Split up comments between leading and trailing

Also make them lazy to allocate the array, and also expose ParseResult#encoding.

lib/prism/parse_result.rb

@@ -21,6 +21,12 @@ def initialize(source, start_line = 1, offsets = [])
       @offsets = offsets # set after parsing is done
     end
 
+    # Returns the encoding of the source code, which is set by parameters to the
+    # parser or by the encoding magic comment.
+    def encoding
+      source.encoding
+    end
+
     # Perform a byteslice on the source code using the given byte offset and
     # byte length.
     def slice(byte_offset, length)
@@ -108,16 +114,46 @@ class Location
     # The length of this location in bytes.
     attr_reader :length
 
-    # The list of comments attached to this location
-    attr_reader :comments
-
     # Create a new location object with the given source, start byte offset, and
     # byte length.
     def initialize(source, start_offset, length)
       @source = source
       @start_offset = start_offset
       @length = length
-      @comments = []
+
+      # These are used to store comments that are associated with this location.
+      # They are initialized to `nil` to save on memory when there are no
+      # comments to be attached and/or the comment-related APIs are not used.
+      @leading_comments = nil
+      @trailing_comments = nil
+    end
+
+    # These are the comments that are associated with this location that exist
+    # before the start of this location.
+    def leading_comments
+      @leading_comments ||= []
+    end
+
+    # Attach a comment to the leading comments of this location.
+    def leading_comment(comment)
+      leading_comments << comment
+    end
+
+    # These are the comments that are associated with this location that exist
+    # after the end of this location.
+    def trailing_comments
+      @trailing_comments ||= []
+    end
+
+    # Attach a comment to the trailing comments of this location.
+    def trailing_comment(comment)
+      trailing_comments << comment
+    end
+
+    # Returns all comments that are associated with this location (both leading
+    # and trailing comments).
+    def comments
+      (@leading_comments || []).concat(@trailing_comments || [])
     end
 
     # Create a new location object with the given options.
@@ -268,6 +304,11 @@ def initialize(location)
     def deconstruct_keys(keys)
       { location: location }
     end
+
+    # Returns the content of the comment by slicing it from the source code.
+    def slice
+      location.slice
+    end
   end
 
   # InlineComment objects are the most common. They correspond to comments in
@@ -437,6 +478,11 @@ def deconstruct_keys(keys)
       { value: value, comments: comments, magic_comments: magic_comments, data_loc: data_loc, errors: errors, warnings: warnings }
     end
 
+    # Returns the encoding of the source code that was parsed.
+    def encoding
+      source.encoding
+    end
+
     # Returns true if there were no errors during parsing and false if there
     # were.
     def success?

lib/prism/parse_result/comments.rb

@@ -39,8 +39,12 @@ def encloses?(comment)
             comment.location.end_offset <= end_offset
         end
 
-        def <<(comment)
-          node.location.comments << comment
+        def leading_comment(comment)
+          node.location.leading_comment(comment)
+        end
+
+        def trailing_comment(comment)
+          node.location.trailing_comment(comment)
         end
       end
 
@@ -65,8 +69,12 @@ def encloses?(comment)
           false
         end
 
-        def <<(comment)
-          location.comments << comment
+        def leading_comment(comment)
+          location.leading_comment(comment)
+        end
+
+        def trailing_comment(comment)
+          location.trailing_comment(comment)
         end
       end
 
@@ -84,15 +92,16 @@ def initialize(parse_result)
       def attach!
         parse_result.comments.each do |comment|
           preceding, enclosing, following = nearest_targets(parse_result.value, comment)
-          target =
-            if comment.trailing?
-              preceding || following || enclosing || NodeTarget.new(parse_result.value)
-            else
-              # If a comment exists on its own line, prefer a leading comment.
-              following || preceding || enclosing || NodeTarget.new(parse_result.value)
-            end
-
-          target << comment
+
+          if comment.trailing?
+            preceding&.trailing_comment(comment) ||
+              (following || enclosing || NodeTarget.new(parse_result.value)).leading_comment(comment)
+          else
+            # If a comment exists on its own line, prefer a leading comment.
+            following&.leading_comment(comment) ||
+              preceding&.trailing_comment(comment) ||
+              (enclosing || NodeTarget.new(parse_result.value)).leading_comment(comment)
+          end
         end
       end
 

test/prism/magic_comment_test.rb

@@ -30,7 +30,7 @@ class MagicCommentTest < TestCase
 
     def assert_magic_comment(example)
       expected = Ripper.new(example).tap(&:parse).encoding
-      actual = Prism.parse(example).source.source.encoding
+      actual = Prism.parse(example).encoding
       assert_equal expected, actual
     end
   end