Because you don’t control third party libraries

A scope groups the initialization visually together, while adding the let app = app; feels like it just adds clutter - I’d probably just leave it mut in that case.

You can have setters that set private fields, there are also sometimes structs with mixed private and public fields

Yeah if you have the second option, use it, but if the struct has private fields it won’t work.

If you’re ever forced to do something the second way, you can also wrap it in braces, that way you end up with an immutable value again:

let app = {
  let mut app = ...
  ...
  app
};

Definitely the second one.

1. It avoids Mut
2. It makes clear that the initialization is over at the end of of the statement. The first option invites people to change some more properties hundreds of lines down where you won’t see them.