Slice append and shared backing array gotcha

Slices derived from the same array share backing memory. Appending to a sub-slice within capacity mutates data visible to the original slice.

Copy a slice when you need an independent copy, or use the full-slice expression to limit capacity:

orig := []int{1, 2, 3, 4, 5}

// sub shares backing array with orig up to cap(sub)
sub := orig[:3]
sub = append(sub, 99) // overwrites orig[3]!
fmt.Println(orig) // [1 2 3 99 5] — surprise!

// FIX 1: explicit copy
copied := make([]int, 3)
copy(copied, orig[:3])
copied = append(copied, 99) // safe

// FIX 2: limit capacity with full slice expression
limited := orig[:3:3] // cap = 3, forces new allocation on append
limited = append(limited, 99) // safe

A slice is a struct of (pointer, length, capacity). Two slices with the same pointer but different lengths share backing storage. append only allocates new memory when len exceeds cap.