Defines a command wrapper that prints the changes the command makes to the environment.

whatsnew in
theorem foo : 42 = 6 * 7 := rfl

def Mathlib.WhatsNew.diffExtension (old new : Lean.Environment) (ext : Lean.PersistentEnvExtension Lean.EnvExtensionEntry Lean.EnvExtensionEntry Lean.EnvExtensionState) : Lean.CoreM (Option Lean.MessageData)

def Mathlib.WhatsNew.whatsNew (old new : Lean.Environment) : Lean.CoreM Lean.MessageData

def Mathlib.WhatsNew.commandWhatsnewIn__ : Lean.ParserDescr

whatsnew in $command executes the command and then prints the declarations that were added to the environment.