[BoundsSafety] easier external bounds annotation tracking

[mat-c]

Hi,

I am using stable/20250402

While testing -fbounds-safety on some project I found some error forcing some annoying conversion.

For example

int count(char *__counted_by(len) p, int len)
{
        int i = 0;
        while(len--) {
                i+= *p++;
        }
        return i;
}

while produce errors

hello3.c:8:23: error: assignment to 'char *__single __counted_by(len)' (aka 'char *__single') 'p' requires corresponding assignment to 'len'; add self assignment 'len = len' if the value has not changed
    8 |                 i+= *p++;
      |                       ^
hello3.c:7:18: error: assignment to 'len' requires corresponding assignment to 'char *__single __counted_by(len)' (aka 'char *__single') 'p'; add self assignment 'p = p' if the value has not changed
    7 |         while(len--) {
      |                  ^
2 errors generated.

But the following code doesn't produce any error

int count2(char *__counted_by(len2) p2, int len2)
{
        int len = len2;
        char *p = p2;
        int i = 0;
        while(len--) {
                i+= *p++;
        }
        return i;
}

parameter in C are local variable.
Isn't possible that function parameters require annotation to pointer, but that they are treated like local variable ?

In function entry, the compiler know the size of the buffer, and auto track pointer update.
May be a knew __counted_by keyword is need to express that we don't want the pointer p to be always size len, but that we want only to provide buffer size in function entry.
For example a something based of __ended_by (do not work as p is a single pointer)

int count(char *__ended_by(p+len) p, int len)