diagnostic: share prefix check

Library/Homebrew/diagnostic.rb

@@ -791,6 +791,17 @@ module Homebrew
         end.join("\n")
       end
 
+      def check_homebrew_prefix
+        return if HOMEBREW_PREFIX.to_s == Homebrew::DEFAULT_PREFIX
+
+        <<~EOS
+          Your Homebrew's prefix is not #{Homebrew::DEFAULT_PREFIX}.
+          You can install Homebrew anywhere you want but some bottles (binary packages)
+          can only be used with a standard prefix and some formulae (packages)
+          may not build correctly with a non-standard prefix.
+        EOS
+      end
+
       def all
         methods.map(&:to_s).grep(/^check_/)
       end

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

@@ -284,17 +284,6 @@ module Homebrew
         EOS
       end
 
-      def check_homebrew_prefix
-        return if HOMEBREW_PREFIX.to_s == "/usr/local"
-
-        <<~EOS
-          Your Homebrew's prefix is not /usr/local.
-          You can install Homebrew anywhere you want but some bottles (binary packages)
-          can only be used with a /usr/local prefix and some formulae (packages)
-          may not build correctly with a non-/usr/local prefix.
-        EOS
-      end
-
       def check_for_gettext
         find_relative_paths("lib/libgettextlib.dylib",
                             "lib/libintl.dylib",

Library/Homebrew/test/diagnostic_checks_spec.rb

@@ -162,4 +162,10 @@ describe Homebrew::Diagnostic::Checks do
       end
     end
   end
+
+  specify "#check_homebrew_prefix" do
+    # the integration tests are run in a special prefix
+    expect(subject.check_homebrew_prefix)
+      .to match("Your Homebrew's prefix is not #{Homebrew::DEFAULT_PREFIX}")
+  end
 end

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

@@ -32,12 +32,6 @@ describe Homebrew::Diagnostic::Checks do
       .to match("Xcode alone is not sufficient on El Capitan")
   end
 
-  specify "#check_homebrew_prefix" do
-    # the integration tests are run in a special prefix
-    expect(subject.check_homebrew_prefix)
-      .to match("Your Homebrew's prefix is not /usr/local.")
-  end
-
   specify "#check_ruby_version" do
     allow(MacOS).to receive(:version).and_return(OS::Mac::Version.new("10.12"))
     stub_const("RUBY_VERSION", "1.8.6")