Lazily allocate @diagnostics Set in AST nodes

Initialize @diagnostics as the frozen empty set instance to avoid
allocating a Hash for every AST node. Most nodes never receive
diagnostics, so the Set is only created on first use via add_diagnostic.

Author: mame

lib/typeprof/core/ast/base.rb

@@ -9,7 +9,7 @@ def initialize(raw_node, lenv)
         @ret = nil
 
         @changes = ChangeSet.new(self, nil)
-        @diagnostics = Set.empty
+        @diagnostics = Set::EMPTY
       end
 
       attr_reader :lenv
@@ -104,6 +104,7 @@ def install_copy(genv)
         @changes.reuse(self)
         (@prev_node || raise).diagnostics.each do |diag|
           diag.reuse(self)
+          @diagnostics = Set.empty if @diagnostics.equal?(Set::EMPTY)
           @diagnostics << diag
         end
         each_subnode do |subnode|
@@ -129,11 +130,12 @@ def narrowings
       end
 
       def add_diagnostic(diag)
+        @diagnostics = Set.empty if @diagnostics.equal?(Set::EMPTY)
         @diagnostics << diag
       end
 
       def remove_diagnostic(diag)
-        @diagnostics.delete(diag)
+        @diagnostics.delete(diag) unless @diagnostics.equal?(Set::EMPTY)
       end
 
       def diff(prev_node)

lib/typeprof/core/util.rb

@@ -81,5 +81,7 @@ def pretty_print(q)
       end
       q.text "]"
     end
+
+    EMPTY = empty.freeze
   end
 end