set object.property = value

[geek-merlin]

With php's property accessors, property setters will go the way of the dinos.
Property write access will be the idiomatic way.

So Twig should support set object.property = value.

[GromNaN]

What is your use-case of setting a nested object value? You can already assign an object to a variable and merge objects (example)

[rgpublic]

It will be useful for a lot of things. I need it all the time to set properties of objects. Whenever you are dealing with objects in Twig that you got from Drupal via a preprocess function and want to set a property instead of calling a method. The complicated syntax {% set foo = foo|merge({bar: 'quz'}) %} just to set a property is unnecessarily complicated, slow and verbose.