Why can’t I append to a slice that’s the property of a struct in golang?

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Everything in Go is passed by value. And a copy is made of the passed value.

Test3.combo() has value (non-pointer) receiver:

func (c Test3) run() []int {
  c.combo()
  return c.all
}

func (c Test3) combo() {
  for i := 0; i < 2; i++ {
    c.all = append(c.all, i)
    fmt.Println("Test3 step", i + 1, c.all)
  }
}

This means when Test3.combo() is called from Test3.run() like c.combo(), a copy is made of c (which is of type Test3). The combo() method operates on a copy. It properly appends 2 numbers to Test3.all, but when this method returns, the copy is discarded.

So when Test3.run() returns c.all, it returns an empty (nil) slice, because the slice to which Test3.combo() appended, was a field of a copy, and which has been discarded.

Solution: simply use a pointer receiver:

func (c *Test3) combo() {
  for i := 0; i < 2; i++ {
    c.all = append(c.all, i)
    fmt.Println("Test3 step", i + 1, c.all)
  }
}

Output (try it on the Go Playground):

Test1 final: [0 1] 

Test2 final: [0 1] 

Test3 step 1 [0]
Test3 step 2 [0 1]
Test3 final: [0 1]

Note the star * in the receiver: func (c *Test3) combo(). By adding it, you make the receiver a pointer, and so when combo() is called, it only receives a pointer to a value of type Test3, and it will modify the pointed value, the value that Test3.run() has, so when combo() returns, the changes are not lost.

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