print sandbox message

Closes Homebrew/homebrew#42293.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/postinstall.rb

@@ -15,8 +15,12 @@ module Homebrew
       #{formula.path}
     ].concat(ARGV.options_only)
 
-    if Sandbox.available? && ARGV.sandbox? && Sandbox.auto_disable?
+    if Sandbox.available? && ARGV.sandbox?
-      Sandbox.print_autodisable_warning
+      if Sandbox.auto_disable?
+        Sandbox.print_autodisable_warning
+      else
+        Sandbox.print_sandbox_message
+      end
     end
 
     Utils.safe_fork do

Library/Homebrew/cmd/test.rb

@@ -35,8 +35,12 @@ module Homebrew
           #{f.path}
         ].concat(ARGV.options_only)
 
-        if Sandbox.available? && ARGV.sandbox? && Sandbox.auto_disable?
+        if Sandbox.available? && ARGV.sandbox?
-          Sandbox.print_autodisable_warning
+          if Sandbox.auto_disable?
+            Sandbox.print_autodisable_warning
+          else
+            Sandbox.print_sandbox_message
+          end
         end
 
         Utils.safe_fork do

Library/Homebrew/formula_installer.rb

@@ -486,8 +486,12 @@ class FormulaInstaller
       #{formula.path}
     ].concat(build_argv)
 
-    if Sandbox.available? && ARGV.sandbox? && Sandbox.auto_disable?
+    if Sandbox.available? && ARGV.sandbox?
-      Sandbox.print_autodisable_warning
+      if Sandbox.auto_disable?
+        Sandbox.print_autodisable_warning
+      else
+        Sandbox.print_sandbox_message
+      end
     end
 
     Utils.safe_fork do

Library/Homebrew/sandbox.rb

@@ -14,9 +14,16 @@ class Sandbox
   end
 
   def self.print_autodisable_warning
-    unless @print_autodisable_warning
+    unless @printed_autodisable_warning
       opoo "The sandbox cannot be used in debug or interactive mode."
-      @print_autodisable_warning = true
+      @printed_autodisable_warning = true
+    end
+  end
+
+  def self.print_sandbox_message
+    unless @printed_sandbox_message
+      ohai "Using the sandbox"
+      @printed_sandbox_message = true
     end
   end