Allow suggesting single line changes in diffs

app/assets/javascripts/api.js

@@ -25,6 +25,7 @@ const Api = {
   userStatusPath: '/api/:version/users/:id/status',
   userPostStatusPath: '/api/:version/user/status',
   commitPath: '/api/:version/projects/:id/repository/commits',
+  applySuggestionPath: '/api/:version/suggestions/:id/apply',
   commitPipelinesPath: '/:project_id/commit/:sha/pipelines',
   branchSinglePath: '/api/:version/projects/:id/repository/branches/:branch',
   createBranchPath: '/api/:version/projects/:id/repository/branches',
@@ -185,6 +186,12 @@ const Api = {
     });
   },
 
+  applySuggestion(id) {
+    const url = Api.buildUrl(Api.applySuggestionPath).replace(':id', encodeURIComponent(id));
+
+    return axios.put(url);
+  },
+
   commitPipelines(projectId, sha) {
     const encodedProjectId = projectId
       .split('/')

app/assets/javascripts/diffs/components/app.vue

@@ -42,6 +42,11 @@ export default {
       type: Object,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
     changesEmptyStateIllustration: {
       type: String,
       required: false,
@@ -208,6 +213,7 @@ export default {
               v-for="file in diffFiles"
               :key="file.newPath"
               :file="file"
+              :help-page-path="helpPagePath"
               :can-current-user-fork="canCurrentUserFork"
             />
           </template>

app/assets/javascripts/diffs/components/diff_content.vue

@@ -23,6 +23,11 @@ export default {
       type: Object,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     ...mapState({
@@ -74,11 +79,13 @@ export default {
           v-if="isInlineView"
           :diff-file="diffFile"
           :diff-lines="diffFile.highlighted_diff_lines || []"
+          :help-page-path="helpPagePath"
         />
         <parallel-diff-view
           v-if="isParallelView"
           :diff-file="diffFile"
           :diff-lines="diffFile.parallel_diff_lines || []"
+          :help-page-path="helpPagePath"
         />
       </template>
       <diff-viewer

app/assets/javascripts/diffs/components/diff_discussions.vue

@@ -13,6 +13,11 @@ export default {
       type: Array,
       required: true,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
     shouldCollapseDiscussions: {
       type: Boolean,
       required: false,
@@ -23,6 +28,11 @@ export default {
       required: false,
       default: false,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   methods: {
     ...mapActions(['toggleDiscussion']),
@@ -72,6 +82,8 @@ export default {
           :render-diff-file="false"
           :always-expanded="true"
           :discussions-by-diff-order="true"
+          :line="line"
+          :help-page-path="helpPagePath"
           @noteDeleted="deleteNoteHandler"
         >
           <span v-if="renderAvatarBadge" slot="avatar-badge" class="badge badge-pill">

app/assets/javascripts/diffs/components/diff_file.vue

@@ -23,6 +23,11 @@ export default {
       type: Boolean,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     return {
@@ -164,6 +169,7 @@ export default {
       v-if="!isCollapsed && file.renderIt"
       :class="{ hidden: isCollapsed || file.too_large }"
       :diff-file="file"
+      :help-page-path="helpPagePath"
     />
     <gl-loading-icon v-if="showLoadingIcon" class="diff-content loading" />
     <div v-else-if="showExpandMessage" class="nothing-here-block diff-collapsed">

app/assets/javascripts/diffs/components/diff_line_note_form.vue

@@ -94,6 +94,7 @@ export default {
       ref="noteForm"
       :is-editing="true"
       :line-code="line.line_code"
+      :line="line"
       save-button-title="Comment"
       class="diff-comment-form"
       @cancelForm="handleCancelCommentForm"

app/assets/javascripts/diffs/components/inline_diff_comment_row.vue

@@ -16,6 +16,11 @@ export default {
       type: String,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     className() {
@@ -38,7 +43,12 @@ export default {
   <tr v-if="shouldRender" :class="className" class="notes_holder">
     <td class="notes_content" colspan="3">
       <div class="content">
-        <diff-discussions v-if="line.discussions.length" :discussions="line.discussions" />
+        <diff-discussions
+          v-if="line.discussions.length"
+          :line="line"
+          :discussions="line.discussions"
+          :help-page-path="helpPagePath"
+        />
         <diff-line-note-form
           v-if="line.hasForm"
           :diff-file-hash="diffFileHash"

app/assets/javascripts/diffs/components/inline_diff_view.vue

@@ -17,6 +17,11 @@ export default {
       type: Array,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     ...mapGetters('diffs', ['commitId']),
@@ -47,6 +52,7 @@ export default {
           :key="`icr-${index}`"
           :diff-file-hash="diffFile.file_hash"
           :line="line"
+          :help-page-path="helpPagePath"
         />
       </template>
     </tbody>

app/assets/javascripts/diffs/components/parallel_diff_comment_row.vue

@@ -20,6 +20,11 @@ export default {
       type: Number,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     hasExpandedDiscussionOnLeft() {
@@ -87,6 +92,8 @@ export default {
         <diff-discussions
           v-if="line.left.discussions.length"
           :discussions="line.left.discussions"
+          :line="line.left"
+          :help-page-path="helpPagePath"
         />
       </div>
       <diff-line-note-form
@@ -102,6 +109,8 @@ export default {
         <diff-discussions
           v-if="line.right.discussions.length"
           :discussions="line.right.discussions"
+          :line="line.right"
+          :help-page-path="helpPagePath"
         />
       </div>
       <diff-line-note-form

app/assets/javascripts/diffs/components/parallel_diff_view.vue

@@ -17,6 +17,11 @@ export default {
       type: Array,
       required: true,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     ...mapGetters('diffs', ['commitId']),
@@ -49,6 +54,7 @@ export default {
             :line="line"
             :diff-file-hash="diffFile.file_hash"
             :line-index="index"
+            :help-page-path="helpPagePath"
           />
         </template>
       </tbody>

app/assets/javascripts/diffs/index.js

@@ -16,6 +16,7 @@ export default function initDiffsApp(store) {
       return {
         endpoint: dataset.endpoint,
         projectPath: dataset.projectPath,
+        helpPagePath: dataset.helpPagePath,
         currentUser: JSON.parse(dataset.currentUserData) || {},
         changesEmptyStateIllustration: dataset.changesEmptyStateIllustration,
       };
@@ -31,6 +32,7 @@ export default function initDiffsApp(store) {
           endpoint: this.endpoint,
           currentUser: this.currentUser,
           projectPath: this.projectPath,
+          helpPagePath: this.helpPagePath,
           shouldShow: this.activeTab === 'diffs',
           changesEmptyStateIllustration: this.changesEmptyStateIllustration,
         },

app/assets/javascripts/lib/utils/text_markdown.js

@@ -39,7 +39,14 @@ function blockTagText(text, textArea, blockTag, selected) {
   }
 }
 
-function moveCursor({ textArea, tag, positionBetweenTags, removedLastNewLine, select }) {
+function moveCursor({
+  textArea,
+  tag,
+  cursorOffset,
+  positionBetweenTags,
+  removedLastNewLine,
+  select,
+}) {
   var pos;
   if (!textArea.setSelectionRange) {
     return;
@@ -61,11 +68,24 @@ function moveCursor({ textArea, tag, positionBetweenTags, removedLastNewLine, se
       pos -= 1;
     }
 
+    if (cursorOffset) {
+      pos -= cursorOffset;
+    }
+
     return textArea.setSelectionRange(pos, pos);
   }
 }
 
-export function insertMarkdownText({ textArea, text, tag, blockTag, selected, wrap, select }) {
+export function insertMarkdownText({
+  textArea,
+  text,
+  tag,
+  cursorOffset,
+  blockTag,
+  selected,
+  wrap,
+  select,
+}) {
   var textToInsert,
     selectedSplit,
     startChar,
@@ -154,20 +174,30 @@ export function insertMarkdownText({ textArea, text, tag, blockTag, selected, wr
   return moveCursor({
     textArea,
     tag: tag.replace(textPlaceholder, selected),
+    cursorOffset,
     positionBetweenTags: wrap && selected.length === 0,
     removedLastNewLine,
     select,
   });
 }
 
-function updateText({ textArea, tag, blockTag, wrap, select }) {
+function updateText({ textArea, tag, cursorOffset, blockTag, wrap, select, tagContent }) {
   var $textArea, selected, text;
   $textArea = $(textArea);
   textArea = $textArea.get(0);
   text = $textArea.val();
-  selected = selectedText(text, textArea);
+  selected = selectedText(text, textArea) || tagContent;
   $textArea.focus();
-  return insertMarkdownText({ textArea, text, tag, blockTag, selected, wrap, select });
+  return insertMarkdownText({
+    textArea,
+    text,
+    tag,
+    cursorOffset,
+    blockTag,
+    selected,
+    wrap,
+    select,
+  });
 }
 
 export function addMarkdownListeners(form) {
@@ -178,9 +208,11 @@ export function addMarkdownListeners(form) {
       return updateText({
         textArea: $this.closest('.md-area').find('textarea'),
         tag: $this.data('mdTag'),
+        cursorOffset: $this.data('mdCursorOffset'),
         blockTag: $this.data('mdBlock'),
         wrap: !$this.data('mdPrepend'),
         select: $this.data('mdSelect'),
+        tagContent: $this.data('mdTagContent').toString(),
       });
     });
 }

app/assets/javascripts/mr_notes/index.js

@@ -33,6 +33,7 @@ export default function initMrNotes() {
         noteableData,
         currentUserData: JSON.parse(notesDataset.currentUserData),
         notesData: JSON.parse(notesDataset.notesData),
+        helpPagePath: notesDataset.helpPagePath,
       };
     },
     computed: {
@@ -71,6 +72,7 @@ export default function initMrNotes() {
           notesData: this.notesData,
           userData: this.currentUserData,
           shouldShow: this.activeTab === 'show',
+          helpPagePath: this.helpPagePath,
         },
       });
     },

app/assets/javascripts/notes/components/note_body.vue

 <script>
+import { mapActions } from 'vuex';
 import $ from 'jquery';
 import noteEditedText from './note_edited_text.vue';
 import noteAwardsList from './note_awards_list.vue';
 import noteAttachment from './note_attachment.vue';
 import noteForm from './note_form.vue';
 import autosave from '../mixins/autosave';
+import Suggestions from '~/vue_shared/components/markdown/suggestions.vue';
 
 export default {
   components: {
@@ -12,6 +14,7 @@ export default {
     noteAwardsList,
     noteAttachment,
     noteForm,
+    Suggestions,
   },
   mixins: [autosave],
   props: {
@@ -19,6 +22,11 @@ export default {
       type: Object,
       required: true,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
     canEdit: {
       type: Boolean,
       required: true,
@@ -28,11 +36,22 @@ export default {
       required: false,
       default: false,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   computed: {
     noteBody() {
       return this.note.note;
     },
+    hasSuggestion() {
+      return this.note.suggestions && this.note.suggestions.length;
+    },
+    lineType() {
+      return this.line ? this.line.type : null;
+    },
   },
   mounted() {
     this.renderGFM();
@@ -53,6 +72,7 @@ export default {
     }
   },
   methods: {
+    ...mapActions(['submitSuggestion']),
     renderGFM() {
       $(this.$refs['note-body']).renderGFM();
     },
@@ -62,19 +82,35 @@ export default {
     formCancelHandler(shouldConfirm, isDirty) {
       this.$emit('cancelForm', shouldConfirm, isDirty);
     },
+    applySuggestion({ suggestionId, flashContainer, callback }) {
+      const { discussion_id: discussionId, id: noteId } = this.note;
+
+      this.submitSuggestion({ discussionId, noteId, suggestionId, flashContainer, callback });
+    },
   },
 };
 </script>
 
