What does "special compiler magic" mean?

Question

In the Nim system module it says:

Most of the routines listed here use special compiler magic.

What do they mean by "special compiler magic"?

Answer 1

Generally it means that the implementations of the routines in that module cannot be expressed in the source language itself. I.e. the compiler recognizes a special set of names and either uses special semantics or compiles in hand-written assembly, etc. The notice is usually to prevent someone from trying to reason about how the interface could be written in the language itself.