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Use generic function to box values on heap #391

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1c813c9
Use generic function to box values on heap
willbeason Mar 11, 2026
1f2e3ff
Add TODO for go1.27 for update to new(V)
willbeason Mar 11, 2026
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127 changes: 44 additions & 83 deletions cmp/compare_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -104,13 +104,12 @@ func mustFormatGolden(path string, in []struct{ Name, Data string }) {

var now = time.Date(2009, time.November, 10, 23, 00, 00, 00, time.UTC)

// TODO(≥go1.18): Define a generic function that boxes a value on the heap.
func newInt(n int) *int { return &n }
// TODO(≥go1.27): Use new(V) to allocate a value.
func ptr[T any](t T) *T { return &t }

type Stringer string

func newStringer(s string) fmt.Stringer { return (*Stringer)(&s) }
func (s Stringer) String() string { return string(s) }
func (s Stringer) String() string { return string(s) }

type test struct {
label string // Test name
Expand Down Expand Up @@ -319,29 +318,29 @@ func comparerTests() []test {
reason: "unexported fields result in a panic by default",
}, {
label: label + "/PointerStructEqual",
x: &struct{ A *int }{newInt(4)},
y: &struct{ A *int }{newInt(4)},
x: &struct{ A *int }{ptr(4)},
y: &struct{ A *int }{ptr(4)},
wantEqual: true,
reason: "comparison of pointer to struct with equal A field",
}, {
label: label + "/PointerStructInequal",
x: &struct{ A *int }{newInt(4)},
y: &struct{ A *int }{newInt(5)},
x: &struct{ A *int }{ptr(4)},
y: &struct{ A *int }{ptr(5)},
wantEqual: false,
reason: "comparison of pointer to struct with inequal A field",
}, {
label: label + "/PointerStructTrueComparer",
x: &struct{ A *int }{newInt(4)},
y: &struct{ A *int }{newInt(5)},
x: &struct{ A *int }{ptr(4)},
y: &struct{ A *int }{ptr(5)},
opts: []cmp.Option{
cmp.Comparer(func(x, y int) bool { return true }),
},
wantEqual: true,
reason: "comparison of pointer to struct with inequal A field, but treated as equal with always equal comparer",
}, {
label: label + "/PointerStructNonNilComparer",
x: &struct{ A *int }{newInt(4)},
y: &struct{ A *int }{newInt(5)},
x: &struct{ A *int }{ptr(4)},
y: &struct{ A *int }{ptr(5)},
opts: []cmp.Option{
cmp.Comparer(func(x, y *int) bool { return x != nil && y != nil }),
},
Expand Down Expand Up @@ -424,35 +423,15 @@ func comparerTests() []test {
wantEqual: false,
reason: "comparer for *regexp.Regexp applied with inequal regexp strings",
}, {
label: label + "/TriplePointerEqual",
x: func() ***int {
a := 0
b := &a
c := &b
return &c
}(),
y: func() ***int {
a := 0
b := &a
c := &b
return &c
}(),
label: label + "/TriplePointerEqual",
x: ptr(ptr(ptr(0))),
y: ptr(ptr(ptr(0))),
wantEqual: true,
reason: "three layers of pointers to the same value",
}, {
label: label + "/TriplePointerInequal",
x: func() ***int {
a := 0
b := &a
c := &b
return &c
}(),
y: func() ***int {
a := 1
b := &a
c := &b
return &c
}(),
label: label + "/TriplePointerInequal",
x: ptr(ptr(ptr(0))),
y: ptr(ptr(ptr(1))),
wantEqual: false,
reason: "three layers of pointers to different values",
}, {
Expand Down Expand Up @@ -909,35 +888,35 @@ func reporterTests() []test {
reason: "reporter should display the qualified type name to disambiguate between the two values",
}, {
label: label + "/AmbiguousPointer",
x: newInt(0),
y: newInt(0),
x: ptr(0),
y: ptr(0),
opts: []cmp.Option{
