Earlier quoted context omitted.
As unwind suggested, they just get transformed down to global functions with unique names. Here's the actual C code generated: https://gist.github.com/def-/0fe87bf1d35102c62d3b#file-nest-...
What about variables nested within these functions then? Is scope enforced? i.e. you can reach back to variable aa in function a() from nested d() but can't reach variable dd in d() from a()?
I have no idea how it actually works, but it would be easy enough to implement by having nested functions take a parameter that's a pointer to a structure containing the variables that are in scope in the outer function.
In your example, it could be something like this:
struct variables_in_a_that_are_visible_in_d {
int aa;
};
void d_nested_in_a(struct variables_in_a_that_are_visible_in_d *ascope) {
// Variable dd, not accessible from a
int dd = 42;
// Increment a's aa
ascope->aa += 1;
}
void a() {
int aa = 0;
// Pack up variables for d()
struct variables_in_a_that_are_visible_in_d ford;
ford.aa = aa;
// actually call d)
d_nested_in_a(&ford);
// Unpack after calling d()
aa = ford.aa;
// Continue with a()
// ...
}