go-loop-variable-pointer-escape_test.go

package main_test

import (
	"testing"
	"testing/quick"
	"unsafe"
)

func all(xs []*int, f func(x *int) bool) (result bool) {
	result = true
	for _, v := range xs {
		result = result && f(v)
	}
	return result
}

func Test_references_to_loop_variable_outside_the_loop_have_same_value(t *testing.T) {
	f := func(xs []int) bool {
		var loopVariableAddress uintptr
		var zeroOfUintptr uintptr
		var copies = make([]*int, 0, len(xs))
		for _, x := range xs {
			if loopVariableAddress == zeroOfUintptr {
				// Store the loop variable's address for later comparison.
				loopVariableAddress = uintptr(unsafe.Pointer(&x))
			}
			// Copy the address of x into a slice.
			copies = append(copies, &x)
			// The append statement above is most likely a mistake.
			// We probably mean
			// copies = append(copies, &xs[index]);
			// assuming `index` is the ignored loop index.
		}
		return all(copies, func(x *int) bool {
			// All values in `copies` are the same pointer address.
			return uintptr(unsafe.Pointer(x)) == loopVariableAddress && *x == *copies[0]
		})
	}

	if err := quick.Check(f, nil); err != nil {
		t.Fatal(err)
	}
}