The in/out features come into their own when inheritance is in play, i.e. for member functions of classes and interfaces. See https://dlang.org/spec/function.html#in_out_inheritance

`import` is so cool we extended it to be able to import .c files! The D compiler internally translates them to D so they can be used. When this was initially proposed, the reaction was "what's that good for?" It turned out to be incredibly useful and a huge time saver.

The concept is sort of like C++ being a superset of C and so being able to incorporate C code, except unlike C++, the C syntax can be left behind. After all, don't we get tired of:

    struct Tag { ... } Tag;

?

> struct Tag { ... } Tag;

What's the thing with the syntax? If you don't intend to use the type elsewhere don't give it a tag, if you want, you have to give it a name. (Assuming you are annoyed by the duplicate Tag)

Which would you prefer:

    struct Tag { ... }

or:

    typedef struct Tag { ... } Tag;

? It's just simpler and easier to write code in D than in C/C++. For another example, in C/C++:

    int foo();
    int bar() { return foo(); }
    int foo() { return 3; }

The D equivalent:

    int bar() { return foo(); }
    int foo() { return 3; }

If the user of Tag is supposed to know how the internal details:

    struct Tag { ... };

if it needs to rely on the internals, but the user shouldn't care:

    typedef struct { ... } Tag;

if it can be opaque (what I would default to):

    typedef struct {} Tag;

I also think that is a good feature to separate specification from implementation, I like being forced to declare first what I want to implement. Funnily in your special case you wouldn't need the declaration. (But of course it's a bad idea to rely on this "feature")

My C++ compiler also implemented contracts back in the 90s: https://www.digitalmars.com/ctg/contract.html

Modern C++ is slowly adopting D features, many of which came from extensions I added to my C++ compiler.