rubocops/caveats: check for dynamic caveats.

This will avoid issues with the JSON API.

Library/Homebrew/rubocops/caveats.rb

@@ -6,13 +6,25 @@ require "rubocops/extend/formula_cop"
 module RuboCop
   module Cop
     module FormulaAudit
-      # This cop ensures that caveats don't recommend unsupported or unsafe operations.
+      # This cop ensures that caveats don't have problematic text or logic.
       #
       # ### Example
       #
       # ```ruby
       # # bad
       # def caveats
+      #   if File.exist?("/etc/issue")
+      #     "This caveat only when file exists that won't work with JSON API."
+      #   end
+      # end
+      #
+      # # good
+      # def caveats
+      #   "This caveat always works regardless of the JSON API."
+      # end
+      #
+      # # bad
+      # def caveats
       #   <<~EOS
       #     Use `setuid` to allow running the executable by non-root users.
       #   EOS
@@ -35,6 +47,18 @@ module RuboCop
 
             problem "Don't use ANSI escape codes in the caveats." if regex_match_group(n, /\e/)
           end
+
+          # Forbid dynamic logic in caveats (only if/else/unless)
+          caveats_method = find_method_def(@body, :caveats)
+          return unless caveats_method
+
+          dynamic_nodes = caveats_method.each_descendant.select do |descendant|
+            descendant.type == :if
+          end
+          dynamic_nodes.each do |node|
+            @offensive_node = node
+            problem "Don't use dynamic logic (if/else/unless) in caveats."
+          end
         end
       end
     end

Library/Homebrew/test/rubocops/caveats_spec.rb

@@ -42,5 +42,35 @@ RSpec.describe RuboCop::Cop::FormulaAudit::Caveats do
         end
       RUBY
     end
+
+    it "reports an offense if dynamic logic (if/else/unless) is used in caveats" do
+      expect_offense(<<~RUBY, "/homebrew-core/Formula/foo.rb")
+        class Foo < Formula
+          homepage "https://brew.sh/foo"
+          url "https://brew.sh/foo-1.0.tgz"
+          def caveats
+            if true
+            ^^^^^^^ FormulaAudit/Caveats: Don't use dynamic logic (if/else/unless) in caveats.
+              "foo"
+            else
+              "bar"
+            end
+          end
+        end
+      RUBY
+
+      expect_offense(<<~RUBY, "/homebrew-core/Formula/foo.rb")
+        class Foo < Formula
+          homepage "https://brew.sh/foo"
+          url "https://brew.sh/foo-1.0.tgz"
+          def caveats
+            unless false
+            ^^^^^^^^^^^^ FormulaAudit/Caveats: Don't use dynamic logic (if/else/unless) in caveats.
+              "foo"
+            end
+          end
+        end
+      RUBY
+    end
   end
 end