Improve reporting of values with cycles (#217)

Previously, the reporter could handle formatting values with cycles
in that it did not crash with a stack overflow. However, the output
was not particularly understandable as it did not surface to the user
why a particular value was truncated, and if it was truncated due
to a cyclic reference, what was the referent.

This change annotates the reporter tree with pointer information
so that a later pass can inject reference information if it is needed
to produce more understandable output.

Consider the following example:
  map[string]*cmp_test.CycleAlpha{
  	"Foo": &⟪ref#0⟫{
  		Name: "Foo",
  		Bravos: map[string]*cmp_test.CycleBravo{
  			"FooBravo": &{
- 				ID:     101,
+ 				ID:     0,
  				Name:   "FooBravo",
  				Mods:   100,
  				Alphas: {"Foo": &⟪ref#0⟫(...)},
  			},
  		},
  	},
  }

This graph contains a cycle. To ensure that a graph can be formatted,
the cycle is truncated as indicated with: &⟪ref#0⟫(...).
The referent was identified earlier with: &⟪ref#0⟫{...}.

2 files changed, 20 insertions, 0 deletions

diff --git a/cmp/internal/value/pointer_purego.go b/cmp/internal/value/pointer_purego.go
index 0a01c47..e9e384a 100644
--- a/cmp/internal/value/pointer_purego.go
+++ b/cmp/internal/value/pointer_purego.go
@@ -21,3 +21,13 @@ func PointerOf(v reflect.Value) Pointer {
 	// assumes that the GC implementation does not use a moving collector.
 	return Pointer{v.Pointer(), v.Type()}
 }
+
+// IsNil reports whether the pointer is nil.
+func (p Pointer) IsNil() bool {
+	return p.p == 0
+}
+
+// Uintptr returns the pointer as a uintptr.
+func (p Pointer) Uintptr() uintptr {
+	return p.p
+}
diff --git a/cmp/internal/value/pointer_unsafe.go b/cmp/internal/value/pointer_unsafe.go
index da134ae..b50c17e 100644
--- a/cmp/internal/value/pointer_unsafe.go
+++ b/cmp/internal/value/pointer_unsafe.go
@@ -24,3 +24,13 @@ func PointerOf(v reflect.Value) Pointer {
 	// which is necessary if the GC ever uses a moving collector.
 	return Pointer{unsafe.Pointer(v.Pointer()), v.Type()}
 }
+
+// IsNil reports whether the pointer is nil.
+func (p Pointer) IsNil() bool {
+	return p.p == nil
+}
+
+// Uintptr returns the pointer as a uintptr.
+func (p Pointer) Uintptr() uintptr {
+	return uintptr(p.p)
+}