 <template>
   <div ref="note-body" :class="{ 'js-task-list-container': canEdit }" class="note-body">
-    <div class="note-text md" v-html="note.note_html"></div>
+    <suggestions
+      v-if="hasSuggestion && !isEditing"
+      :suggestions="note.suggestions"
+      :note-html="note.note_html"
+      :line-type="lineType"
+      :help-page-path="helpPagePath"
+      @apply="applySuggestion"
+    />
+    <div v-else class="note-text md" v-html="note.note_html"></div>
     <note-form
       v-if="isEditing"
       ref="noteForm"
       :is-editing="isEditing"
       :note-body="noteBody"
       :note-id="note.id"
+      :line="line"
+      :note="note"
+      :help-page-path="helpPagePath"
       :markdown-version="note.cached_markdown_version"
       @handleFormUpdate="handleFormUpdate"
       @cancelForm="formCancelHandler"

app/assets/javascripts/notes/components/note_form.vue

 <script>
+import { mergeUrlParams } from '~/lib/utils/url_utility';
 import { mapGetters, mapActions } from 'vuex';
 import eventHub from '../event_hub';
 import issueWarning from '../../vue_shared/components/issue/issue_warning.vue';
@@ -53,6 +54,21 @@ export default {
       required: false,
       default: false,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
+    note: {
+      type: Object,
+      required: false,
+      default: null,
+    },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     return {
@@ -79,7 +95,8 @@ export default {
       return '#';
     },
     markdownPreviewPath() {
-      return this.getNoteableDataByProp('preview_note_path');
+      const notable = this.getNoteableDataByProp('preview_note_path');
+      return mergeUrlParams({ preview_suggestions: true }, notable);
     },
     markdownDocsPath() {
       return this.getNotesDataByProp('markdownDocsPath');
@@ -93,6 +110,18 @@ export default {
     isDisabled() {
       return !this.updatedNoteBody.length || this.isSubmitting;
     },
+    discussionNote() {
+      const discussionNote = this.discussion.id
+        ? this.getDiscussionLastNote(this.discussion)
+        : this.note;
+      return discussionNote || {};
+    },
+    canSuggest() {
+      return (
+        this.getNoteableData.can_receive_suggestion &&
+        (this.line && this.line.can_receive_suggestion)
+      );
+    },
   },
   watch: {
     noteBody() {
@@ -171,7 +200,11 @@ export default {
         :markdown-docs-path="markdownDocsPath"
         :markdown-version="markdownVersion"
         :quick-actions-docs-path="quickActionsDocsPath"
+        :line="line"
+        :note="discussionNote"
+        :can-suggest="canSuggest"
         :add-spacing-classes="false"
+        :help-page-path="helpPagePath"
       >
         <textarea
           id="note_note"

app/assets/javascripts/notes/components/noteable_discussion.vue

@@ -49,6 +49,11 @@ export default {
       type: Object,
       required: true,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
     renderDiffFile: {
       type: Boolean,
       required: false,
@@ -64,6 +69,11 @@ export default {
       required: false,
       default: false,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     const { diff_discussion: isDiffDiscussion, resolved } = this.discussion;
@@ -194,6 +204,13 @@ export default {
         false,
       );
     },
+    diffLine() {
+      if (this.discussion.diff_discussion && this.discussion.truncated_diff_lines) {
+        return this.discussion.truncated_diff_lines.slice(-1)[0];
+      }
+
+      return this.line;
+    },
   },
   watch: {
     isReplying() {
@@ -357,6 +374,8 @@ Please check your network connection and try again.`;
                   <component
                     :is="componentName(initialDiscussion)"
                     :note="componentData(initialDiscussion)"
+                    :line="line"
+                    :help-page-path="helpPagePath"
                     @handleDeleteNote="deleteNoteHandler"
                   >
                     <slot slot="avatar-badge" name="avatar-badge"></slot>
@@ -373,6 +392,8 @@ Please check your network connection and try again.`;
                       v-for="note in replies"
                       :key="note.id"
                       :note="componentData(note)"
+                      :help-page-path="helpPagePath"
+                      :line="line"
                       @handleDeleteNote="deleteNoteHandler"
                     />
                   </template>
@@ -383,6 +404,8 @@ Please check your network connection and try again.`;
                     v-for="(note, index) in discussion.notes"
                     :key="note.id"
                     :note="componentData(note)"
+                    :help-page-path="helpPagePath"
+                    :line="diffLine"
                     @handleDeleteNote="deleteNoteHandler"
                   >
                     <slot v-if="index === 0" slot="avatar-badge" name="avatar-badge"></slot>
@@ -447,6 +470,7 @@ Please check your network connection and try again.`;
                   ref="noteForm"
                   :discussion="discussion"
                   :is-editing="false"
+                  :line="diffLine"
                   save-button-title="Comment"
                   @handleFormUpdate="saveReply"
                   @cancelForm="cancelReplyForm"

app/assets/javascripts/notes/components/noteable_note.vue

@@ -27,6 +27,16 @@ export default {
       type: Object,
       required: true,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     return {
@@ -220,8 +230,10 @@ export default {
       <note-body
         ref="noteBody"
         :note="note"
+        :line="line"
         :can-edit="note.current_user.can_edit"
         :is-editing="isEditing"
+        :help-page-path="helpPagePath"
         @handleFormUpdate="formUpdateHandler"
         @cancelForm="formCancelHandler"
       />

app/assets/javascripts/notes/components/notes_app.vue

@@ -49,6 +49,11 @@ export default {
       required: false,
       default: 0,
     },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     return {
@@ -206,6 +211,7 @@ export default {
           :key="discussion.id"
           :discussion="discussion"
           :render-diff-file="true"
+          :help-page-path="helpPagePath"
         />
       </template>
     </ul>

app/assets/javascripts/notes/services/notes_service.js

 import Vue from 'vue';
+import Api from '~/api';
 import VueResource from 'vue-resource';
 import * as constants from '../constants';
 
@@ -44,4 +45,7 @@ export default {
   toggleIssueState(endpoint, data) {
     return Vue.http.put(endpoint, data);
   },
+  applySuggestion(id) {
+    return Api.applySuggestion(id);
+  },
 };

app/assets/javascripts/notes/stores/actions.js

@@ -405,5 +405,25 @@ export const startTaskList = ({ dispatch }) =>
 export const updateResolvableDiscussonsCounts = ({ commit }) =>
   commit(types.UPDATE_RESOLVABLE_DISCUSSIONS_COUNTS);
 
+export const submitSuggestion = (
+  { commit },
+  { discussionId, noteId, suggestionId, flashContainer, callback },
+) => {
+  service
+    .applySuggestion(suggestionId)
+    .then(() => {
+      commit(types.APPLY_SUGGESTION, { discussionId, noteId, suggestionId });
+      callback();
+    })
+    .catch(() => {
+      Flash(
+        __('Something went wrong while applying the suggestion. Please try again.'),
+        'alert',
+        flashContainer,
+      );
+      callback();
+    });
+};
+
 // prevent babel-plugin-rewire from generating an invalid default during karma tests
 export default () => {};

app/assets/javascripts/notes/stores/modules/index.js

@@ -20,6 +20,7 @@ export default () => ({
     userData: {},
     noteableData: {
       current_user: {},
+      preview_note_path: 'path/to/preview',
     },
     commentsDisabled: false,
     resolvableDiscussionsCount: 0,

app/assets/javascripts/notes/stores/mutation_types.js

@@ -16,6 +16,7 @@ export const SET_DISCUSSION_DIFF_LINES = 'SET_DISCUSSION_DIFF_LINES';
 export const SET_NOTES_FETCHED_STATE = 'SET_NOTES_FETCHED_STATE';
 export const SET_NOTES_LOADING_STATE = 'SET_NOTES_LOADING_STATE';
 export const DISABLE_COMMENTS = 'DISABLE_COMMENTS';
+export const APPLY_SUGGESTION = 'APPLY_SUGGESTION';
 
 // DISCUSSION
 export const COLLAPSE_DISCUSSION = 'COLLAPSE_DISCUSSION';

app/assets/javascripts/notes/stores/mutations.js

@@ -197,6 +197,17 @@ export default {
     }
   },
 
+  [types.APPLY_SUGGESTION](state, { noteId, discussionId, suggestionId }) {
+    const noteObj = utils.findNoteObjectById(state.discussions, discussionId);
+    const comment = utils.findNoteObjectById(noteObj.notes, noteId);
+
+    comment.suggestions = comment.suggestions.map(suggestion => ({
+      ...suggestion,
+      applied: suggestion.applied || suggestion.id === suggestionId,
+      appliable: false,
+    }));
+  },
+
   [types.UPDATE_DISCUSSION](state, noteData) {
     const note = noteData;
     const selectedDiscussion = state.discussions.find(disc => disc.id === note.id);

app/assets/javascripts/vue_shared/components/markdown/field.vue

 <script>
 import $ from 'jquery';
+import _ from 'underscore';
 import { __ } from '~/locale';
+import { stripHtml } from '~/lib/utils/text_utility';
 import Flash from '../../../flash';
 import GLForm from '../../../gl_form';
 import markdownHeader from './header.vue';
 import markdownToolbar from './toolbar.vue';
 import icon from '../icon.vue';
+import Suggestions from '~/vue_shared/components/markdown/suggestions.vue';
 
 export default {
   components: {
     markdownHeader,
     markdownToolbar,
     icon,
+    Suggestions,
   },
   props: {
     markdownPreviewPath: {
@@ -48,12 +52,33 @@ export default {
       required: false,
       default: true,
     },
+    line: {
+      type: Object,
+      required: false,
+      default: null,
+    },
+    note: {
+      type: Object,
+      required: false,
+      default: () => ({}),
+    },
+    canSuggest: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    helpPagePath: {
+      type: String,
+      required: false,
+      default: '',
+    },
   },
   data() {
     return {
       markdownPreview: '',
       referencedCommands: '',
       referencedUsers: '',
+      hasSuggestion: false,
       markdownPreviewLoading: false,
       previewMarkdown: false,
     };
@@ -63,6 +88,39 @@ export default {
       const referencedUsersThreshold = 10;
       return this.referencedUsers.length >= referencedUsersThreshold;
     },
+    lineContent() {
+      const FIRST_CHAR_REGEX = /^(\+|-)/;
+      const [firstSuggestion] = this.suggestions;
+      if (firstSuggestion) {
+        return firstSuggestion.from_content;
+      }
+
+      if (this.line) {
+        const { rich_text: richText, text } = this.line;
+
+        if (text) {
+          return text.replace(FIRST_CHAR_REGEX, '');
+        }
+
+        return _.unescape(stripHtml(richText).replace(/\n/g, ''));
+      }
+
+      return '';
+    },
+    lineNumber() {
+      let lineNumber;
+      if (this.line) {
+        const { new_line: newLine, old_line: oldLine } = this.line;
+        lineNumber = newLine || oldLine;
+      }
+      return lineNumber;
+    },
+    suggestions() {
+      return this.note.suggestions || [];
+    },
+    lineType() {
+      return this.line ? this.line.type : '';
+    },
   },
   mounted() {
     /*
@@ -122,6 +180,7 @@ export default {
       if (data.references) {
         this.referencedCommands = data.references.commands;
         this.referencedUsers = data.references.users;
+        this.hasSuggestion = data.references.suggestions && data.references.suggestions.length;
       }
 
