refactor(rust): try to better compose cross toolchains

rust/flake.nix

@@ -35,8 +35,10 @@
               overlays = [ (final: prev: { ${packageName} = self.packages.${system}.${packageName}; }) ];
             };
 
-            fenix-pkgs = fenix.packages.${system};
+            fenixPkgsFor = pkgs: fenix.packages.${pkgs.system};
-            fenix-channel = fenix-pkgs.toolchainOf {
+            fenixChannelFor =
+              pkgs:
+              (fenixPkgsFor pkgs).toolchainOf {
                 channel = "nightly";
                 date =
                   builtins.replaceStrings [ "nightly-" ] [ "" ]
@@ -44,31 +46,39 @@
                 sha256 = "sha256-SzEeSoO54GiBQ2kfANPhWrt0EDRxqEvhIbTt2uJt/TQ=";
               };
 
-            makeCrossPackage =
+            toolchainFor =
-              packageName: pkgsCross:
+              pkgs:
               let
-                pkgs = builtins.throw "defining cross pkg, accessing pkgs is a bug";
+                fenix-pkgs = fenixPkgsFor pkgs;
               in
-              {
-                "${packageName}-cross-${pkgsCross.stdenv.hostPlatform.config}${
-                  if pkgsCross.stdenv.hostPlatform.isStatic then "-static" else ""
-                }" =
-                  let
-                    toolchain =
               with fenix-pkgs;
               combine [
                 minimal.cargo
                 minimal.rustc
-                        targets.${pkgsCross.rust.lib.toRustTarget pkgsCross.stdenv.targetPlatform}.latest.rust-std
+                targets.${pkgs.rust.lib.toRustTarget pkgs.stdenv.targetPlatform}.latest.rust-std
               ];
+
+            rustPlatformFor =
+              pkgs:
+              let
+                toolchain = toolchainFor pkgs;
               in
-                  pkgsCross.callPackage (./. + "/nix/packages/${packageName}.nix") {
+              pkgs.makeRustPlatform {
-                    inherit cargoMeta;
-                    flake-self = self;
-                    rustPlatform = pkgsCross.makeRustPlatform {
                 cargo = toolchain;
                 rustc = toolchain;
               };
+
+            crossPackageFor =
+              pkgs:
+              let
+                rustPlatform = rustPlatformFor pkgs;
+              in
+              {
+                "${packageName}-cross-${pkgs.stdenv.hostPlatform.config}${
+                  if pkgs.stdenv.hostPlatform.isStatic then "-static" else ""
+                }" = pkgs.callPackage (./. + "/nix/packages/${packageName}.nix") {
+                  inherit cargoMeta rustPlatform;
+                  flake-self = self;
                 };
               };
           in
@@ -76,9 +86,11 @@
             inherit
               system
               pkgs
-              fenix-pkgs
+              fenixPkgsFor
-              fenix-channel
+              fenixChannelFor
-              makeCrossPackage
+              toolchainFor
+              rustPlatformFor
+              crossPackageFor
               ;
           }
         );
@@ -89,11 +101,14 @@
       packages = forSystems (
         {
           pkgs,
-          fenix-channel,
+          fenixChannelFor,
           system,
-          makeCrossPackage,
+          crossPackageFor,
           ...
         }:
+        let
+          fenix-channel = fenixChannelFor pkgs;
+        in
         {
           ${packageName} = pkgs.callPackage (./. + "/nix/packages/${packageName}.nix") {
             inherit cargoMeta;
@@ -105,21 +120,17 @@
           };
           default = self.packages.${system}.${packageName};
         }
-        // makeCrossPackage packageName pkgs.pkgsCross.musl64.pkgsStatic
+        // crossPackageFor pkgs.pkgsCross.musl64.pkgsStatic
-        // makeCrossPackage packageName pkgs.pkgsCross.musl32.pkgsStatic
+        // crossPackageFor pkgs.pkgsCross.musl32.pkgsStatic
-        // makeCrossPackage packageName pkgs.pkgsCross.aarch64-multiplatform-musl.pkgsStatic
+        // crossPackageFor pkgs.pkgsCross.aarch64-multiplatform-musl.pkgsStatic
-        // makeCrossPackage packageName pkgs.pkgsCross.armv7l-hf-multiplatform.pkgsStatic
+        // crossPackageFor pkgs.pkgsCross.armv7l-hf-multiplatform.pkgsStatic
-        // makeCrossPackage packageName pkgs.pkgsCross.mingwW64.pkgsStatic
+        // crossPackageFor pkgs.pkgsCross.mingwW64.pkgsStatic
       );
 
       devShells = forSystems (
-        {
+        { pkgs, fenixChannelFor, ... }:
-          pkgs,
-          fenix-pkgs,
-          fenix-channel,
-          ...
-        }:
         let
+          fenix-channel = fenixChannelFor pkgs;
           fenixRustToolchain = fenix-channel.withComponents [
             "cargo"
             "clippy-preview"