From 5721c4fa47f6aa99c039bbceab40392d5110794e Mon Sep 17 00:00:00 2001
From: Randy Eckenrode <randy@largeandhighquality.com>
Date: Mon, 30 Sep 2024 09:12:53 -0400
Subject: [PATCH] darwin: add libSystem

The new Darwin SDK pattern relies on an effectively empty, stub libc
implementation. The actual libSystem to be linked is located dynamically
based on the active SDK for the target. Independent build, host, and
target SDKs are all supported by Darwin.

The stub libSystem contains empty `include` and `lib` folders to avoid
warnings from wrappers that add those paths unconditionally, which can
turn into errors when a package is building with warnings-as-errors.

While it would be nice if a fallback libc could be provided, SDK headers
are not compatible between framework versions. Providing a fallback
risks mixing headers from different frameworks, which can result in hard
to diagnose errors involving semicolons or other punctuation.
---
 pkgs/os-specific/darwin/libSystem/default.nix | 13 +++++++++++++
 pkgs/top-level/darwin-packages.nix            |  2 ++
 2 files changed, 15 insertions(+)
 create mode 100644 pkgs/os-specific/darwin/libSystem/default.nix

diff --git a/pkgs/os-specific/darwin/libSystem/default.nix b/pkgs/os-specific/darwin/libSystem/default.nix
new file mode 100644
index 000000000000..1dea8e846fdf
--- /dev/null
+++ b/pkgs/os-specific/darwin/libSystem/default.nix
@@ -0,0 +1,13 @@
+{ stdenvNoCC }:
+
+# Darwin dynamically determines the `libSystem` to use based on the SDK found at `DEVELOPER_DIR`.
+# By default, this will be `apple-sdk` or one of the versioned variants.
+stdenvNoCC.mkDerivation {
+  pname = "libSystem";
+  version = "B";
+
+  # Silence linker warnings due a missing `lib` (which is added by cc-wrapper).
+  buildCommand = ''
+    mkdir -p "$out/include" "$out/lib"
+  '';
+}
diff --git a/pkgs/top-level/darwin-packages.nix b/pkgs/top-level/darwin-packages.nix
index 30d5ef83061a..0d3cf4033efa 100644
--- a/pkgs/top-level/darwin-packages.nix
+++ b/pkgs/top-level/darwin-packages.nix
@@ -148,6 +148,8 @@ impure-cmds // appleSourcePackages // chooseLibs // {
   # Removes propagated packages from the stdenv, so those packages can be built without depending upon themselves.
   bootstrapStdenv = mkBootstrapStdenv pkgs.stdenv;
 
+  libSystem = callPackage ../os-specific/darwin/libSystem { };
+
   # TODO(@connorbaker): See https://github.com/NixOS/nixpkgs/issues/229389.
   cf-private = self.apple_sdk.frameworks.CoreFoundation;