       this.$nextTick(() => {
@@ -147,6 +206,8 @@ export default {
   >
     <markdown-header
       :preview-markdown="previewMarkdown"
+      :line-content="lineContent"
+      :can-suggest="canSuggest"
       @preview-markdown="showPreviewTab"
       @write-markdown="showWriteTab"
     />
@@ -163,19 +224,39 @@ export default {
         />
       </div>
     </div>
-    <div
-      v-show="previewMarkdown"
-      ref="markdown-preview"
-      class="md-preview js-vue-md-preview md md-preview-holder"
-      v-html="markdownPreview"
-    ></div>
+    <template v-if="hasSuggestion">
+      <div
+        v-show="previewMarkdown"
+        ref="markdown-preview"
+        class="md-preview js-vue-md-preview md md-preview-holder"
+      >
+        <suggestions
+          v-if="hasSuggestion"
+          :note-html="markdownPreview"
+          :from-line="lineNumber"
+          :from-content="lineContent"
+          :line-type="lineType"
+          :disabled="true"
+          :suggestions="suggestions"
+          :help-page-path="helpPagePath"
+        />
+      </div>
+    </template>
+    <template v-else>
+      <div
+        v-show="previewMarkdown"
+        ref="markdown-preview"
+        class="md-preview js-vue-md-preview md md-preview-holder"
+        v-html="markdownPreview"
+      ></div>
+    </template>
     <template v-if="previewMarkdown && !markdownPreviewLoading">
       <div v-if="referencedCommands" class="referenced-commands" v-html="referencedCommands"></div>
       <div v-if="shouldShowReferencedUsers" class="referenced-users">
         <span>
-          <i class="fa fa-exclamation-triangle" aria-hidden="true"> </i> You are about to add
+          <i class="fa fa-exclamation-triangle" aria-hidden="true"></i> You are about to add
           <strong>
-            <span class="js-referenced-users-count"> {{ referencedUsers.length }} </span>
+            <span class="js-referenced-users-count">{{ referencedUsers.length }}</span>
           </strong>
           people to the discussion. Proceed with caution.
         </span>

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -17,6 +17,16 @@ export default {
       type: Boolean,
       required: true,
     },
+    lineContent: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    canSuggest: {
+      type: Boolean,
+      required: false,
+      default: true,
+    },
   },
   computed: {
     mdTable() {
@@ -27,6 +37,9 @@ export default {
         '| cell | cell |',
       ].join('\n');
     },
+    mdSuggestion() {
+      return ['```suggestion', `{text}`, '```'].join('\n');
+    },
   },
   mounted() {
     $(document).on('markdown-preview:show.vue', this.previewMarkdownTab);
@@ -119,6 +132,16 @@ export default {
           :button-title="__('Add a table')"
           icon="table"
         />
+        <toolbar-button
+          v-if="canSuggest"
+          :tag="mdSuggestion"
+          :prepend="true"
+          :button-title="__('Insert suggestion')"
+          :cursor-offset="4"
+          :tag-content="lineContent"
+          icon="doc-code"
+          class="qa-suggestion-btn"
+        />
         <button
           v-gl-tooltip
           aria-label="Go full screen"

app/assets/javascripts/vue_shared/components/markdown/suggestion_diff.vue

+<script>
+import SuggestionDiffHeader from './suggestion_diff_header.vue';
+
+export default {
+  components: {
+    SuggestionDiffHeader,
+  },
+  props: {
+    newLines: {
+      type: Array,
+      required: true,
+    },
+    fromContent: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    fromLine: {
+      type: Number,
+      required: true,
+    },
+    suggestion: {
+      type: Object,
+      required: true,
+    },
+    disabled: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    helpPagePath: {
+      type: String,
+      required: true,
+    },
+  },
+  methods: {
+    applySuggestion(callback) {
+      this.$emit('apply', { suggestionId: this.suggestion.id, callback });
+    },
+  },
+};
+</script>
+
+<template>
+  <div>
+    <suggestion-diff-header
+      class="qa-suggestion-diff-header"
+      :can-apply="suggestion.appliable && suggestion.current_user.can_apply && !disabled"
+      :is-applied="suggestion.applied"
+      :help-page-path="helpPagePath"
+      @apply="applySuggestion"
+    />
+    <table class="mb-3 md-suggestion-diff">
+      <tbody>
+        <!-- Old Line -->
+        <tr class="line_holder old">
+          <td class="diff-line-num old_line qa-old-diff-line-number old">{{ fromLine }}</td>
+          <td class="diff-line-num new_line old"></td>
+          <td class="line_content old">
+            <span>{{ fromContent }}</span>
+          </td>
+        </tr>
+        <!-- New Line(s) -->
+        <tr v-for="(line, key) of newLines" :key="key" class="line_holder new">
+          <td class="diff-line-num old_line new"></td>
+          <td class="diff-line-num new_line qa-new-diff-line-number new">{{ line.lineNumber }}</td>
+          <td class="line_content new">
+            <span>{{ line.content }}</span>
+          </td>
+        </tr>
+      </tbody>
+    </table>
+  </div>
+</template>

app/assets/javascripts/vue_shared/components/markdown/suggestion_diff_header.vue

+<script>
+import Icon from '~/vue_shared/components/icon.vue';
+
+export default {
+  components: { Icon },
+  props: {
+    canApply: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    isApplied: {
+      type: Boolean,
+      required: true,
+      default: false,
+    },
+    helpPagePath: {
+      type: String,
+      required: true,
+    },
+  },
+  data() {
+    return {
+      isAppliedSuccessfully: false,
+      isApplying: false,
+    };
+  },
+  methods: {
+    applySuggestion() {
+      if (!this.canApply) return;
+      this.isApplying = true;
+      this.$emit('apply', this.applySuggestionCallback);
+    },
+    applySuggestionCallback() {
+      this.isApplying = false;
+    },
+  },
+};
+</script>
+
+<template>
+  <div class="md-suggestion-header border-bottom-0 mt-2">
+    <div class="qa-suggestion-diff-header font-weight-bold">
+      {{ __('Suggested change') }}
+      <a v-if="helpPagePath" :href="helpPagePath" :aria-label="__('Help')">
+        <icon name="question-o" css-classes="link-highlight" />
+      </a>
+    </div>
+    <span v-if="isApplied" class="badge badge-success">{{ __('Applied') }}</span>
+    <button
+      v-if="canApply"
+      type="button"
+      class="btn qa-apply-btn"
+      :disabled="isApplying"
+      @click="applySuggestion"
+    >
+      {{ __('Apply suggestion') }}
+    </button>
+  </div>
+</template>

app/assets/javascripts/vue_shared/components/markdown/suggestions.vue

+<script>
+import Vue from 'vue';
+import SuggestionDiff from './suggestion_diff.vue';
+import Flash from '~/flash';
+
+export default {
+  components: { SuggestionDiff },
+  props: {
+    fromLine: {
+      type: Number,
+      required: false,
+      default: 0,
+    },
+    fromContent: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    lineType: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    suggestions: {
+      type: Array,
+      required: false,
+      default: () => [],
+    },
+    noteHtml: {
+      type: String,
+      required: true,
+    },
+    disabled: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    helpPagePath: {
+      type: String,
+      required: true,
+    },
+  },
+  data() {
+    return {
+      isRendered: false,
+    };
+  },
+  watch: {
+    suggestions() {
+      this.reset();
+    },
+    noteHtml() {
+      this.reset();
+    },
+  },
+  mounted() {
+    this.renderSuggestions();
+  },
+  methods: {
+    renderSuggestions() {
+      // swaps out suggestion(s) markdown with rich diff components
+      // (while still keeping non-suggestion markdown in place)
+
+      if (!this.noteHtml) return;
+      const { container } = this.$refs;
+      const suggestionElements = container.querySelectorAll('.js-render-suggestion');
+
+      if (this.lineType === 'old') {
+        Flash('Unable to apply suggestions to a deleted line.', 'alert', this.$el);
+      }
+
+      suggestionElements.forEach((suggestionEl, i) => {
+        const suggestionParentEl = suggestionEl.parentElement;
+        const newLines = this.extractNewLines(suggestionParentEl);
+        const diffComponent = this.generateDiff(newLines, i);
+        diffComponent.$mount(suggestionParentEl);
+      });
+
+      this.isRendered = true;
+    },
+    extractNewLines(suggestionEl) {
+      // extracts the suggested lines from the markdown
+      // calculates a line number for each line
+
+      const FIRST_CHAR_REGEX = /^(\+|-)/;
+      const newLines = suggestionEl.querySelectorAll('.line');
+      const fromLine = this.suggestions.length ? this.suggestions[0].from_line : this.fromLine;
+      const lines = [];
+
+      newLines.forEach((line, i) => {
+        const content = `${line.innerText.replace(FIRST_CHAR_REGEX, '')}\n`;
+        const lineNumber = fromLine + i;
+        lines.push({ content, lineNumber });
+      });
+
+      return lines;
+    },
+    generateDiff(newLines, suggestionIndex) {
+      // generates the diff <suggestion-diff /> component
+      // all `suggestion` markdown will be swapped out by this component
+
+      const { suggestions, disabled, helpPagePath } = this;
+      const suggestion =
+        suggestions && suggestions[suggestionIndex] ? suggestions[suggestionIndex] : {};
+      const fromContent = suggestion.from_content || this.fromContent;
+      const fromLine = suggestion.from_line || this.fromLine;
+      const SuggestionDiffComponent = Vue.extend(SuggestionDiff);
+      const suggestionDiff = new SuggestionDiffComponent({
+        propsData: { newLines, fromLine, fromContent, disabled, suggestion, helpPagePath },
+      });
+
+      suggestionDiff.$on('apply', ({ suggestionId, callback }) => {
+        this.$emit('apply', { suggestionId, callback, flashContainer: this.$el });
+      });
+
+      return suggestionDiff;
+    },
+    reset() {
+      // resets the container HTML (replaces it with the updated noteHTML)
+      // calls `renderSuggestions` once the updated noteHTML is added to the DOM
+
+      this.$refs.container.innerHTML = this.noteHtml;
+      this.isRendered = false;
+      this.renderSuggestions();
+      this.$nextTick(() => this.renderSuggestions());
+    },
+  },
+};
+</script>
+
+<template>
+  <div>
+    <div class="flash-container mt-3"></div>
+    <div v-show="isRendered" ref="container" v-html="noteHtml"></div>
+  </div>
+</template>

