[GNAT] This restriction prevents the compiler from building 'trampolines'. This is a structure that is built on the stack and contains dynamic code to be executed at run time. On some targets, a trampoline is built for the following features: Access, Unrestricted_Access, or Address of a nested subprogram; nested task bodies; primitive operations of nested tagged types. Trampolines do not work on machines that prevent execution of stack data. For example, on windows systems, enabling DEP (data execution protection) will cause trampolines to raise an exception. Trampolines are also quite slow at run time.
On many targets, trampolines have been largely eliminated. Look at the version of system.ads for your target — if it has Always_Compatible_Rep equal to False, then trampolines are largely eliminated. In particular, a trampoline is built for the following features: Address of a nested subprogram; Access or Unrestricted_Access of a nested subprogram, but only if pragma Favor_Top_Level applies, or the access type has a foreign-language convention; primitive operations of nested tagged types.