Invalid Capture Detection

As shown in the motivation section in README, traditional C/C++ macro might capture the incorrect declaration due to its primitive text substitution system:

#define foo(arg) {\
    int x = 0;  \
    return arg + x; \
}

// ERROR: caller will always return 0 
// regardless the argument value
int caller(int x) {
    foo(x)
}

In nacro rule, the equivalent code:

// input.c
#pragma nacro rule foo
(arg:$expr) -> {
  int x = 0;
  return arg + x;
}

int caller(int x) {
  foo(x)
}

will throw error messages:

$ clang-nacro -fsyntax-only input.c
./input.c:9:7: error: a potential declaration leak detected
  foo(x)
      ^
./input.c:9:7: note: the reference to 'x' that comes from outside a nacro
./input.c:4:7: note: is bind to declaration within a nacro
  int x = 0;
      ^
1 error generated.
$

For now, we can do nothing but throw error messages, unfortunately. Fixing it automatically involve internal support from the parser. However, Clang's parser doesn't provide any plugin or customizations points.