app/assets/javascripts/vue_shared/components/markdown/toolbar_button.vue

@@ -37,6 +37,16 @@ export default {
       required: false,
       default: false,
     },
+    tagContent: {
+      type: String,
+      required: false,
+      default: '',
+    },
+    cursorOffset: {
+      type: Number,
+      required: false,
+      default: 0,
+    },
   },
 };
 </script>
@@ -45,8 +55,10 @@ export default {
   <button
     v-gl-tooltip
     :data-md-tag="tag"
+    :data-md-cursor-offset="cursorOffset"
     :data-md-select="tagSelect"
     :data-md-block="tagBlock"
+    :data-md-tag-content="tagContent"
     :data-md-prepend="prepend"
     :title="buttonTitle"
     :aria-label="buttonTitle"

app/assets/stylesheets/framework/markdown_area.scss

@@ -277,6 +277,27 @@
   }
 }
 
+.md-suggestion-diff {
+  display: table !important;
+  border: 1px solid $border-color !important;
+}
+
+.md-suggestion-header {
+  height: $suggestion-header-height;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  background-color: $gray-light;
+  border: 1px solid $border-color;
+  padding: $gl-padding;
+  border-radius: $border-radius-default $border-radius-default 0 0;
+
+  svg {
+    vertical-align: middle;
+    margin-bottom: 3px;
+  }
+}
+
 @include media-breakpoint-down(xs) {
   .atwho-view-ul {
     width: 350px;

app/assets/stylesheets/framework/variables.scss

@@ -252,6 +252,7 @@ $browserScrollbarSize: 10px;
  * Misc
  */
 $header-height: 40px;
+$suggestion-header-height: 46px;
 $ide-statusbar-height: 25px;
 $fixed-layout-width: 1280px;
 $limited-layout-width: 990px;

app/controllers/concerns/preview_markdown.rb

@@ -12,7 +12,7 @@ module PreviewMarkdown
       when 'wikis'    then { pipeline: :wiki, project_wiki: @project_wiki, page_slug: params[:id] }
       when 'snippets' then { skip_project_check: true }
       when 'groups'   then { group: group }
-      when 'projects' then { issuable_state_filter_enabled: true }
+      when 'projects' then projects_filter_params
       else {}
       end
 
@@ -22,9 +22,17 @@ module PreviewMarkdown
       body: view_context.markdown(result[:text], markdown_params),
       references: {
         users: result[:users],
+        suggestions: result[:suggestions],
         commands: view_context.markdown(result[:commands])
       }
     }
   end
+
+  def projects_filter_params
+    {
+      issuable_state_filter_enabled: true,
+      suggestions_filter_enabled: params[:preview_suggestions].present?
+    }
+  end
   # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/models/concerns/noteable.rb

@@ -26,6 +26,10 @@ module Noteable
     DiscussionNote.noteable_types.include?(base_class_name)
   end
 
+  def supports_suggestion?
+    false
+  end
+
   def discussions_rendered_on_frontend?
     false
   end

app/models/diff_note.rb

@@ -66,10 +66,23 @@ class DiffNote < Note
     self.original_position.diff_refs == diff_refs
   end
 
+  def supports_suggestion?
+    return false unless noteable.supports_suggestion? && on_text?
+    # We don't want to trigger side-effects of `diff_file` call.
+    return false unless file = fetch_diff_file
+    return false unless line = file.line_for_position(self.original_position)
+
+    line&.suggestible?
+  end
+
   def discussion_first_note?
     self == discussion.first_note
   end
 
+  def banzai_render_context(field)
+    super.merge(suggestions_filter_enabled: supports_suggestion?)
+  end
+
   private
 
   def enqueue_diff_file_creation_job

app/models/merge_request.rb

@@ -363,6 +363,11 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
+  def supports_suggestion?
+    # Should be `true` when removing the FF.
+    Suggestion.feature_enabled?
+  end
+
   # Calls `MergeWorker` to proceed with the merge process and
   # updates `merge_jid` with the MergeWorker#jid.
   # This helps tracking enqueued and ongoing merge jobs.

app/models/note.rb

@@ -69,6 +69,12 @@ class Note < ActiveRecord::Base
   belongs_to :last_edited_by, class_name: 'User'
 
   has_many :todos
+
+  # The delete_all definition is required here in order
+  # to generate the correct DELETE sql for
+  # suggestions.delete_all calls
+  has_many :suggestions, -> { order(:relative_order) },
+    inverse_of: :note, dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
   has_many :events, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_one :system_note_metadata
   has_one :note_diff_file, inverse_of: :diff_note, foreign_key: :diff_note_id
@@ -110,7 +116,7 @@ class Note < ActiveRecord::Base
   scope :inc_author, -> { includes(:author) }
   scope :inc_relations_for_view, -> do
     includes(:project, { author: :status }, :updated_by, :resolved_by, :award_emoji,
-             :system_note_metadata, :note_diff_file)
+             :system_note_metadata, :note_diff_file, :suggestions)
   end
 
   scope :with_notes_filter, -> (notes_filter) do
@@ -226,6 +232,10 @@ class Note < ActiveRecord::Base
     Gitlab::HookData::NoteBuilder.new(self).build
   end
 
+  def supports_suggestion?
+    false
+  end
+
   def for_commit?
     noteable_type == "Commit"
   end

app/models/suggestion.rb

+# frozen_string_literal: true
+
+class Suggestion < ApplicationRecord
+  FEATURE_FLAG = :diff_suggestions
+
+  belongs_to :note, inverse_of: :suggestions
+  validates :note, presence: true
+  validates :commit_id, presence: true, if: :applied?
+
+  delegate :original_position, :position, :diff_file,
+    :noteable, to: :note
+
+  def self.feature_enabled?
+    Feature.enabled?(FEATURE_FLAG)
+  end
+
+  def project
+    noteable.source_project
+  end
+
+  def branch
+    noteable.source_branch
+  end
+
+  # For now, suggestions only serve as a way to send patches that
+  # will change a single line (being able to apply multiple in the same place),
+  # which explains `from_line` and `to_line` being the same line.
+  # We'll iterate on that in https://gitlab.com/gitlab-org/gitlab-ce/issues/53310
+  # when allowing multi-line suggestions.
+  def from_line
+    position.new_line
+  end
+  alias_method :to_line, :from_line
+
+  def from_original_line
+    original_position.new_line
+  end
+  alias_method :to_original_line, :from_original_line
+
+  # `from_line_index` and `to_line_index` represents diff/blob line numbers in
+  # index-like way (N-1).
+  def from_line_index
+    from_line - 1
+  end
+  alias_method :to_line_index, :from_line_index
+
+  def appliable?
+    return false unless note.supports_suggestion?
+
+    !applied? &&
+      noteable.opened? &&
+      different_content? &&
+      note.active?
+  end
+
+  private
+
+  def different_content?
+    from_content != to_content
+  end
+end

app/policies/suggestion_policy.rb

+# frozen_string_literal: true
+
+class SuggestionPolicy < BasePolicy
+  delegate { @subject.project }
+
+  condition(:can_push_to_branch) do
+    Gitlab::UserAccess.new(@user, project: @subject.project).can_push_to_branch?(@subject.branch)
+  end
+
+  rule { can_push_to_branch }.enable :apply_suggestion
+end

app/serializers/diff_line_entity.rb

@@ -11,4 +11,6 @@ class DiffLineEntity < Grape::Entity
   expose :rich_text do |line|
     ERB::Util.html_escape(line.rich_text || line.text)
   end
+
+  expose :suggestible?, as: :can_receive_suggestion
 end

app/serializers/merge_request_widget_entity.rb

@@ -238,6 +238,8 @@ class MergeRequestWidgetEntity < IssuableEntity
     end
   end
 
+  expose :supports_suggestion?, as: :can_receive_suggestion
+
   private
 
   delegate :current_user, to: :request

app/serializers/note_entity.rb

@@ -36,6 +36,7 @@ class NoteEntity < API::Entities::Note
     end
   end
 
+  expose :suggestions, using: SuggestionEntity
   expose :resolved?, as: :resolved
   expose :resolvable?, as: :resolvable
 

app/serializers/suggestion_entity.rb

+# frozen_string_literal: true
+
+class SuggestionEntity < API::Entities::Suggestion
+  include RequestAwareEntity
+
+  expose :current_user do
+    expose :can_apply do |suggestion|
+      Ability.allowed?(current_user, :apply_suggestion, suggestion)
+    end
+  end
+
+  private
+
+  def current_user
+    request.current_user
+  end
+end

app/services/notes/create_service.rb

@@ -36,6 +36,7 @@ module Notes
       if !only_commands && note.save
         todo_service.new_note(note, current_user)
         clear_noteable_diffs_cache(note)
+        Suggestions::CreateService.new(note).execute
       end
 
       if command_params.present?

app/services/notes/update_service.rb

@@ -14,6 +14,17 @@ module Notes
         TodoService.new.update_note(note, current_user, old_mentioned_users)
       end
 
+      if note.supports_suggestion?
+        Suggestion.transaction do
+          note.suggestions.delete_all
+          Suggestions::CreateService.new(note).execute
+        end
+
+        # We need to refresh the previous suggestions call cache
+        # in order to get the new records.
+        note.reload
+      end
+
       note
     end
   end

app/services/preview_markdown_service.rb

@@ -4,10 +4,12 @@ class PreviewMarkdownService < BaseService
   def execute
     text, commands = explain_quick_actions(params[:text])
     users = find_user_references(text)
+    suggestions = find_suggestions(text)
 
     success(
       text: text,
       users: users,
+      suggestions: suggestions,
       commands: commands.join(' '),
       markdown_engine: markdown_engine
     )
@@ -28,6 +30,12 @@ class PreviewMarkdownService < BaseService
     extractor.users.map(&:username)
   end
 
