Lazily compute code_units_cache in FileContext

Instead of eagerly computing the UTF-16 code units cache during parsing,
store the Prism source and defer computation until first access. This
avoids expensive String#encode calls when code_range positions are not
immediately needed.

Author: mame

lib/typeprof/core/ast.rb

@@ -11,7 +11,7 @@ def self.parse_rb(path, src)
       raise unless raw_scope.type == :program_node
 
       prism_source = result.source
-      file_context = FileContext.new(path, prism_source.code_units_cache(Encoding::UTF_16LE), result.comments)
+      file_context = FileContext.new(path, prism_source, result.comments)
 
       cref = CRef::Toplevel
       lenv = LocalEnv.new(file_context, cref, {}, [])

lib/typeprof/core/env.rb

@@ -305,12 +305,17 @@ class Hash[K, V]
   end
 
   class FileContext
-    attr_reader :path, :code_units_cache, :comments
-    def initialize(path, code_units_cache = nil, comments = nil)
+    attr_reader :path, :comments
+    def initialize(path, prism_source = nil, comments = nil)
       @path = path
-      @code_units_cache = code_units_cache
+      @prism_source = prism_source
+      @code_units_cache = nil
       @comments = comments
     end
+
+    def code_units_cache
+      @code_units_cache ||= @prism_source&.code_units_cache(Encoding::UTF_16LE)
+    end
   end
 
   class LocalEnv