formula: add linux and macos only function blocks

Library/Homebrew/extend/os/formula.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+if OS.mac?
+  require "extend/os/mac/formula"
+elsif OS.linux?
+  require "extend/os/linux/formula"
+end

Library/Homebrew/extend/os/linux/formula.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class Formula
+  class << self
+    undef on_linux
+
+    def on_linux(&_block)
+      yield
+    end
+  end
+end

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

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class Formula
+  class << self
+    undef on_macos
+
+    def on_macos(&_block)
+      yield
+    end
+  end
+end

Library/Homebrew/formula.rb

@@ -2423,10 +2423,25 @@ class Formula
       specs.each { |spec| spec.depends_on(dep) }
     end
 
+    # Indicates use of dependencies provided by macOS.
+    # On macOS this is a no-op (as we use the system libraries there).
+    # On Linux this will act as `depends_on`.
     def uses_from_macos(dep)
       specs.each { |spec| spec.uses_from_macos(dep) }
     end
 
+    # Block executed only executed on macOS. No-op on Linux.
+    # <pre>on_macos do
+    #   depends_on "mac_only_dep"
+    # end</pre>
+    def on_macos(&_block); end
+
+    # Block executed only executed on Linux. No-op on macOS.
+    # <pre>on_linux do
+    #   depends_on "linux_only_dep"
+    # end</pre>
+    def on_linux(&_block); end
+
     # @!attribute [w] option
     # Options can be used as arguments to `brew install`.
     # To switch features on/off: `"with-something"` or `"with-otherthing"`.
@@ -2657,3 +2672,5 @@ class Formula
     end
   end
 end
+
+require "extend/os/formula"

Library/Homebrew/test/os/linux/formula_spec.rb

@@ -20,4 +20,82 @@ describe Formula do
       expect(f.class.head.deps.first.name).to eq("foo")
     end
   end
+
+  describe "#on_linux" do
+    it "defines an url on Linux only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        on_macos do
+          url "https://brew.sh/test-macos-0.1.tbz"
+          sha256 TEST_SHA256
+        end
+
+        on_linux do
+          url "https://brew.sh/test-linux-0.1.tbz"
+          sha256 TEST_SHA256
+        end
+      end
+
+      expect(f.stable.url).to eq("https://brew.sh/test-linux-0.1.tbz")
+    end
+  end
+
+  describe "#on_linux" do
+    it "adds a dependency on Linux only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        depends_on "hello_both"
+
+        on_macos do
+          depends_on "hello_macos"
+        end
+
+        on_linux do
+          depends_on "hello_linux"
+        end
+      end
+
+      expect(f.class.stable.deps[0].name).to eq("hello_both")
+      expect(f.class.stable.deps[1].name).to eq("hello_linux")
+      expect(f.class.stable.deps[2]).to eq(nil)
+    end
+  end
+
+  describe "#on_linux" do
+    it "adds a patch on Linux only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        patch do
+          url "patch_both"
+        end
+
+        on_macos do
+          patch do
+            url "patch_macos"
+          end
+        end
+
+        on_linux do
+          patch do
+            url "patch_linux"
+          end
+        end
+      end
+
+      expect(f.patchlist.length).to eq(2)
+      expect(f.patchlist.first.strip).to eq(:p1)
+      expect(f.patchlist.first.url).to eq("patch_both")
+      expect(f.patchlist.second.strip).to eq(:p1)
+      expect(f.patchlist.second.url).to eq("patch_linux")
+    end
+  end
 end

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

@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+require "formula"
+
+describe Formula do
+  describe "#on_macos" do
+    it "defines an url on macos only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        on_macos do
+          url "https://brew.sh/test-macos-0.1.tbz"
+          sha256 TEST_SHA256
+        end
+
+        on_linux do
+          url "https://brew.sh/test-linux-0.1.tbz"
+          sha256 TEST_SHA256
+        end
+      end
+
+      expect(f.stable.url).to eq("https://brew.sh/test-macos-0.1.tbz")
+    end
+  end
+
+  describe "#on_macos" do
+    it "adds a dependency on macos only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        depends_on "hello_both"
+
+        on_macos do
+          depends_on "hello_macos"
+        end
+
+        on_linux do
+          depends_on "hello_linux"
+        end
+      end
+
+      expect(f.class.stable.deps[0].name).to eq("hello_both")
+      expect(f.class.stable.deps[1].name).to eq("hello_macos")
+      expect(f.class.stable.deps[2]).to eq(nil)
+    end
+  end
+
+  describe "#on_macos" do
+    it "adds a patch on macos only" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        patch do
+          url "patch_both"
+        end
+
+        on_macos do
+          patch do
+            url "patch_macos"
+          end
+        end
+
+        on_linux do
+          patch do
+            url "patch_linux"
+          end
+        end
+      end
+
+      expect(f.patchlist.length).to eq(2)
+      expect(f.patchlist.first.strip).to eq(:p1)
+      expect(f.patchlist.first.url).to eq("patch_both")
+      expect(f.patchlist.second.strip).to eq(:p1)
+      expect(f.patchlist.second.url).to eq("patch_macos")
+    end
+  end
+end