+  def find_suggestions(text)
+    return [] unless params[:preview_suggestions]
+
+    Banzai::SuggestionsParser.parse(text)
+  end
+
   def find_commands_target
     QuickActions::TargetService
       .new(project, current_user)

app/services/suggestions/apply_service.rb

+# frozen_string_literal: true
+
+module Suggestions
+  class ApplyService < ::BaseService
+    def initialize(current_user)
+      @current_user = current_user
+    end
+
+    def execute(suggestion)
+      unless suggestion.appliable?
+        return error('Suggestion is not appliable')
+      end
+
+      params = file_update_params(suggestion)
+      result = ::Files::UpdateService.new(suggestion.project, @current_user, params).execute
+
+      if result[:status] == :success
+        suggestion.update(commit_id: result[:result], applied: true)
+      end
+
+      result
+    end
+
+    private
+
+    def file_update_params(suggestion)
+      diff_file = suggestion.diff_file
+
+      file_path = diff_file.file_path
+      branch_name = suggestion.noteable.source_branch
+      file_content = new_file_content(suggestion)
+      commit_message = "Apply suggestion to #{file_path}"
+
+      {
+        file_path: file_path,
+        branch_name: branch_name,
+        start_branch: branch_name,
+        commit_message: commit_message,
+        file_content: file_content
+      }
+    end
+
+    def new_file_content(suggestion)
+      range = suggestion.from_line_index..suggestion.to_line_index
+      blob = suggestion.diff_file.new_blob
+
+      blob.load_all_data!
+      content = blob.data.lines
+      content[range] = suggestion.to_content
+
+      content.join
+    end
+  end
+end

app/services/suggestions/create_service.rb

+# frozen_string_literal: true
+
+module Suggestions
+  class CreateService
+    def initialize(note)
+      @note = note
+    end
+
+    def execute
+      return unless @note.supports_suggestion?
+
+      suggestions = Banzai::SuggestionsParser.parse(@note.note)
+
+      # For single line suggestion we're only looking forward to
+      # change the line receiving the comment. Though, in
+      # https://gitlab.com/gitlab-org/gitlab-ce/issues/53310
+      # we'll introduce a ```suggestion:L<x>-<y>, so this will
+      # slightly change.
+      comment_line = @note.position.new_line
+
+      rows =
+        suggestions.map.with_index do |suggestion, index|
+          from_content = changing_lines(comment_line, comment_line)
+
+          # The parsed suggestion doesn't have information about the correct
+          # ending characters (we may have a line break, or not), so we take
+          # this information from the last line being changed (last
+          # characters).
+          endline_chars = line_break_chars(from_content.lines.last)
+          to_content = "#{suggestion}#{endline_chars}"
+
+          {
+            note_id: @note.id,
+            from_content: from_content,
+            to_content: to_content,
+            relative_order: index
+          }
+        end
+
+      rows.in_groups_of(100, false) do |rows|
+        Gitlab::Database.bulk_insert('suggestions', rows)
+      end
+    end
+
+    private
+
+    def changing_lines(from_line, to_line)
+      @note.diff_file.new_blob_lines_between(from_line, to_line).join
+    end
+
+    def line_break_chars(line)
+      match = /\r\n|\r|\n/.match(line)
+      match[0] if match
+    end
+  end
+end

app/views/projects/merge_requests/show.html.haml

@@ -67,6 +67,7 @@
                 noteable_data: serialize_issuable(@merge_request),
                 noteable_type: 'MergeRequest',
                 target_type: 'merge_request',
+                help_page_path: nil,
                 current_user_data: UserSerializer.new(project: @project).represent(current_user, {}, MergeRequestUserEntity).to_json} }
 
       #commits.commits.tab-pane
@@ -76,6 +77,7 @@
           = render 'projects/commit/pipelines_list', disable_initialization: true, endpoint: pipelines_project_merge_request_path(@project, @merge_request)
       #js-diffs-app.diffs.tab-pane{ data: { "is-locked" => @merge_request.discussion_locked?,
         endpoint: diffs_project_merge_request_path(@project, @merge_request, 'json', request.query_parameters),
+        help_page_path: nil,
         current_user_data: UserSerializer.new(project: @project).represent(current_user, {}, MergeRequestUserEntity).to_json,
         project_path: project_path(@merge_request.project),
         changes_empty_state_illustration: image_path('illustrations/merge_request_changes_empty.svg') } }

changelogs/unreleased/suggest-change-to-diff-line.yml

+---
+title: Add ability to render suggestions
+merge_request: 23147
+author:
+type: added

db/migrate/20181123144235_create_suggestions.rb

+# frozen_string_literal: true
+
+class CreateSuggestions < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def change
+    create_table :suggestions, id: :bigserial do |t|
+      t.references :note, foreign_key: { on_delete: :cascade }, null: false
+      t.integer :relative_order, null: false, limit: 2
+      t.boolean :applied, null: false, default: false
+      t.string :commit_id
+      t.text :from_content, null: false
+      t.text :to_content, null: false
+
+      t.index [:note_id, :relative_order],
+        name: 'index_suggestions_on_note_id_and_relative_order',
+        unique: true
+    end
+  end
+end

db/schema.rb

@@ -1956,6 +1956,16 @@ ActiveRecord::Schema.define(version: 20181204154019) do
     t.index ["subscribable_id", "subscribable_type", "user_id", "project_id"], name: "index_subscriptions_on_subscribable_and_user_id_and_project_id", unique: true, using: :btree
   end
 
+  create_table "suggestions", id: :bigserial, force: :cascade do |t|
+    t.integer "note_id", null: false
+    t.integer "relative_order", limit: 2, null: false
+    t.boolean "applied", default: false, null: false
+    t.string "commit_id"
+    t.text "from_content", null: false
+    t.text "to_content", null: false
+    t.index ["note_id", "relative_order"], name: "index_suggestions_on_note_id_and_relative_order", unique: true, using: :btree
+  end
+
   create_table "system_note_metadata", force: :cascade do |t|
     t.integer "note_id", null: false
     t.integer "commit_count"
@@ -2432,6 +2442,7 @@ ActiveRecord::Schema.define(version: 20181204154019) do
   add_foreign_key "services", "projects", name: "fk_71cce407f9", on_delete: :cascade
   add_foreign_key "snippets", "projects", name: "fk_be41fd4bb7", on_delete: :cascade
   add_foreign_key "subscriptions", "projects", on_delete: :cascade
+  add_foreign_key "suggestions", "notes", on_delete: :cascade
   add_foreign_key "system_note_metadata", "notes", name: "fk_d83a918cb1", on_delete: :cascade
   add_foreign_key "term_agreements", "application_setting_terms", column: "term_id"
   add_foreign_key "term_agreements", "users", on_delete: :cascade

lib/api/api.rb

@@ -149,6 +149,7 @@ module API
     mount ::API::Snippets
     mount ::API::Submodules
     mount ::API::Subscriptions
+    mount ::API::Suggestions
     mount ::API::SystemHooks
     mount ::API::Tags
     mount ::API::Templates

lib/api/entities.rb

@@ -1495,5 +1495,17 @@ module API
       expose :label, using: Entities::LabelBasic
       expose :action
     end
+
+    class Suggestion < Grape::Entity
+      expose :id
+      expose :from_original_line
+      expose :to_original_line
+      expose :from_line
+      expose :to_line
+      expose :appliable?, as: :appliable
+      expose :applied
+      expose :from_content
+      expose :to_content
+    end
   end
 end

lib/api/suggestions.rb

+# frozen_string_literal: true
+
+module API
+  class Suggestions < Grape::API
+    before { authenticate! }
+
+    resource :suggestions do
+      desc 'Apply suggestion patch in the Merge Request it was created' do
+        success Entities::Suggestion
+      end
+      params do
+        requires :id, type: String, desc: 'The suggestion ID'
+      end
+      put ':id/apply' do
+        suggestion = Suggestion.find_by_id(params[:id])
+
+        not_found! unless suggestion
+        authorize! :apply_suggestion, suggestion
+
+        result = ::Suggestions::ApplyService.new(current_user).execute(suggestion)
+
+        if result[:status] == :success
+          present suggestion, with: Entities::Suggestion, current_user: current_user
+        else
+          http_status = result[:http_status] || 400
+          render_api_error!(result[:message], http_status)
+        end
+      end
+    end
+  end
+end

lib/banzai/filter/suggestion_filter.rb

+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    class SuggestionFilter < HTML::Pipeline::Filter
+      # Class used for tagging elements that should be rendered
+      TAG_CLASS = 'js-render-suggestion'.freeze
+
+      def call
+        return doc unless Suggestion.feature_enabled?
+        return doc unless suggestions_filter_enabled?
+
+        doc.search('pre.suggestion > code').each do |node|
+          node.add_class(TAG_CLASS)
+        end
+
+        doc
+      end
+
+      def suggestions_filter_enabled?
+        context[:suggestions_filter_enabled]
+      end
+    end
+  end
+end

lib/banzai/filter/syntax_highlight_filter.rb

@@ -69,7 +69,7 @@ module Banzai
       end
 
       def use_rouge?(language)
-        %w(math mermaid plantuml).exclude?(language)
+        %w(math mermaid plantuml suggestion).exclude?(language)
       end
     end
   end

lib/banzai/pipeline/gfm_pipeline.rb

@@ -29,6 +29,7 @@ module Banzai
           Filter::TableOfContentsFilter,
           Filter::AutolinkFilter,
           Filter::ExternalLinkFilter,
+          Filter::SuggestionFilter,
 
           *reference_filters,
 

lib/banzai/pipeline/post_process_pipeline.rb

@@ -14,7 +14,8 @@ module Banzai
         [
           Filter::RedactorFilter,
           Filter::RelativeLinkFilter,
-          Filter::IssuableStateFilter
+          Filter::IssuableStateFilter,
+          Filter::SuggestionFilter
         ]
       end
 

lib/banzai/suggestions_parser.rb

+# frozen_string_literal: true
+
+module Banzai
+  module SuggestionsParser
+    # Returns the content of each suggestion code block.
+    #
+    def self.parse(text)
+      html = Banzai.render(text, project: nil, no_original_data: true)
+      doc = Nokogiri::HTML(html)
+
+      doc.search('pre.suggestion').map { |node| node.text }
+    end
+  end
+end

lib/gitlab/diff/file.rb

@@ -122,6 +122,16 @@ module Gitlab
         old_blob_lazy&.itself
       end
 
+      def new_blob_lines_between(from_line, to_line)
+        return [] unless new_blob
+
+        from_index = from_line - 1
+        to_index = to_line - 1
+
+        new_blob.load_all_data!
+        new_blob.data.lines[from_index..to_index]
+      end
+
       def content_sha
         new_content_sha || old_content_sha
       end

lib/gitlab/diff/line.rb

@@ -73,6 +73,10 @@ module Gitlab
         !meta?
       end
 
+      def suggestible?
+        !removed?
+      end
+
       def rich_text
         @parent_file.try(:highlight_lines!) if @parent_file && !@rich_text
 

lib/gitlab/usage_data.rb

@@ -85,6 +85,8 @@ module Gitlab
             releases: count(Release),
             remote_mirrors: count(RemoteMirror),
             snippets: count(Snippet),
+            suggestions: count(Suggestion),
+            todos: count(Todo),
             uploads: count(Upload),
             web_hooks: count(WebHook)
           }.merge(services_usage).merge(approximate_counts)

locale/gitlab.pot

@@ -657,6 +657,12 @@ msgstr ""
 msgid "Applications"
 msgstr ""
 
+msgid "Applied"
+msgstr ""
+
+msgid "Apply suggestion"
+msgstr ""
+
 msgid "Apr"
 msgstr ""
 
@@ -3597,6 +3603,9 @@ msgstr ""
 msgid "Input your repository URL"
 msgstr ""
 
+msgid "Insert suggestion"
+msgstr ""
+
 msgid "Install GitLab Runner"
 msgstr ""
 
@@ -6080,6 +6089,9 @@ msgstr ""
 msgid "Something went wrong when toggling the button"
 msgstr ""
 
+msgid "Something went wrong while applying the suggestion. Please try again."
+msgstr ""
+
 msgid "Something went wrong while closing the %{issuable}. Please try again later"
 msgstr ""
 
@@ -6347,6 +6359,9 @@ msgstr ""
 msgid "Subscribed"
 msgstr ""
 
