Initialistion from initializer lists in C++ basically rely on the = operator being able to do arbitrary work, e.g. allocations and generally manipulating the heap in arbitrary ways. Rust tries to avoid implicit performance sinks like that, and so doesn't have constructors in the same manner as C++.

That said, in future, it is likely to be possible for things like `Vec::from_array([1, 2, 3, 4])` and `HashMap::from_array([[1, 2], [3, 4]])` to work.

Rust doesn't need to take the C++ approach to implement it, but it can still use the same syntax which is simply much neater. HashMap::from_array([[1, 2], [3, 4]]) looks horrible in comparison.

Exact implementation is another question. That's why asked, whether it's feasible or it's something hard to fit into the language.

I don't see how Rust can get the same syntax as C++ without having the implicit performance sink I mentioned: creating vectors and hashmaps needs to allocate, and so letting this happen implicitly is not great. In fact, the syntax was pretty much the whole meat of my comment.

Why can't Rust do what it does already with macros, except using the simpler syntax described above? I.e. that syntax can mean explicit allocation (it's just a short form for it).

I'm not sure, other than that because {} evaluates to a value, this already looks very close to valid Rust; I thought it was for a minute, but realized that it's not quite.

I'm not sure I personally feel that the first is particularly better than the latter, but it should have to be by a large margin to justify adding new syntax to the language.

I can explain why I think initializer lists style is better. You can have a standard / uniform syntax for initialization. And it's using already defined syntax for types to actually indicate what you are creating. Compare it to the current one. It's necessary to create a new syntax for each type (that you want to enable for such initialization). I.e. a macro. This macro also is different from already existing syntax which indicates the type (i.e. you have Vec and you have vec!). That's why I think current approach is more cumbersome and initializer lists style is more streamlined and elegant.

> it should have to be by a large margin to justify adding new syntax to the language.

C++11 put some effort into simplifying some cumbersome syntax constructs. It makes things easier to read and write which pays off.