You're right in your first part. Ada 83 is less complex than modern C++ or Rust. However Ada kept evolving, and a lot of complexity was added in later revisions, such as Ada 95, which added a kind of bastardized and very complex Java style object model layer.

Ada features that are hard to compile are very common in the language. It is generally a language that is hard to compile to efficient code, because rules were conceived in an abstract notion of what safety is. But in general Ada is an extremely over specified language, which leaves very little space to interpretation. You can check the Ada reference manual if you want, which is a use 1000 pages book (http://www.ada-auth.org/arm.html)

* Array types are very powerful and very complicated * Tasking & threading are specified in the language, which seems good on paper, but the abstractions are not very efficient and of tremendous complexity to implement. * Ada's generic model is very hard to compile efficiently. It was designed in a way that tried to make it possible to compile down both to a "shared implementation" approach, as well as down to a monomorphized approach. Mistakes were done down the line wrt the specification of generics which made compiling them to shared generics almost impossible, which is why some compiler vendors didn't support some features of the language at all. * Ada scoping & module system is of immense complexity * The type system is very vast. Ada's name & type resolution algorithm is extremely complex to implement. functions can be overloaded on both parameters & return types, and there is a enclosing context that determines which overloads will be used in the end. On top of that you have preferences rules for some functions & types, subtyping, derived types, etc ...

This is just what comes to mind on a late Friday evening :) I would say that the language is so complex that writing a new compiler is one of those herculean efforts that reach similar heights as writing a new C++ compiler. That's just a fe