Interpolating a single string results in that string.

spec/std/string_spec.cr

@@ -2516,8 +2516,7 @@ describe "String" do
     it "of a single string" do
       string = "hello"
       interpolated = String.interpolation(string)
-      interpolated.should_not be(string)
-      interpolated.should eq(string)
+      interpolated.should be(string)
     end
 
     it "of a single non-string" do

src/string.cr

@@ -4438,15 +4438,11 @@ class String
   # "#{value}" # same as String.interpolation(value)
   # ```
   #
-  # In this case the implementation just returns a new string with the contents
-  # of `value`.
+  # In this case the implementation just returns the same string.
   #
   # NOTE: there should never be a need to call this method instead of using string interpolation.
   def self.interpolation(value : String)
-    String.new(value.bytesize) do |buffer|
-      buffer.copy_from(value.to_unsafe, value.bytesize)
-      {value.bytesize, value.size_known? ? value.size : 0}
-    end
+    value
   end
 
   # Implementation of string interpolation of a single non-string value.