Call #instance_exec on DSL object instead of proxy (#39)

when the DSL object is the same object as the block context object.

This short-circuits prior to performing setup (like copying ivars
from the block context to the proxy) that is unnecessary when the block
context and DSL object "match".

Add test that runs non-mutative code on frozen DSL object to check that
no FrozenError is raised, as it would have been if a
FallbackContextProxy tried to dynamically add methods to the DSL object.

HISTORY.md

@@ -2,7 +2,9 @@
 
 ## [Unreleased changes](http://github.com/ms-ati/docile/compare/v1.3.5...master)
 
-  ...
+  - Short-circuit to calling #instance_exec directly on the DSL object (prior to
+    constructing a proxy object) when the DSL object and block context object are
+    identical
 
 ## [v1.3.5 (Jan 13, 2021)](http://github.com/ms-ati/docile/compare/v1.3.4...v1.3.5)
 

lib/docile/execution.rb

@@ -16,6 +16,13 @@ module Execution
     # @return           [Object] the return value of the block
     def exec_in_proxy_context(dsl, proxy_type, *args, &block)
       block_context = eval("self", block.binding)
+
+      # Use #equal? to test strict object identity (assuming that this dictum
+      # from the Ruby docs holds: "[u]nlike ==, the equal? method should never
+      # be overridden by subclasses as it is used to determine object
+      # identity")
+      return dsl.instance_exec(*args, &block) if dsl.equal?(block_context)
+
       proxy_context = proxy_type.new(dsl, block_context)
       begin
         block_context.instance_variables.each do |ivar|

spec/docile_spec.rb

@@ -414,6 +414,12 @@ def dsl_eval_string(string)
         expect(dsl.foo).to eq(0)
         expect(dsl.bar).to eq(1)
       end
+
+      context "when the DSL object is frozen" do
+        it "can call non-mutative code without raising an exception" do
+          expect { dsl.freeze.dsl_eval_string('1 + 2') }.not_to raise_error
+        end
+      end
     end
 
     context "when NoMethodError is raised" do