assumeSafeAppend inconsistent when multiple slices in use

[simendsjo]

I don't know if this is an actual problem, but I don't understand the
behavior.
When one slice calls assumeSafeAppend, both slices is "given control",
that is, gets the parents capacity.
It's best explained with an example:


	int[] a;
	a.length = 4;

	int[] b = a[0..1];
	int[] c = a[0..1];

	// appending to b or c reallocates
	assert(a.capacity >  0);
	assert(b.capacity == 0);
	assert(c.capacity == 0);

	// gives a's capacity to b AND c
	b.assumeSafeAppend();
	assert(a.capacity == 0);
	assert(b.capacity >  0);
	assert(c.capacity >  0); // why does c get capacity?

	// .. until one of the slices modifies. Just changing
assumeSafeAppend does not change this behavior
	b ~= 1;
	// now b has full control
	assert(a.capacity == 0);
	assert(b.capacity >  0);
	assert(c.capacity == 0);

	// c full control
	c.assumeSafeAppend();
	assert(a.capacity == 0);
	assert(b.capacity == 0);
	assert(c.capacity >  0);

	// a full control
	a.assumeSafeAppend();
	assert(a.capacity >  0);
	assert(b.capacity == 0);
	assert(c.capacity == 0);

	// but now ONLY b gets full control. Not both b and c as the
first assumeSafeAppend
	b.assumeSafeAppend();
	assert(a.capacity == 0);
	assert(b.capacity >  0);
	assert(c.capacity == 0);