs.callPackage crateExpression { };
 
         # How to launch binary
         plcom = pkgs.writeShellScriptBin "plcom" ''
@@ -123,6 +202,7 @@
           # Meta
           name = metadata.pname;
           tag = metadata.version;
+          architecture = "arm64"; # TODO: dynamic thing
           created = builtins.substring 0 8 self.lastModifiedDate;
 
           # Content of image
@@ -160,7 +240,7 @@
           inherit
             # Build the crate as part of `nix flake check` for convenience
             plcom
-            plcomClippy
+            #plcomClippy
             ;
         };