12 files declare their use of Breadcrumb

BookBreadcrumbBuilder.php in core/modules/book/src/BookBreadcrumbBuilder.php
BreadcrumbManagerTest.php in core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbManagerTest.php
BreadcrumbTest.php in core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbTest.php
CommentBreadcrumbBuilder.php in core/modules/comment/src/CommentBreadcrumbBuilder.php
ForumBreadcrumbBuilderBase.php in core/modules/forum/src/Breadcrumb/ForumBreadcrumbBuilderBase.php
HelpBreadcrumbBuilder.php in core/modules/help_topics/src/HelpBreadcrumbBuilder.php
LayoutBuilderBreadcrumbAlterTest.php in core/modules/layout_builder/tests/src/Kernel/LayoutBuilderBreadcrumbAlterTest.php
layout_builder.module in core/modules/layout_builder/layout_builder.module
Provides hook implementations for Layout Builder.
layout_builder_test.module in core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.module
Provides hook implementations for Layout Builder tests.
menu_ui.module in core/modules/menu_ui/menu_ui.module
Allows administrators to customize the site's navigation menus.
PathBasedBreadcrumbBuilder.php in core/modules/system/src/PathBasedBreadcrumbBuilder.php
TermBreadcrumbBuilder.php in core/modules/taxonomy/src/TermBreadcrumbBuilder.php

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