Use reflect.Value.Pointer() to compare pointers

[AlexanderYastrebov]

Summary

Use reflect.Value.Pointer() to compare pointers

Changes

Updates samePointers implementation and adds test cases

Motivation

This is needed to check if map, slice and other built-in types point to the same value

Related issues

Fixes #1076

[stevenh]

LGTM

[stevenh]

@dolmen quick ping as @AlexanderYastrebov has updated based on your feedback

[brackendawson]

The docstring for Same would need to be changed:

Same asserts that two pointers reference the same object.

  assert.Same(t, ptr1, ptr2)

Both arguments must be pointer variables. Pointer variable sameness is
determined based on the equality of both type and value.

• This now works for more than pointer variables.
• This now works for types which are convertable to the same type.
• It needs to be clear that only slices pointing to the start of the same array are the same, and not slices pointing to different indices of the same array:

var (
	a   [8]int
	sa0 = a[:]
	sa3 = a[3:]
)
assert.Same(t, sa0, sa3)

[brackendawson]

This is a change in behaviour and we must decide if it's safe to do in v1.

No invocation of Same could previously pass with any non-pointer types.

Previously NotSame will always pass if non-pointer type/s are passed it, it can now fail of those reference the same object. In this case I think it would fix a test the author would have intended to fail.

I'd like some more opinions before merging this.

[brackendawson]

This is ambiguous for slices in this case:

var (
	a   [8]int
	sa0 = a[:]
	sa3 = a[3:]
)
assert.Same(t, sa0, sa3)

sa0 and sa3 reference the same object if you take object to mean array, but not if you mean contiguous segment of an underlying array. The language specification uses both terms at various points. We should make it clear that Same only considers slices to be the same if they reference the same contiguous segment of an underlying array.

[AlexanderYastrebov]

IMO reverse is also surprising

var (
	a   [8]int
	sa0 = a[:]
	sa1 = a[:1]
)
assert.Same(t, sa0, sa1) // passes