Hints

millions slice's backing array uses 65 MB of memory.
make a new slice with 10 elements with a new backing array.
- copy the last 10 elements of the millions slice to the new slice.
- So you will have slice with a new backing array only with 10 elements.
- Then overwrite the millions slice by simply assigning last10 slice to it.
Remember: slice ~= pointer to a backing array.

If you overwrite the slice, it will lose that pointer. So Go can collect the unused memory.