Built-in clear for maps and slices

m := map[string]int{"a": 1, "b": 2}

// *** before: ***
for k := range m {
    delete(m, k)
}

// *** in version 1.21: ***
clear(m) // one call empties the map

s := []int{1, 2, 3}
clear(s) // the slice elements are zeroed, the length stays