derive(new)

The constructor pattern is well-used in Rust, but is very boiler-platey. I propose allowing #[derive(new)] to be added to structs to provide the common constructor pattern.

When placed on a struct called Foo with fields f_0 to f_n with types T_0 to T_n it would create an inherent impl for that struct with a single function called new which would take n args, with types T_0 to T_n and return a Foo, it would simply create a new Foo object mapping args to fields. It would be pub if Foo was pub. The impl would have the same type and lifetime parameters as the struct.

This is a bit different from existing derive functionallity, since it takes a function name rather than a trait name as input and creates an inherent impl, rather than a trait impl. But it would work in basically the same way and have all the same benefits.

No implementation files or tests are named. Start by reading the proposed #[derive(new)] behavior and the RFC process for Rust language changes; done means reaching a settled design decision about generated constructors, visibility, generics, lifetimes, and compatibility with existing derive behavior.