+msgid "Suggested change"
+msgstr ""
+
 msgid "Switch branch/tag"
 msgstr ""
 

spec/db/schema_spec.rb

@@ -54,7 +54,8 @@ describe 'Database schema' do
     user_agent_details: %w[subject_id],
     users: %w[color_scheme_id created_by_id theme_id],
     users_star_projects: %w[user_id],
-    web_hooks: %w[service_id]
+    web_hooks: %w[service_id],
+    suggestions: %w[commit_id]
   }.with_indifferent_access.freeze
 
   context 'for table' do

spec/factories/suggestions.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :suggestion do
+    relative_order 0
+    association :note, factory: :diff_note_on_merge_request
+    from_content "    vars = {\n"
+    to_content "    vars = [\n"
+
+    trait :unappliable do
+      from_content "foo"
+      to_content "foo"
+    end
+
+    trait :applied do
+      applied true
+      commit_id { RepoHelpers.sample_commit.id }
+    end
+  end
+end

spec/features/merge_request/user_suggests_changes_on_diff_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'User comments on a diff', :js do
+  include MergeRequestDiffHelpers
+  include RepoHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:merge_request) do
+    create(:merge_request_with_diffs, source_project: project, target_project: project, source_branch: 'merge-test')
+  end
+  let(:user) { create(:user) }
+
+  before do
+    project.add_maintainer(user)
+    sign_in(user)
+
+    visit(diffs_project_merge_request_path(project, merge_request))
+  end
+
+  context 'single suggestion note' do
+    it 'suggestion is presented' do
+      click_diff_line(find("[id='#{sample_compare.changes[1][:line_code]}']"))
+
+      page.within('.js-discussion-note-form') do
+        fill_in('note_note', with: "```suggestion\n# change to a comment\n```")
+        click_button('Comment')
+      end
+
+      wait_for_requests
+
+      page.within('.diff-discussions') do
+        expect(page).to have_button('Apply suggestion')
+        expect(page).to have_content('Suggested change')
+        expect(page).to have_content('	url = https://github.com/gitlabhq/gitlab-shell.git')
+        expect(page).to have_content('# change to a comment')
+      end
+    end
+
+    it 'suggestion is appliable' do
+      click_diff_line(find("[id='#{sample_compare.changes[1][:line_code]}']"))
+
+      page.within('.js-discussion-note-form') do
+        fill_in('note_note', with: "```suggestion\n# change to a comment\n```")
+        click_button('Comment')
+      end
+
+      wait_for_requests
+
+      page.within('.diff-discussions') do
+        expect(page).not_to have_content('Applied')
+
+        click_button('Apply suggestion')
+        wait_for_requests
+
+        expect(page).to have_content('Applied')
+      end
+    end
+  end
+
+  context 'multiple suggestions in a single note' do
+    it 'suggestions are presented' do
+      click_diff_line(find("[id='#{sample_compare.changes[1][:line_code]}']"))
+
+      page.within('.js-discussion-note-form') do
+        fill_in('note_note', with: "```suggestion\n# change to a comment\n```\n```suggestion\n# or that\n```")
+        click_button('Comment')
+      end
+
+      wait_for_requests
+
+      page.within('.diff-discussions') do
+        suggestion_1 = page.all(:css, '.md-suggestion-diff')[0]
+        suggestion_2 = page.all(:css, '.md-suggestion-diff')[1]
+
+        expect(suggestion_1).to have_content('	url = https://github.com/gitlabhq/gitlab-shell.git')
+        expect(suggestion_1).to have_content('# change to a comment')
+
+        expect(suggestion_2).to have_content('	url = https://github.com/gitlabhq/gitlab-shell.git')
+        expect(suggestion_2).to have_content('# or that')
+      end
+    end
+  end
+end

spec/fixtures/api/schemas/entities/diff_line.json

@@ -8,7 +8,8 @@
     "new_line": { "type": ["integer", "null"] },
     "text": { "type": ["string"] },
     "rich_text": { "type": ["string"] },
-    "meta_data": { "type": ["object", "null"] }
+    "meta_data": { "type": ["object", "null"] },
+    "can_receive_suggestion": { "type": "boolean" }
   },
   "additionalProperties": false
 }

spec/fixtures/api/schemas/entities/merge_request_widget.json

@@ -119,7 +119,8 @@
     "can_push_to_source_branch": { "type": "boolean" },
     "rebase_path": { "type": ["string", "null"] },
     "squash": { "type": "boolean" },
-    "test_reports_path": { "type": ["string", "null"] }
+    "test_reports_path": { "type": ["string", "null"] },
+    "can_receive_suggestion": { "type": "boolean" }
   },
   "additionalProperties": false
 }

spec/javascripts/diffs/components/diff_content_spec.js

@@ -17,6 +17,7 @@ describe('DiffContent', () => {
       current_user: {
         can_create_note: false,
       },
+      preview_note_path: 'path/to/preview',
     };
 
     vm = mountComponentWithStore(Component, {

spec/javascripts/diffs/mock_data/diff_discussions.js

@@ -487,8 +487,19 @@ export default {
     ],
   },
   diff_discussion: true,
-  truncated_diff_lines:
-    '<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="1">\n1\n</td>\n<td class="line_content new noteable_line"><span id="LC1" class="line" lang="plaintext">  - Bad dates</span>\n</td>\n</tr>\n<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="2">\n2\n</td>\n<td class="line_content new noteable_line"><span id="LC2" class="line" lang="plaintext"></span>\n</td>\n</tr>\n',
+  truncated_diff_lines: [
+    {
+      text: 'line',
+      rich_text:
+        '<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="1">\n1\n</td>\n<td class="line_content new noteable_line"><span id="LC1" class="line" lang="plaintext">  - Bad dates</span>\n</td>\n</tr>\n<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="2">\n2\n</td>\n<td class="line_content new noteable_line"><span id="LC2" class="line" lang="plaintext"></span>\n</td>\n</tr>\n',
+      can_receive_suggestion: true,
+      line_code: '6f209374f7e565f771b95720abf46024c41d1885_1_1',
+      type: 'new',
+      old_line: null,
+      new_line: 1,
+      meta_data: null,
+    },
+  ],
 };
 
 export const imageDiffDiscussions = [

spec/javascripts/vue_shared/components/markdown/header_spec.js

@@ -28,6 +28,7 @@ describe('Markdown field header component', () => {
       'Add a numbered list',
       'Add a task list',
       'Add a table',
+      'Insert suggestion',
       'Go full screen',
     ];
     const elements = vm.$el.querySelectorAll('.toolbar-btn');
@@ -93,4 +94,18 @@ describe('Markdown field header component', () => {
       '| header | header |\n| ------ | ------ |\n| cell | cell |\n| cell | cell |',
     );
   });
+
+  it('renders suggestion template', () => {
+    vm.lineContent = 'Some content';
+
+    expect(vm.mdSuggestion).toEqual('```suggestion\n{text}\n```');
+  });
+
+  it('does not render suggestion button if `canSuggest` is set to false', () => {
+    vm.canSuggest = false;
+
+    Vue.nextTick(() => {
+      expect(vm.$el.querySelector('.qa-suggestion-btn')).toBe(null);
+    });
+  });
 });

spec/javascripts/vue_shared/components/markdown/suggestion_diff_header_spec.js

+import Vue from 'vue';
+import SuggestionDiffHeaderComponent from '~/vue_shared/components/markdown/suggestion_diff_header.vue';
+
+const MOCK_DATA = {
+  canApply: true,
+  isApplied: false,
+  helpPagePath: 'path_to_docs',
+};
+
+describe('Suggestion Diff component', () => {
+  let vm;
+
+  function createComponent(propsData) {
+    const Component = Vue.extend(SuggestionDiffHeaderComponent);
+
+    return new Component({
+      propsData,
+    }).$mount();
+  }
+
+  beforeEach(done => {
+    vm = createComponent(MOCK_DATA);
+    Vue.nextTick(done);
+  });
+
+  describe('init', () => {
+    it('renders a suggestion header', () => {
+      const header = vm.$el.querySelector('.qa-suggestion-diff-header');
+
+      expect(header).not.toBeNull();
+      expect(header.innerHTML.includes('Suggested change')).toBe(true);
+    });
+
+    it('renders an apply button', () => {
+      const applyBtn = vm.$el.querySelector('.qa-apply-btn');
+
+      expect(applyBtn).not.toBeNull();
+      expect(applyBtn.innerHTML.includes('Apply suggestion')).toBe(true);
+    });
+
+    it('does not render an apply button if `canApply` is set to false', () => {
+      const props = Object.assign(MOCK_DATA, { canApply: false });
+
+      vm = createComponent(props);
+
+      expect(vm.$el.querySelector('.qa-apply-btn')).toBeNull();
+    });
+  });
+
+  describe('applySuggestion', () => {
+    it('emits when the apply button is clicked', () => {
+      const props = Object.assign(MOCK_DATA, { canApply: true });
+
+      vm = createComponent(props);
+      spyOn(vm, '$emit');
+      vm.applySuggestion();
+
+      expect(vm.$emit).toHaveBeenCalled();
+    });
+
+    it('does not emit when the canApply is set to false', () => {
+      spyOn(vm, '$emit');
+      vm.canApply = false;
+      vm.applySuggestion();
+
+      expect(vm.$emit).not.toHaveBeenCalled();
+    });
+  });
+});

spec/javascripts/vue_shared/components/markdown/suggestion_diff_spec.js

+import Vue from 'vue';
+import SuggestionDiffComponent from '~/vue_shared/components/markdown/suggestion_diff.vue';
+
+const MOCK_DATA = {
+  canApply: true,
+  newLines: [
+    { content: 'Line 1\n', lineNumber: 1 },
+    { content: 'Line 2\n', lineNumber: 2 },
+    { content: 'Line 3\n', lineNumber: 3 },
+  ],
+  fromLine: 1,
+  fromContent: 'Old content',
+  suggestion: {
+    id: 1,
+  },
+  helpPagePath: 'path_to_docs',
+};
+
+describe('Suggestion Diff component', () => {
+  let vm;
+
+  beforeEach(done => {
+    const Component = Vue.extend(SuggestionDiffComponent);
+
+    vm = new Component({
+      propsData: MOCK_DATA,
+    }).$mount();
+
+    Vue.nextTick(done);
+  });
+
+  describe('init', () => {
+    it('renders a suggestion header', () => {
+      expect(vm.$el.querySelector('.qa-suggestion-diff-header')).not.toBeNull();
+    });
+
+    it('renders a diff table', () => {
+      expect(vm.$el.querySelector('table.md-suggestion-diff')).not.toBeNull();
+    });
+
+    it('renders the oldLineNumber', () => {
+      const fromLine = vm.$el.querySelector('.qa-old-diff-line-number').innerHTML;
+
+      expect(parseInt(fromLine, 10)).toBe(vm.fromLine);
+    });
+
+    it('renders the oldLineContent', () => {
+      const fromContent = vm.$el.querySelector('.line_content.old').innerHTML;
+
+      expect(fromContent.includes(vm.fromContent)).toBe(true);
+    });
+
+    it('renders the contents of newLines', () => {
+      const newLines = vm.$el.querySelectorAll('.line_holder.new');
+
+      newLines.forEach((line, i) => {
+        expect(newLines[i].innerHTML.includes(vm.newLines[i].content)).toBe(true);
+      });
+    });
+
+    it('renders a line number for each line', () => {
+      const newLineNumbers = vm.$el.querySelectorAll('.qa-new-diff-line-number');
+
+      newLineNumbers.forEach((line, i) => {
+        expect(newLineNumbers[i].innerHTML.includes(vm.newLines[i].lineNumber)).toBe(true);
+      });
+    });
+  });
+
+  describe('applySuggestion', () => {
+    it('emits apply event when applySuggestion is called', () => {
+      const callback = () => {};
+      spyOn(vm, '$emit');
+      vm.applySuggestion(callback);
+
+      expect(vm.$emit).toHaveBeenCalledWith('apply', { suggestionId: vm.suggestion.id, callback });
+    });
+  });
+});

