Askama's let operator tries to move out of self

[msrd0]

For code like {% let foo = bar %}, askama emits let foo = self.bar in rust code, which only works if bar is of a type that implements Copy. Error message looks similar to this:

 --> tests/dashboard.rs:8:10
  |
8 | #[derive(Template)]
  |          ^^^^^^^^
  |          |
  |          move occurs because `self.style.dashboard` has type `std::string::String`, which does not implement the `Copy` trait
  |          help: consider borrowing here: `&Template`
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

[vallentin]

I considered this issue while working on #396. I ended up not implicitly borrowing for let declarations. The reason being that it semantically mimics Rust more closure. I felt it would be more confusing to have to call clone() when self.bar is already copyable. Thereby it would be more natural to call e.g. as_str() or as_ref() when knowing that it otherwise would get moved.

Ideas are welcome. The main issue in relation to this, is that the generator does not have any type information available.