Impose verbosity limit when formatting map keys (#248)

Map keys should have a sensible verbosity limit imposed,
otherwise the reporter can end up printing a massive data structure
that cannot reasonably fit in memory.

cmp/compare_test.go

@@ -1276,6 +1276,17 @@ using the AllowUnexported option.`, "\n"),
 		y:         MyComposite{},
 		wantEqual: false,
 		reason:    "batched diffing for empty slices and nil slices",
+	}, {
+		label: label + "/LargeMapKey",
+		x: map[*[]byte]int{func() *[]byte {
+			b := make([]byte, 1<<20, 1<<20)
+			return &b
+		}(): 0},
+		y: map[*[]byte]int{func() *[]byte {
+			b := make([]byte, 1<<20, 1<<20)
+			return &b
+		}(): 0},
+		reason: "printing map keys should have some verbosity limit imposed",
 	}}
 }
 

cmp/report_reflect.go

@@ -351,6 +351,8 @@ func formatMapKey(v reflect.Value, disambiguate bool, ptrs *pointerReferences) s
 	opts.PrintAddresses = disambiguate
 	opts.AvoidStringer = disambiguate
 	opts.QualifiedNames = disambiguate
+	opts.VerbosityLevel = maxVerbosityPreset
+	opts.LimitVerbosity = true
 	s := opts.FormatValue(v, reflect.Map, ptrs).String()
 	return strings.TrimSpace(s)
 }

cmp/testdata/diffs

@@ -1008,6 +1008,12 @@
 + 	FloatsC: nil,
   }
 >>> TestDiff/Reporter/EmptySlices
+<<< TestDiff/Reporter/LargeMapKey
+  map[*[]uint8]int{
+- 	&⟪0xdeadf00f⟫⟪ptr:0xdeadf00f, len:1048576, cap:1048576⟫{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, ...}: 0,
++ 	&⟪0xdeadf00f⟫⟪ptr:0xdeadf00f, len:1048576, cap:1048576⟫{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, ...}: 0,
+  }
+>>> TestDiff/Reporter/LargeMapKey
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{