spec/javascripts/vue_shared/components/markdown/suggestions_spec.js

+import Vue from 'vue';
+import SuggestionsComponent from '~/vue_shared/components/markdown/suggestions.vue';
+
+const MOCK_DATA = {
+  fromLine: 1,
+  fromContent: 'Old content',
+  suggestions: [],
+  noteHtml: `
+    <div class="suggestion">
+      <div class="line">Suggestion 1</div>
+    </div>    
+    
+    <div class="suggestion">
+      <div class="line">Suggestion 2</div>
+    </div>
+  `,
+  isApplied: false,
+  helpPagePath: 'path_to_docs',
+};
+
+const generateLine = content => {
+  const line = document.createElement('div');
+  line.className = 'line';
+  line.innerHTML = content;
+
+  return line;
+};
+
+const generateMockLines = () => {
+  const line1 = generateLine('Line 1');
+  const line2 = generateLine('Line 2');
+  const line3 = generateLine('Line 3');
+  const container = document.createElement('div');
+
+  container.appendChild(line1);
+  container.appendChild(line2);
+  container.appendChild(line3);
+
+  return container;
+};
+
+describe('Suggestion component', () => {
+  let vm;
+  let extractedLines;
+  let diffTable;
+
+  beforeEach(done => {
+    const Component = Vue.extend(SuggestionsComponent);
+
+    vm = new Component({
+      propsData: MOCK_DATA,
+    }).$mount();
+
+    extractedLines = vm.extractNewLines(generateMockLines());
+    diffTable = vm.generateDiff(extractedLines).$mount().$el;
+
+    spyOn(vm, 'renderSuggestions');
+    vm.renderSuggestions();
+    Vue.nextTick(done);
+  });
+
+  describe('mounted', () => {
+    it('renders a flash container', () => {
+      expect(vm.$el.querySelector('.flash-container')).not.toBeNull();
+    });
+
+    it('renders a container for suggestions', () => {
+      expect(vm.$refs.container).not.toBeNull();
+    });
+
+    it('renders suggestions', () => {
+      expect(vm.renderSuggestions).toHaveBeenCalled();
+      expect(vm.$el.innerHTML.includes('Suggestion 1')).toBe(true);
+      expect(vm.$el.innerHTML.includes('Suggestion 2')).toBe(true);
+    });
+  });
+
+  describe('extractNewLines', () => {
+    it('extracts suggested lines', () => {
+      const expectedReturn = [
+        { content: 'Line 1\n', lineNumber: 1 },
+        { content: 'Line 2\n', lineNumber: 2 },
+        { content: 'Line 3\n', lineNumber: 3 },
+      ];
+
+      expect(vm.extractNewLines(generateMockLines())).toEqual(expectedReturn);
+    });
+
+    it('increments line number for each extracted line', () => {
+      expect(extractedLines[0].lineNumber).toEqual(1);
+      expect(extractedLines[1].lineNumber).toEqual(2);
+      expect(extractedLines[2].lineNumber).toEqual(3);
+    });
+
+    it('returns empty array if no lines are found', () => {
+      const el = document.createElement('div');
+
+      expect(vm.extractNewLines(el)).toEqual([]);
+    });
+  });
+
+  describe('generateDiff', () => {
+    it('generates a diff table', () => {
+      expect(diffTable.querySelector('.md-suggestion-diff')).not.toBeNull();
+    });
+
+    it('generates a diff table that contains contents of `oldLineContent`', () => {
+      expect(diffTable.innerHTML.includes(vm.fromContent)).toBe(true);
+    });
+
+    it('generates a diff table that contains contents the suggested lines', () => {
+      extractedLines.forEach((line, i) => {
+        expect(diffTable.innerHTML.includes(extractedLines[i].content)).toBe(true);
+      });
+    });
+
+    it('generates a diff table with the correct line number for each suggested line', () => {
+      const lines = diffTable.getElementsByClassName('qa-new-diff-line-number');
+
+      expect([...lines][0].innerHTML).toBe('1');
+      expect([...lines][1].innerHTML).toBe('2');
+      expect([...lines][2].innerHTML).toBe('3');
+    });
+  });
+});

spec/lib/banzai/filter/suggestion_filter_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Banzai::Filter::SuggestionFilter do
+  include FilterSpecHelper
+
+  let(:input) { "<pre class='code highlight js-syntax-highlight suggestion'><code>foo\n</code></pre>" }
+  let(:default_context) do
+    { suggestions_filter_enabled: true }
+  end
+
+  it 'includes `js-render-suggestion` class' do
+    doc = filter(input, default_context)
+    result = doc.css('code').first
+
+    expect(result[:class]).to include('js-render-suggestion')
+  end
+
+  it 'includes no `js-render-suggestion` when feature disabled' do
+    stub_feature_flags(diff_suggestions: false)
+
+    doc = filter(input, default_context)
+    result = doc.css('code').first
+
+    expect(result[:class]).to be_nil
+  end
+
+  it 'includes no `js-render-suggestion` when filter is disabled' do
+    doc = filter(input)
+    result = doc.css('code').first
+
+    expect(result[:class]).to be_nil
+  end
+end

spec/lib/banzai/suggestions_parser_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Banzai::SuggestionsParser do
+  describe '.parse' do
+    it 'returns a list of suggestion contents' do
+      markdown = <<-MARKDOWN.strip_heredoc
+        ```suggestion
+          foo
+          bar
+        ```
+
+        ```
+          nothing
+        ```
+
+        ```suggestion
+          xpto
+          baz
+        ```
+
+        ```thing
+          this is not a suggestion, it's a thing
+        ```
+      MARKDOWN
+
+      expect(described_class.parse(markdown)).to eq(["  foo\n  bar",
+                                                     "  xpto\n  baz"])
+    end
+  end
+end

spec/lib/gitlab/import_export/all_models.yml

@@ -37,6 +37,7 @@ notes:
 - events
 - system_note_metadata
 - note_diff_file
+- suggestions
 label_links:
 - target
 - label

spec/lib/gitlab/usage_data_spec.rb

@@ -117,6 +117,7 @@ describe Gitlab::UsageData do
         releases
         remote_mirrors
         snippets
+        suggestions
         todos
         uploads
         web_hooks

spec/models/diff_note_spec.rb

@@ -318,6 +318,24 @@ describe DiffNote do
     end
   end
 
+  describe '#supports_suggestion?' do
+    context 'when noteable does not support suggestions' do
+      it 'returns false' do
+        allow(subject.noteable).to receive(:supports_suggestion?) { false }
+
+        expect(subject.supports_suggestion?).to be(false)
+      end
+    end
+
+    context 'when line is not suggestible' do
+      it 'returns false' do
+        allow_any_instance_of(Gitlab::Diff::Line).to receive(:suggestible?) { false }
+
+        expect(subject.supports_suggestion?).to be(false)
+      end
+    end
+  end
+
   describe "image diff notes" do
     let(:path) { "files/images/any_image.png" }
 

spec/models/suggestion_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Suggestion do
+  let(:suggestion) { create(:suggestion) }
+
+  describe 'associations' do
+    it { is_expected.to belong_to(:note) }
+  end
+
+  describe 'validations' do
+    it { is_expected.to validate_presence_of(:note) }
+
+    context 'when suggestion is applied' do
+      before do
+        allow(subject).to receive(:applied?).and_return(true)
+      end
+
+      it { is_expected.to validate_presence_of(:commit_id) }
+    end
+  end
+
+  describe '#appliable?' do
+    context 'when note does not support suggestions' do
+      it 'returns false' do
+        expect_next_instance_of(DiffNote) do |note|
+          allow(note).to receive(:supports_suggestion?) { false }
+        end
+
+        expect(suggestion).not_to be_appliable
+      end
+    end
+
+    context 'when patch is already applied' do
+      let(:suggestion) { create(:suggestion, :applied) }
+
+      it 'returns false' do
+        expect(suggestion).not_to be_appliable
+      end
+    end
+
+    context 'when merge request is not opened' do
+      let(:merge_request) { create(:merge_request, :merged) }
+      let(:note) do
+        create(:diff_note_on_merge_request, project: merge_request.project,
+                                            noteable: merge_request)
+      end
+
+      let(:suggestion) { create(:suggestion, note: note) }
+
+      it 'returns false' do
+        expect(suggestion).not_to be_appliable
+      end
+    end
+  end
+end

spec/requests/api/suggestions_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe API::Suggestions do
+  let(:project) { create(:project, :repository) }
+  let(:user) { create(:user) }
+
+  let(:merge_request) do
+    create(:merge_request, source_project: project,
+                           target_project: project)
+  end
+
+  let(:position) do
+    Gitlab::Diff::Position.new(old_path: "files/ruby/popen.rb",
+                               new_path: "files/ruby/popen.rb",
+                               old_line: nil,
+                               new_line: 9,
+                               diff_refs: merge_request.diff_refs)
+  end
+
+  let(:diff_note) do
+    create(:diff_note_on_merge_request, noteable: merge_request,
+                                        position: position,
+                                        project: project)
+  end
+
+  describe "PUT /suggestions/:id/apply" do
+    let(:url) { "/suggestions/#{suggestion.id}/apply" }
+
+    context 'when successfully applies patch' do
+      let(:suggestion) do
+        create(:suggestion, note: diff_note,
+                            from_content: "      raise RuntimeError, \"System commands must be given as an array of strings\"\n",
+                            to_content: "      raise RuntimeError, 'Explosion'\n      # explosion?")
+      end
+
+      it 'returns 200 with json content' do
+        project.add_maintainer(user)
+
+        put api(url, user), id: suggestion.id
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response)
+          .to include('id', 'from_original_line', 'to_original_line',
+                      'from_line', 'to_line', 'appliable', 'applied',
+                      'from_content', 'to_content')
+      end
+    end
+
+    context 'when not able to apply patch' do
+      let(:suggestion) do
+        create(:suggestion, :unappliable, note: diff_note)
+      end
+
+      it 'returns 400 with json content' do
+        project.add_maintainer(user)
+
+        put api(url, user), id: suggestion.id
+
+        expect(response).to have_gitlab_http_status(400)
+        expect(json_response).to eq({ 'message' => 'Suggestion is not appliable' })
+      end
+    end
+
+    context 'when unauthorized' do
+      let(:suggestion) do
+        create(:suggestion, note: diff_note,
+                            from_content: "      raise RuntimeError, \"System commands must be given as an array of strings\"\n",
+                            to_content: "      raise RuntimeError, 'Explosion'\n      # explosion?")
+      end
+
+      it 'returns 403 with json content' do
+        project.add_reporter(user)
+
+        put api(url, user), id: suggestion.id
+
+        expect(response).to have_gitlab_http_status(403)
+        expect(json_response).to eq({ 'message' => '403 Forbidden' })
+      end
+    end
+  end
+end

