Warning: // 1990s style generic code: from the article describes me and C++, though I do understand more modern C++ at a basic level, I'm not an expert. You might say I'm.... rusty...

ahem.

One huge difference in general between C++ templates and Rust's generics (I _believe_ Java/C# are more like Rust here, and I'm not sure about Swift) in that C++ templates generally don't check that their calling code actually supports the type that's passed into the template. In other words, the check happens after the template is expanded. (This is the sort example on page 3). Rust's generics system does the opposite: it requires that if you call a method on some generic parameter, that generic parameter is constrained by a trait that has that method. This happens before expansion. So concepts are, in some ways, an attempt to make the C++ system closer to Rust's system. (This is ignoring the actual chronology here, of course: concepts have been in development a long time).

The way "requires" works here is very different than in Rust, as well. In Rust, you define a trait the same way you'd define regular methods. If those methods have a body, then it's used as a default implementation, if it does not, then you must implement that method. The equivalent of "must have an iterator type" is associated types, you add "type Name;" instead of a method signature.

So the equivalent of the "Sequence" type on page 7 would be

  trait Sequence {
      type Value;
      type Iterator<Item=Value>; // Iterator itself has an associated type, Value
    
      fn begin() -> Self::Iterator;
      fn end() -> Self::Iterator;
  }

roughly speaking. I don't think this exactly works, but just to give you some idea of how the syntaxes roughly compare. In general, I know that a Sequence trait in Rust would require higher kinded types or associated type constructors, which Rust does not yet have, and so the concept is probably not directly expressible.

That's my impressions after reading this. I have been meaning to read about concepts for a while, but since I only have so much time, have mostly waited to see how they shake out before really digging in. It's slightly slower going since my C++ isn't spectacular. This post mostly represents my understanding of the differences, I'm not trying to make any judgements here, etc.