Configuration
variable_get() has been removed.
Documentation
Files Affected
Entity API
node_load() has been removed.
Documentation
Files Affected
Form API
form_set_error() is now a method of FormStateInterface.
Documentation
Files Affected
The signature of hook_form_alter() has changed in Drupal 8.
Documentation
Files Affected
Info File
Module info files' core key must have a value of 8.x.
Documentation
Files Affected
Info files must contain a type key.
Documentation
Files Affected
Many common dependencies have moved into core.
Documentation
Files Affected
Menu/Routing
hook_menu() has been removed from Drupal 8.
Documentation
Files Affected
Miscellaneous
Many common functions, shared variables, and constants have been renamed.
Documentation
Automated web tests must be in a PSR-4 namespace, and unit tests must be converted to PHPUnit.
Documentation
Rendering
element_children() is now a method of the Element class.
Documentation
Files Affected
System
user_access() is now AccountInterface::hasPermission().
Documentation
Files Affected
Static permissions are now defined in MODULE.permissions.yml.
Documentation
Files Affected

This issue can be fixed automatically.