Inline diff line rendering

Renders diff lines inline, rather than rendering through a partial.
Also removes numerous conditionals to speed up rendering by focusing
only on the primary use-case.

Changelog: performance

app/helpers/notes_helper.rb

@@ -48,7 +48,7 @@ module NotesHelper
       data[:note_type] = LegacyDiffNote.name
     else
       data[:note_type] = DiffNote.name
-      data[:position] = position.to_json
+      data[:position] = Gitlab::Json.dump(position)
     end
 
     data

app/views/notify/_note_email.html.haml

@@ -26,12 +26,10 @@
     = stylesheet_link_tag 'mailers/highlighted_diff_email'
 
   %table
-    = render partial: "projects/diffs/line",
+    = render partial: "projects/diffs/email_line",
       collection: discussion.truncated_diff_lines(diff_limit: diff_limit),
       as: :line,
-      locals: { diff_file: discussion.diff_file,
-        plain: true,
-        email: true }
+      locals: { diff_file: discussion.diff_file }
 
 %div{ style: note_style }
   = markdown(note.note, pipeline: :email, author: note.author, current_user: @recipient, issuable_reference_expansion_enabled: true)

app/views/notify/repository_push_email.html.haml

@@ -74,7 +74,7 @@
             - blob = diff_file.blob
             - if blob && blob.readable_text?
               %table.code.white
-                = render partial: "projects/diffs/line", collection: diff_file.highlighted_diff_lines, as: :line, locals: { diff_file: diff_file, plain: true, email: true }
+                = render partial: "projects/diffs/email_line", collection: diff_file.highlighted_diff_lines, as: :line, locals: { diff_file: diff_file }
             - else
               No preview for this file type
           %br

app/views/projects/diffs/_email_line.html.haml

+%tr.line_holder{ class: line.type }
+  - case line.type
+  - when 'match'
+    = diff_match_line line.old_pos, line.new_pos, text: line.text
+  - when 'old-nonewline', 'new-nonewline'
+    %td.old_line.diff-line-num
+    %td.new_line.diff-line-num
+    %td.line_content.match= line.text
+  - else
+    %td.old_line.diff-line-num{ class: line.type, data: { linenumber: line.old_pos } }
+      = diff_link_number(line.type, "new", line.old_pos)
+
+    %td.new_line.diff-line-num{ class: line.type, data: { linenumber: line.new_pos } }
+      = diff_link_number(line.type, "old", line.new_pos)
+
+    %td.line_content{ class: line.type }<
+      %pre= line.rich_text

app/views/projects/diffs/_line.html.haml

+-# This file is deprecated in favour of inline rendering:
+-# https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57237
 - plain = local_assigns.fetch(:plain, false)
 - discussions = local_assigns.fetch(:discussions, nil)
 - line_code = diff_file.line_code(line)

app/views/projects/diffs/_text_file.html.haml

@@ -4,10 +4,38 @@
     %a.show-suppressed-diff.cursor-pointer.js-show-suppressed-diff= _("Changes suppressed. Click to show.")
 
 %table.text-file.diff-wrap-lines.code.code-commit.js-syntax-highlight.commit-diff{ data: diff_view_data, class: too_big ? 'hide' : '' }
-  = render partial: "projects/diffs/line",
-    collection: diff_file.highlighted_diff_lines,
-    as: :line,
-    locals: { diff_file: diff_file, discussions: @grouped_diff_discussions }
+  - if Feature.enabled?(:inline_diff_line_rendering, @project, default_enabled: :yaml)
+    - diff_file.highlighted_diff_lines.each do |line|
+      - line_code = diff_file.line_code(line)
+
+      %tr.line_holder{ class: line.type, id: line_code }
+        - case line.type
+        - when 'match'
+          = diff_match_line line.old_pos, line.new_pos, text: line.text
+        - when 'old-nonewline', 'new-nonewline'
+          %td.old_line.diff-line-num
+          %td.new_line.diff-line-num
+          %td.line_content.match= line.text
+        - else
+          %td.old_line.diff-line-num{ class: "#{line.type} js-avatar-container", data: { linenumber: line.old_pos } }
+            = add_diff_note_button(line_code, diff_file.position(line), line.type)
+            %a{ href: "##{line_code}", data: { linenumber: diff_link_number(line.type, "new", line.old_pos) } }
+
+          %td.new_line.diff-line-num{ class: line.type, data: { linenumber: line.new_pos } }
+            %a{ href: "##{line_code}", data: { linenumber: diff_link_number(line.type, "old", line.new_pos) } }
+
+          %td.line_content{ class: line.type }<
+            = diff_line_content(line.rich_text)
+
+      - if line.discussable? && @grouped_diff_discussions.present? && @grouped_diff_discussions[line_code]
+        - line_discussions = @grouped_diff_discussions[line_code]
+        = render "discussions/diff_discussion", discussions: line_discussions, expanded: line_discussions.any?(&:expanded?)
+
+  - else
+    = render partial: "projects/diffs/line",
+      collection: diff_file.highlighted_diff_lines,
+      as: :line,
+      locals: { diff_file: diff_file, discussions: @grouped_diff_discussions }
 
   - if !diff_file.new_file? && !diff_file.deleted_file? && diff_file.highlighted_diff_lines.any?
     - last_line = diff_file.highlighted_diff_lines.last

config/feature_flags/development/inline_diff_line_rendering.yml

+---
+name: inline_diff_line_rendering
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57237
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330582
+milestone: '14.6'
+type: development
+group: group::source code
+default_enabled: false