erb.rb: Add locals option to set local variables

to `ERB#result` method.

[ruby-core:55985] [Feature #8631]

lib/erb.rb

@@ -885,8 +885,18 @@ def run(b=new_toplevel)
   #
   # _b_ accepts a Binding object which is used to set the context of
   # code evaluation.
+  # _locals_ accepts a Hash object which is used to set local variables
+  # on the Binding object.
   #
-  def result(b=new_toplevel)
+  def result(b=new_toplevel, locals: nil)
+    if locals
+      shadows = locals.keys.select { |l| b.local_variable_defined?(l) }
+      b = eval("proc { |#{shadows.join(',')}| self.send(:binding) }.call", b)
+      locals.each_pair do |key, value|
+        b.local_variable_set(key, value)
+      end
+    end
+
     if @safe_level
       proc {
         $SAFE = @safe_level

test/erb/test_erb.rb

@@ -540,6 +540,37 @@ def test_frozen_string_literal
       assert_equal(flag, erb.result)
     end
   end
+
+  def test_result_with_locals
+    erb = @erb.new("<%= foo %>")
+    assert_equal("1", erb.result(locals: { foo: "1" }))
+
+    erb = @erb.new("<%= foo %>")
+    b = Struct.new(:foo).new("1").instance_eval { binding }
+    assert_equal("2", erb.result(b, locals: { foo: "2" }))
+
+    erb = @erb.new("<%= foo %><%= bar %>")
+    b = Struct.new(:foo).new("1").instance_eval { binding }
+    assert_equal("12", erb.result(b, locals: { bar: "2" }))
+  end
+
+  def test_result_with_locals_does_not_modify_given_binding
+    EnvUtil.suppress_warning do
+      a = 1
+      assert_equal("2", @erb.new("<%= a %>").result(binding, locals: { a: 2 }))
+      assert_equal(1, a)
+    end
+  end
+
+  def test_result_with_locals_named_binding
+    b = binding
+    binding = 1
+    assert_equal("2", @erb.new("<%= binding %>").result(b, locals: { binding: 2 }))
+  end
+
+  def test_result_with_invalid_name_locals
+    assert_raise(NameError) { @erb.new("").result(locals: { :'foo-bar' => 1 }) }
+  end
 end
 
 class TestERBCoreWOStrScan < TestERBCore