Use from_fn for non-Copyable Rust types

[lucic71]

In Rust, a struct is Copyable if it explicitly derives or implements Copy. All fields being Copy is not enough.

Add the derives attribute in the IR that lists all traits that a mapped Rust type derives. For example rustls_slice_bytes, a struct with Copyable fields, doesn't derive Copy. So derives contains only Default.

User defined types and builtin types also have their derives field filled. The entire PR is built around the following check:

    if (auto *rec = element_type->getAsRecordDecl()) {
      if (!RecordDerivesCopy(rec)) {
        return std::format("std::array::from_fn::<_, {}, _>(|_| {})",
                           size_as_string.c_str(), element_type_as_string);
      }
    }

Arrays of non-copyable types cannot be initialized with [non_copyable_default, N], they have to use the std::array:from_fn format.

[nunoplopes]

Uhm, I think we can use just let _arr: [NonCopy; 3] = Default::default();. No need for from_fn?

[lucic71]

Unfortunately no, Default::default() only works for array sizes < 32: https://godbolt.org/z/c5T6boGbq

[nunoplopes]

Ahah, ok