cc-wrapper: make availability warnings into errors (#445119)

doc/redirects.json

@@ -222,6 +222,9 @@
   "sec-building-packages-with-llvm-using-clang-stdenv": [
     "index.html#sec-building-packages-with-llvm-using-clang-stdenv"
   ],
+  "sec-darwin-availability-checks": [
+    "index.html#sec-darwin-availability-checks"
+  ],
   "sec-darwin-libcxx-deployment-targets": [
     "index.html#sec-darwin-libcxx-deployment-targets"
   ],

doc/stdenv/platform-notes.chapter.md

@@ -47,6 +47,17 @@ See below for how to use a newer deployment target.
 For example, `std::print` depends on features that are only available on macOS 13.3 or newer.
 To make them available, set the deployment target to 13.3 using `darwinMinVersionHook`.
 
+#### Package fails to build due to missing API availability checks {#sec-darwin-availability-checks}
+
+This is normally a bug in the package or a misconfigured deployment target.
+* If it is using an API from a newer release (e.g., from macOS 26.0 while targeting macOS 14.0), it needs to use an availability check.
+  The code should be patched to use [`__builtin_available`](https://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available).
+  Note that while the linked documentation is for Objective-C, it is applicable to C and C++ except that you use `__builtin_available` in place of `@available`.
+* If the package intends to require the newer platform (i.e., it does not support running on older versions with reduced functionality), use `darwinMinVersionHook` to set the deployment target to the required version.
+  See below for how to use a newer deployment target.
+* If the package actually handles this through some other mechanism (e.g., MoltenVK relies on the running platform’s MSL version), the error can be suppressed.
+  To suppress the error, add `-Wno-error=unguarded-availability` to `env.NIX_CFLAGS_COMPILE`.
+
 #### Package requires a non-default SDK or fails to build due to missing frameworks or symbols {#sec-darwin-troubleshooting-using-sdks}
 
 In some cases, you may have to use a non-default SDK.

pkgs/build-support/cc-wrapper/add-clang-cc-cflags-before.sh

@@ -35,3 +35,7 @@ elif [[ $0 != *cpp ]]; then
         extraBefore+=(-mabi=@explicitAbiValue@)
     fi
 fi
+
+if [[ "@darwinMinVersion@" ]]; then
+    extraBefore+=(-Werror=unguarded-availability)
+fi

pkgs/by-name/mo/moltenvk/package.nix

@@ -51,6 +51,13 @@ stdenv.mkDerivation (finalAttrs: {
   };
 
   postPatch = ''
+    # Update the deployment target for the minimum target used by nixpkgs.
+    while IFS= read -d "" proj; do
+      echo "Updating deployment target to ${stdenv.hostPlatform.darwinMinVersion}: $proj"
+      substituteInPlace "$proj" \
+        --replace-fail 'MACOSX_DEPLOYMENT_TARGET = 10.15' "MACOSX_DEPLOYMENT_TARGET = $MACOSX_DEPLOYMENT_TARGET"
+    done < <(grep -Z -rl --include=project.pbxproj MACOSX_DEPLOYMENT_TARGET)
+
     # Move `mvkGitRevDerived.h` to a stable location
     substituteInPlace Scripts/gen_moltenvk_rev_hdr.sh \
       --replace-fail '$'''{BUILT_PRODUCTS_DIR}' "$NIX_BUILD_TOP/$sourceRoot/build/include" \
@@ -91,7 +98,11 @@ stdenv.mkDerivation (finalAttrs: {
   '';
 
   env.NIX_CFLAGS_COMPILE = toString (
-    lib.optional (stdenv.cc.libcxx != null) "-isystem ${lib.getInclude stdenv.cc.libcxx}/include/c++/v1"
+    # MoltenVK does its own checks for availability by probing the version at runtime and checking the MSL version.
+    [ "-Wno-error=unguarded-availability" ]
+    ++ lib.optional (
+      stdenv.cc.libcxx != null
+    ) "-isystem ${lib.getInclude stdenv.cc.libcxx}/include/c++/v1"
     ++ [
       "-I${lib.getDev spirv-cross}/include/spirv_cross"
       "-I${lib.getDev spirv-headers}/include/spirv/unified1"