Add cop to stop bin in service block

The preferred method is opt_bin because that works
with the API and is more portable (works between versions).

Also removed the last example from the docs of `bin/"name"`
from the service block section

Library/Homebrew/rubocops/all.rb

@@ -24,6 +24,7 @@ require_relative "lines"
 require_relative "livecheck"
 require_relative "options"
 require_relative "patches"
+require_relative "service"
 require_relative "text"
 require_relative "urls"
 require_relative "uses_from_macos"

Library/Homebrew/rubocops/service.rb

@@ -0,0 +1,31 @@
+# typed: true
+# frozen_string_literal: true
+
+require "rubocops/extend/formula_cop"
+
+module RuboCop
+  module Cop
+    module FormulaAudit
+      # This cop audits the service block.
+      #
+      # @api private
+      class Service < FormulaCop
+        extend AutoCorrector
+
+        def audit_formula(_node, _class_node, _parent_class_node, body_node)
+          service_node = find_block(body_node, :service)
+          return if service_node.blank?
+
+          # This check ensures that `bin` is not referenced because
+          # `opt_bin` is more portable and works with the API.
+          find_every_method_call_by_name(service_node, :bin).each do |bin_node|
+            offending_node(bin_node)
+            problem "Use `opt_bin` instead of `bin` in service blocks." do |corrector|
+              corrector.replace(bin_node.source_range, "opt_bin")
+            end
+          end
+        end
+      end
+    end
+  end
+end

Library/Homebrew/test/rubocops/service_spec.rb

@@ -0,0 +1,43 @@
+# typed: false
+# frozen_string_literal: true
+
+require "rubocops/service"
+
+describe RuboCop::Cop::FormulaAudit::Service do
+  subject(:cop) { described_class.new }
+
+  it "reports an offense when a formula's service block uses `bin`" do
+    expect_offense(<<~RUBY)
+      class Foo < Formula
+        url "https://brew.sh/foo-1.0.tgz"
+
+        service do
+          run [bin/"foo", "run", "-config", etc/"foo/config.json"]
+               ^^^ Use `opt_bin` instead of `bin` in service blocks.
+        end
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      class Foo < Formula
+        url "https://brew.sh/foo-1.0.tgz"
+
+        service do
+          run [opt_bin/"foo", "run", "-config", etc/"foo/config.json"]
+        end
+      end
+    RUBY
+  end
+
+  it "reports no offenses when a formula's service block uses `opt_bin`" do
+    expect_no_offenses(<<~RUBY)
+      class Bin < Formula
+        url "https://brew.sh/foo-1.0.tgz"
+
+        service do
+          run [opt_bin/"bin", "run", "-config", etc/"bin/config.json"]
+        end
+      end
+    RUBY
+  end
+end

docs/Formula-Cookbook.md

@@ -882,7 +882,7 @@ There are two ways to add `launchd` plists and `systemd` services to a formula,
 
    ```ruby
    service do
-     run bin/"script"
+     run opt_bin/"script"
    end
    ```