spec/serializers/suggestion_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe SuggestionEntity do
+  include RepoHelpers
+
+  let(:user) { create(:user) }
+  let(:request) { double('request', current_user: user) }
+  let(:suggestion) { create(:suggestion) }
+  let(:entity) { described_class.new(suggestion, request: request) }
+
+  subject { entity.as_json }
+
+  it 'exposes correct attributes' do
+    expect(subject).to include(:id, :from_original_line, :to_original_line, :from_line,
+                               :to_line, :appliable, :applied, :from_content, :to_content)
+  end
+
+  it 'exposes current user abilities' do
+    expect(subject[:current_user]).to include(:can_apply)
+  end
+end

spec/services/notes/update_service_spec.rb

@@ -20,6 +20,29 @@ describe Notes::UpdateService do
       @note.reload
     end
 
+    context 'suggestions' do
+      it 'refreshes note suggestions' do
+        markdown = <<-MARKDOWN.strip_heredoc
+          ```suggestion
+            foo
+          ```
+
+          ```suggestion
+            bar
+          ```
+        MARKDOWN
+
+        suggestion = create(:suggestion)
+        note = suggestion.note
+
+        expect { described_class.new(project, user, note: markdown).execute(note) }
+          .to change { note.suggestions.count }.from(1).to(2)
+
+        expect(note.suggestions.order(:relative_order).map(&:to_content))
+          .to eq(["  foo\n", "  bar\n"])
+      end
+    end
+
     context 'todos' do
       let!(:todo) { create(:todo, :assigned, user: user, project: project, target: issue, author: user2) }
 

spec/services/preview_markdown_service_spec.rb

@@ -19,6 +19,31 @@ describe PreviewMarkdownService do
     end
   end
 
+  describe 'suggestions' do
+    let(:params) { { text: "```suggestion\nfoo\n```", preview_suggestions: preview_suggestions } }
+    let(:service) { described_class.new(project, user, params) }
+
+    context 'when preview markdown param is present' do
+      let(:preview_suggestions) { true }
+
+      it 'returns users referenced in text' do
+        result = service.execute
+
+        expect(result[:suggestions]).to eq(['foo'])
+      end
+    end
+
+    context 'when preview markdown param is not present' do
+      let(:preview_suggestions) { false }
+
+      it 'returns users referenced in text' do
+        result = service.execute
+
+        expect(result[:suggestions]).to eq([])
+      end
+    end
+  end
+
   context 'new note with quick actions' do
     let(:issue) { create(:issue, project: project) }
     let(:params) do

spec/services/suggestions/apply_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Suggestions::ApplyService do
+  include ProjectForksHelper
+
+  let(:project) { create(:project, :repository) }
+  let(:user) { create(:user, :commit_email) }
+
+  let(:position) do
+    Gitlab::Diff::Position.new(old_path: "files/ruby/popen.rb",
+                               new_path: "files/ruby/popen.rb",
+                               old_line: nil,
+                               new_line: 9,
+                               diff_refs: merge_request.diff_refs)
+  end
+
+  let(:suggestion) do
+    create(:suggestion, note: diff_note,
+                        from_content: "      raise RuntimeError, \"System commands must be given as an array of strings\"\n",
+                        to_content: "      raise RuntimeError, 'Explosion'\n      # explosion?\n")
+  end
+
+  subject { described_class.new(user) }
+
+  context 'patch is appliable' do
+    let(:expected_content) do
+      <<-CONTENT.strip_heredoc
+          require 'fileutils'
+          require 'open3'
+
+          module Popen
+            extend self
+
+            def popen(cmd, path=nil)
+              unless cmd.is_a?(Array)
+                raise RuntimeError, 'Explosion'
+                # explosion?
+              end
+
+              path ||= Dir.pwd
+
+              vars = {
+                "PWD" => path
+              }
+
+              options = {
+                chdir: path
+              }
+
+              unless File.directory?(path)
+                FileUtils.mkdir_p(path)
+              end
+
+              @cmd_output = ""
+              @cmd_status = 0
+
+              Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
+                @cmd_output << stdout.read
+                @cmd_output << stderr.read
+                @cmd_status = wait_thr.value.exitstatus
+              end
+
+              return @cmd_output, @cmd_status
+            end
+          end
+      CONTENT
+    end
+
+    context 'non-fork project' do
+      let(:merge_request) do
+        create(:merge_request, source_project: project,
+                               target_project: project)
+      end
+
+      let!(:diff_note) do
+        create(:diff_note_on_merge_request, noteable: merge_request,
+                                            position: position,
+                                            project: project)
+      end
+
+      before do
+        project.add_maintainer(user)
+      end
+
+      it 'updates the file with the new contents' do
+        subject.execute(suggestion)
+
+        blob = project.repository.blob_at_branch(merge_request.source_branch,
+                                                 position.new_path)
+
+        expect(blob.data).to eq(expected_content)
+      end
+
+      it 'returns success status' do
+        result = subject.execute(suggestion)
+
+        expect(result[:status]).to eq(:success)
+      end
+
+      it 'updates suggestion applied and commit_id columns' do
+        expect { subject.execute(suggestion) }
+          .to change(suggestion, :applied)
+          .from(false).to(true)
+          .and change(suggestion, :commit_id)
+          .from(nil)
+      end
+
+      it 'created commit has users email and name' do
+        subject.execute(suggestion)
+
+        commit = project.repository.commit
+
+        expect(user.commit_email).not_to eq(user.email)
+        expect(commit.author_email).to eq(user.commit_email)
+        expect(commit.committer_email).to eq(user.commit_email)
+        expect(commit.author_name).to eq(user.name)
+      end
+    end
+
+    context 'fork-project' do
+      let(:project) { create(:project, :public, :repository) }
+
+      let(:forked_project) do
+        fork_project_with_submodules(project, user)
+      end
+
+      let(:merge_request) do
+        create(:merge_request,
+               source_branch: 'conflict-resolvable-fork', source_project: forked_project,
+               target_branch: 'conflict-start', target_project: project)
+      end
+
+      let!(:diff_note) do
+        create(:diff_note_on_merge_request, noteable: merge_request, position: position, project: project)
+      end
+
+      before do
+        project.add_maintainer(user)
+      end
+
+      it 'updates file in the source project' do
+        expect(Files::UpdateService).to receive(:new)
+          .with(merge_request.source_project, user, anything)
+          .and_call_original
+
+        subject.execute(suggestion)
+      end
+    end
+  end
+
+  context 'no permission' do
+    let(:merge_request) do
+      create(:merge_request, source_project: project,
+                             target_project: project)
+    end
+
+    let(:diff_note) do
+      create(:diff_note_on_merge_request, noteable: merge_request,
+                                          position: position,
+                                          project: project)
+    end
+
+    context 'user cannot write in project repo' do
+      before do
+        project.add_reporter(user)
+      end
+
+      it 'returns error' do
+        result = subject.execute(suggestion)
+
+        expect(result).to eq(message: "You are not allowed to push into this branch",
+                             status: :error)
+      end
+    end
+  end
+
+  context 'patch is not appliable' do
+    let(:merge_request) do
+      create(:merge_request, source_project: project,
+                             target_project: project)
+    end
+
+    let(:diff_note) do
+      create(:diff_note_on_merge_request, noteable: merge_request,
+                                          position: position,
+                                          project: project)
+    end
+
+    before do
+      project.add_maintainer(user)
+    end
+
+    context 'suggestion was already applied' do
+      it 'returns success status' do
+        result = subject.execute(suggestion)
+
+        expect(result[:status]).to eq(:success)
+      end
+    end
+
+    context 'note is outdated' do
+      before do
+        allow(diff_note).to receive(:active?) { false }
+      end
+
+      it 'returns error message' do
+        result = subject.execute(suggestion)
+
+        expect(result).to eq(message: 'Suggestion is not appliable',
+                             status: :error)
+      end
+    end
+
+    context 'suggestion was already applied' do
+      before do
+        suggestion.update!(applied: true, commit_id: 'sha')
+      end
+
+      it 'returns error message' do
+        result = subject.execute(suggestion)
+
+        expect(result).to eq(message: 'Suggestion is not appliable',
+                             status: :error)
+      end
+    end
+  end
+end

spec/services/suggestions/create_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Suggestions::CreateService do
+  let(:project_with_repo) { create(:project, :repository) }
+  let(:merge_request) do
+    create(:merge_request, source_project: project_with_repo,
+                           target_project: project_with_repo)
+  end
+
+  let(:position) do
+    Gitlab::Diff::Position.new(old_path: "files/ruby/popen.rb",
+                               new_path: "files/ruby/popen.rb",
+                               old_line: nil,
+                               new_line: 14,
+                               diff_refs: merge_request.diff_refs)
+  end
+
+  let(:markdown) do
+    <<-MARKDOWN.strip_heredoc
+        ```suggestion
+          foo
+          bar
+        ```
+
+        ```
+          nothing
+        ```
+
+        ```suggestion
+          xpto
+          baz
+        ```
+
+        ```thing
+          this is not a suggestion, it's a thing
+        ```
+    MARKDOWN
+  end
+
+  subject { described_class.new(note) }
+
+  describe '#execute' do
+    context 'should not try to parse suggestions' do
+      context 'when not a diff note for merge requests' do
+        let(:note) do
+          create(:diff_note_on_commit, project: project_with_repo,
+                                       note: markdown)
+        end
+
+        it 'does not try to parse suggestions' do
+          expect(Banzai::SuggestionsParser).not_to receive(:parse)
+
+          subject.execute
+        end
+      end
+
+      context 'when diff note is not for text' do
+        let(:note) do
+          create(:diff_note_on_merge_request, project: project_with_repo,
+                                              noteable: merge_request,
+                                              position: position,
+                                              note: markdown)
+        end
+
+        it 'does not try to parse suggestions' do
+          allow(note).to receive(:on_text?) { false }
+
+          expect(Banzai::SuggestionsParser).not_to receive(:parse)
+
+          subject.execute
+        end
+      end
+    end
+
+    context 'should create suggestions' do
+      let(:note) do
+        create(:diff_note_on_merge_request, project: project_with_repo,
+                                            noteable: merge_request,
+                                            position: position,
+                                            note: markdown)
+      end
+
+      context 'single line suggestions' do
+        it 'persists suggestion records' do
+          expect { subject.execute }
+            .to change { note.suggestions.count }
+            .from(0)
+            .to(2)
+        end
+
+        it 'persists original from_content lines and suggested lines' do
+          subject.execute
+
+          suggestions = note.suggestions.order(:relative_order)
+
+          suggestion_1 = suggestions.first
+          suggestion_2 = suggestions.last
+
+          expect(suggestion_1).to have_attributes(from_content: "    vars = {\n",
+                                                  to_content: "  foo\n  bar\n")
+
+          expect(suggestion_2).to have_attributes(from_content: "    vars = {\n",
+                                                  to_content: "  xpto\n  baz\n")
+        end
+      end
+    end
+  end
+end