Memory leak with dynamic array

[bearophile]

This can help confuse your mind a bit:

import std.stdio: writeln;
void main() {
    {
        int[] arr = [0, 1, 2, 3, 4, 5, 6].dup;
        int[] slice = arr[2 .. 4];
        writeln("arr, slice: ", arr, " | ", slice); // arr, slice: 0 1 2 3 4 5 6 | 2 3
        slice ~= 10; slice ~= 20;
        writeln("arr.capacity, slice.capacity: ", arr.capacity, " ", slice.capacity); // arr.capacity, slice.capacity: 7 7
        writeln("arr, slice: ", arr, " | ", slice); // arr, slice: 0 1 2 3 4 5 6 | 2 3 10 20
    }

    {
        int[] arr = [0, 1, 2, 3, 4, 5, 6].dup;
        int[] slice = arr[2 .. 4];
        writeln("arr, slice: ", arr, " | ", slice); // arr, slice: 0 1 2 3 4 5 6 | 2 3

        slice.assumeSafeAppend;
        slice ~= 10; slice ~= 20; // causes stomping
        writeln("arr.capacity, slice.capacity: ", arr.capacity, " ", slice.capacity); // arr.capacity, slice.capacity: 0 5
        writeln("arr, slice: ", arr, " | ", slice); // arr, slice: 0 1 2 3 10 20 6 | 2 3 10 20
        slice ~= 30; slice ~= 40;
        writeln("arr.capacity, slice.capacity: ", arr.capacity, " ", slice.capacity); // arr.capacity, slice.capacity: 7 7
        writeln("arr, slice: ", arr, " | ", slice); // arr, slice: 0 1 2 3 10 20 30 | 2 3 10 20 30 40
    }
}


The  slice.capacity = 7 in the first case is just a coincidence, it's the result of the overallocation.

But I don't know why arr.capacity is zero and then seven in the second and third case.

Bye,
bearophile