diff --git a/lib/vagrant/action/builtin/mixin_synced_folders.rb b/lib/vagrant/action/builtin/mixin_synced_folders.rb
index d2dc76730..0f3889a0d 100644
--- a/lib/vagrant/action/builtin/mixin_synced_folders.rb
+++ b/lib/vagrant/action/builtin/mixin_synced_folders.rb
@@ -1,4 +1,5 @@
 require "json"
+require "set"
 
 require 'vagrant/util/scoped_hash_override'
 
@@ -151,6 +152,49 @@ module Vagrant
           return folders
         end
 
+        # This finds the difference between two lists of synced folder
+        # definitions.
+        #
+        # This will return a hash with three keys: "added", "removed",
+        # and "modified". These will contain a set of IDs of folders
+        # that were added, removed, or modified, respectively.
+        #
+        # The parameters should be results from the {#synced_folders} call.
+        #
+        # @return [hash]
+        def synced_folders_diff(one, two)
+          existing_ids = {}
+          one.each do |impl, fs|
+            fs.each do |id, data|
+              existing_ids[id] = data
+            end
+          end
+
+          result = Hash.new { |h, k| h[k] = Set.new }
+          two.each do |impl, fs|
+            fs.each do |id, data|
+              existing = existing_ids.delete(id)
+              if !existing
+                result[:added] << id
+                next
+              end
+
+              # Exists, so we have to compare the host and guestpath, which
+              # is most important...
+              if existing[:hostpath] != data[:hostpath] ||
+                existing[:guestpath] != data[:guestpath]
+                result[:modified] << id
+              end
+            end
+          end
+
+          existing_ids.each do |k, _|
+            result[:removed] << k
+          end
+
+          result
+        end
+
         protected
 
         def cached_synced_folders(machine)
diff --git a/test/unit/vagrant/action/builtin/mixin_synced_folders_test.rb b/test/unit/vagrant/action/builtin/mixin_synced_folders_test.rb
index d588879b9..a9c4711d2 100644
--- a/test/unit/vagrant/action/builtin/mixin_synced_folders_test.rb
+++ b/test/unit/vagrant/action/builtin/mixin_synced_folders_test.rb
@@ -191,4 +191,64 @@ describe Vagrant::Action::Builtin::MixinSyncedFolders do
       })
     end
   end
+
+  describe "#synced_folders_diff" do
+    it "sees two equal " do
+      one = {
+        default: { "foo" => {} },
+      }
+
+      two = {
+        default: { "foo" => {} },
+      }
+
+      expect(subject.synced_folders_diff(one, two)).to be_empty
+    end
+
+    it "sees modifications" do
+      one = {
+        default: { "foo" => {} },
+      }
+
+      two = {
+        default: { "foo" => { hostpath: "foo" } },
+      }
+
+      result = subject.synced_folders_diff(one, two)
+      expect(result[:modified]).to_not be_empty
+    end
+
+    it "sees adding" do
+      one = {
+        default: { "foo" => {} },
+      }
+
+      two = {
+        default: {
+          "foo" => {},
+          "bar" => {},
+        },
+      }
+
+      result = subject.synced_folders_diff(one, two)
+      expect(result[:added]).to_not be_empty
+      expect(result[:removed]).to be_empty
+      expect(result[:modified]).to be_empty
+    end
+
+    it "sees removing" do
+      one = {
+        default: { "foo" => {} },
+      }
+
+      two = {
+        default: {},
+      }
+
+      result = subject.synced_folders_diff(one, two)
+      expect(result[:added]).to be_empty
+      expect(result[:removed]).to_not be_empty
+      expect(result[:modified]).to be_empty
+    end
+  end
 end