formula: add on_linux and on_macos blocks

for install and others

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

@@ -2,6 +2,14 @@
 # frozen_string_literal: true
 
 class Formula
+  undef on_linux
+
+  def on_linux(&_block)
+    raise "No block content defined for on_linux block" unless block_given?
+
+    yield
+  end
+
   undef shared_library
 
   def shared_library(name, version = nil)
@@ -12,6 +20,8 @@ class Formula
     undef on_linux
 
     def on_linux(&_block)
+      raise "No block content defined for on_linux block" unless block_given?
+
       yield
     end
 

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

@@ -2,10 +2,20 @@
 # frozen_string_literal: true
 
 class Formula
+  undef on_macos
+
+  def on_macos(&_block)
+    raise "No block content defined for on_macos block" unless block_given?
+
+    yield
+  end
+
   class << self
     undef on_macos
 
     def on_macos(&_block)
+      raise "No block content defined for on_macos block" unless block_given?
+
       yield
     end
   end

Library/Homebrew/formula.rb

@@ -1366,6 +1366,22 @@ class Formula
     "#<Formula #{name} (#{active_spec_sym}) #{path}>"
   end
 
+  # Block only executed on macOS. No-op on Linux.
+  # <pre>on_macos do
+  #   Do something mac specific
+  # end</pre>
+  def on_macos(&_block)
+    raise "No block content defined for on_macos block" unless block_given?
+  end
+
+  # Block only executed on Linux. No-op on macOS.
+  # <pre>on_linux do
+  #   Do something linux specific
+  # end</pre>
+  def on_linux(&_block)
+    raise "No block content defined for on_linux block" unless block_given?
+  end
+
   # Standard parameters for cargo builds.
   def std_cargo_args
     ["--locked", "--root", prefix, "--path", "."]
@@ -2512,13 +2528,17 @@ class Formula
     # <pre>on_macos do
     #   depends_on "mac_only_dep"
     # end</pre>
-    def on_macos(&_block); end
+    def on_macos(&_block)
+      raise "No block content defined for on_macos block" unless block_given?
+    end
 
     # Block only executed on Linux. No-op on macOS.
     # <pre>on_linux do
     #   depends_on "linux_only_dep"
     # end</pre>
-    def on_linux(&_block); end
+    def on_linux(&_block)
+      raise "No block content defined for on_linux block" unless block_given?
+    end
 
     # @!attribute [w] option
     # Options can be used as arguments to `brew install`.

Library/Homebrew/test/formula_spec.rb

@@ -1363,4 +1363,50 @@ describe Formula do
       expect(f.any_installed_version).to eq(PkgVersion.parse("1.0_1"))
     end
   end
+
+  describe "#on_macos", :needs_macos do
+    let(:f) do
+      Class.new(Testball) do
+        @test = 0
+        attr_reader :test
+
+        def install
+          on_macos do
+            @test = 1
+          end
+          on_linux do
+            @test = 2
+          end
+        end
+      end.new
+    end
+
+    it "only calls code within on_macos" do
+      f.brew { f.install }
+      expect(f.test).to eq(1)
+    end
+  end
+
+  describe "#on_linux", :needs_linux do
+    let(:f) do
+      Class.new(Testball) do
+        @test = 0
+        attr_reader :test
+
+        def install
+          on_macos do
+            @test = 1
+          end
+          on_linux do
+            @test = 2
+          end
+        end
+      end.new
+    end
+
+    it "only calls code within on_linux" do
+      f.brew { f.install }
+      expect(f.test).to eq(2)
+    end
+  end
 end