Issue #2463321 by amateescu: Serializing the database connection is dangerous and error-prone, make it unserializable again

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2364157 by mpdonadio, dawehner, martin107, Berdir, pcambra, naveenvalecha, tim.plunkett: Replace most existing _url calls with Url objects

Issue #2309737 by rpayanm, hussainweb, max-kuzomko, herom, millerbennett, ianthomas_uk, quietone, Sutharsan, sumitmadan, toddmbloom: Remove deprecated format_plural usage

Issue #2226863 by cs_shadow, filijonka, joelpittet, pguillard: Update stale references to theme functions that have been converted to Twig

Issue #2362227 by dawehner, mpdonadio, znerol, tim.plunkett, skipyT: Replace all instances of current_path()

Issue #2329901 by lanchez, lauriii | davidhernandez: Move form classes from preprocess to templates.

Issue #2343661 by larowlan, tim.plunkett | xjm: Rename l() to _l() and url() to _url(), and document replacements.

Issue #2337709 by tim.plunkett: Convert form_state_values_clean() to FormStateInterface::cleanValues().

Issue #2335659 by tim.plunkett, dawehner, effulgentsia, sun: Remove FormState ArrayAccess usage from core.

Issue #2283675 by er.pushpinderrana, amitgoyal, mparker17, joachim, mmarquez: Document how optgroups are generated in form_select_options()

Issue #2334405 by almaudoh, drunken monkey: Clean up documentation links to functions moved to render element classes

Issue #2300875 by joshi.rohit100, amitgoyal, penyaskito: Remove drupal_redirect_form() as it is deprecated.

Issue #2152227 by JeroenT, joelpittet, steinmb, steveoliver, hussainweb, shanethehat, jenlampton, kpa, AnythonyR, EVIIILJ, kgoel, Cottser, dsdeiz, hanpersand: Convert theme_tableselect() to #theme table__tableselect.

Issue #2301593 by er.pushpinderrana, joshi.rohit100: Remove drupal_retrieve_form() as it is deprecated.

Issue #314385 by penyaskito, Pancho, mgifford, jjcarrion, jlbellido, akalata, alimac, jday, Risse, stBorchert | emmajane: Make position of #description configurable via the API.

Issue #2318099 by tim.plunkett: Replace $form_state['complete_form'] with $form_state->getCompleteForm().

Issue #2313397 by tim.plunkett: Replace FormBuilderInterface::setValue() with FormStateInterface::setValueForElement().