Answer by Andrea Proone for How to update an existing object with Diesel? "`T: Identifiable` is not satisfied"

From the documentation on Identifiable:

This must be implemented to use associations. Additionally, implementing this trait allows you to pass your struct to update (update(&your_struct) is equivalent to update(YourStruct::table().find(&your_struct.primary_key())).

This trait is usually implemented on a reference to a struct, not the struct itself.

So you simply need to borrow user:

diesel::update(&user).set(...);