Implement pkg-config macOS SDK mismatch diagnostic check

Co-authored-by: MikeMcQuaid <125011+MikeMcQuaid@users.noreply.github.com>

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -506,6 +506,42 @@ module OS
 
           nil
         end
+
+        def check_pkgconf_macos_sdk_mismatch
+          # We don't provide suitable bottles for these versions.
+          return if OS::Mac.version.prerelease? || OS::Mac.version.outdated_release?
+
+          pkgconf = begin
+            ::Formula["pkgconf"]
+          rescue FormulaUnavailableError
+            nil
+          end
+          return unless pkgconf
+          return unless pkgconf.any_version_installed?
+
+          tab = Tab.for_formula(pkgconf)
+          return unless tab.built_on
+
+          built_on_version = tab.built_on["os_version"]
+                                &.delete_prefix("macOS ")
+                                &.sub(/\.\d+$/, "")
+          return unless built_on_version
+
+          current_version = MacOS.version.to_s
+          return if built_on_version == current_version
+
+          <<~EOS
+            You have pkgconf installed that was built on macOS #{built_on_version}
+                     but you are running macOS #{current_version}.
+
+            This can cause issues with packages that depend on system libraries, such as libffi.
+            To fix this issue, reinstall pkgconf:
+              brew reinstall pkgconf
+
+            For more information, see: https://github.com/Homebrew/brew/issues/16137
+            We'd welcome a PR to automatically mitigate this instead of just warning about it.
+          EOS
+        end
       end
     end
   end

Library/Homebrew/test/os/mac/diagnostic_spec.rb

@@ -95,4 +95,52 @@ RSpec.describe Homebrew::Diagnostic::Checks do
         .to include("The contents of the SDKs in your Xcode installation do not match the SDK folder names")
     end
   end
+
+  describe "#check_pkgconf_macos_sdk_mismatch" do
+    let(:pkg_config_formula) { instance_double(Formula, any_version_installed?: true) }
+    let(:tab) { instance_double(Tab, built_on: { "os_version" => "13" }) }
+
+    before do
+      allow(Formula).to receive(:[]).with("pkgconf").and_return(pkg_config_formula)
+      allow(Tab).to receive(:for_formula).with(pkg_config_formula).and_return(tab)
+    end
+
+    it "doesn't trigger when pkgconf is not installed" do
+      allow(Formula).to receive(:[]).with("pkgconf").and_raise(FormulaUnavailableError.new("pkgconf"))
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to be_nil
+    end
+
+    it "doesn't trigger when no versions are installed" do
+      allow(pkg_config_formula).to receive(:any_version_installed?).and_return(false)
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to be_nil
+    end
+
+    it "doesn't trigger when built_on information is missing" do
+      allow(tab).to receive(:built_on).and_return(nil)
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to be_nil
+    end
+
+    it "doesn't trigger when os_version information is missing" do
+      allow(tab).to receive(:built_on).and_return({ "cpu_family" => "x86_64" })
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to be_nil
+    end
+
+    it "doesn't trigger when versions match" do
+      current_version = MacOS.version.to_s
+      allow(tab).to receive(:built_on).and_return({ "os_version" => current_version })
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to be_nil
+    end
+
+    it "triggers when built_on version differs from current macOS version" do
+      allow(MacOS).to receive(:version).and_return(MacOSVersion.new("14"))
+      allow(tab).to receive(:built_on).and_return({ "os_version" => "13" })
+
+      expect(checks.check_pkgconf_macos_sdk_mismatch).to include("brew reinstall pkgconf")
+    end
+  end
 end