cmp.Comparer(func(x, y *int) bool { return x == y }),
},
wantEqual: false,
reason: "reporter should display the address to disambiguate between the two values",
}, {
label: label + "/AmbiguousPointerStruct",
x: struct{ I *int }{newInt(0)},
y: struct{ I *int }{newInt(0)},
x: struct{ I *int }{ptr(0)},
y: struct{ I *int }{ptr(0)},
opts: []cmp.Option{
cmp.Comparer(func(x, y *int) bool { return x == y }),
},
wantEqual: false,
reason: "reporter should display the address to disambiguate between the two struct fields",
}, {
label: label + "/AmbiguousPointerSlice",
x: []*int{newInt(0)},
y: []*int{newInt(0)},
x: []*int{ptr(0)},
y: []*int{ptr(0)},
opts: []cmp.Option{
cmp.Comparer(func(x, y *int) bool { return x == y }),
},
wantEqual: false,
reason: "reporter should display the address to disambiguate between the two slice elements",
}, {
label: label + "/AmbiguousPointerMap",
x: map[string]*int{"zero": newInt(0)},
y: map[string]*int{"zero": newInt(0)},
x: map[string]*int{"zero": ptr(0)},
y: map[string]*int{"zero": ptr(0)},
opts: []cmp.Option{
cmp.Comparer(func(x, y *int) bool { return x == y }),
},
Expand All @@ -946,25 +925,25 @@ func reporterTests() []test {
}, {
label: label + "/AmbiguousStringer",
x: Stringer("hello"),
y: newStringer("hello"),
y: ptr[Stringer]("hello"),
wantEqual: false,
reason: "reporter should avoid calling String to disambiguate between the two values",
}, {
label: label + "/AmbiguousStringerStruct",
x: struct{ S fmt.Stringer }{Stringer("hello")},
y: struct{ S fmt.Stringer }{newStringer("hello")},
y: struct{ S fmt.Stringer }{ptr[Stringer]("hello")},
wantEqual: false,
reason: "reporter should avoid calling String to disambiguate between the two struct fields",
}, {
label: label + "/AmbiguousStringerSlice",
x: []fmt.Stringer{Stringer("hello")},
y: []fmt.Stringer{newStringer("hello")},
y: []fmt.Stringer{ptr[Stringer]("hello")},
wantEqual: false,
reason: "reporter should avoid calling String to disambiguate between the two slice elements",
}, {
label: label + "/AmbiguousStringerMap",
x: map[string]fmt.Stringer{"zero": Stringer("hello")},
y: map[string]fmt.Stringer{"zero": newStringer("hello")},
y: map[string]fmt.Stringer{"zero": ptr[Stringer]("hello")},
wantEqual: false,
reason: "reporter should avoid calling String to disambiguate between the two map values",
}, {
Expand All @@ -984,15 +963,15 @@ func reporterTests() []test {
foo1.Bar{"fizz"}: "buzz",
},
y: map[interface{}]string{
newStringer("hello"): "goodbye",
foo2.Bar{"fizz"}: "buzz",
ptr[Stringer]("hello"): "goodbye",
foo2.Bar{"fizz"}: "buzz",
},
wantEqual: false,
reason: "reporter should avoid calling String to disambiguate between the two map keys",
}, {
label: label + "/NonAmbiguousStringerMapKey",
x: map[interface{}]string{Stringer("hello"): "goodbye"},
y: map[interface{}]string{newStringer("fizz"): "buzz"},
y: map[interface{}]string{ptr[Stringer]("fizz"): "buzz"},
wantEqual: false,
reason: "reporter should call String as there is no ambiguity between the two map keys",
}, {
Expand Down Expand Up @@ -1279,15 +1258,9 @@ using the AllowUnexported option.`, "\n"),
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)
return &b
}(): 0},
y: map[*[]byte]int{func() *[]byte {
b := make([]byte, 1<<20)
return &b
}(): 0},
label: label + "/LargeMapKey",
x: map[*[]byte]int{ptr(make([]byte, 1<<20)): 0},
y: map[*[]byte]int{ptr(make([]byte, 1<<20)): 0},
reason: "printing map keys should have some verbosity limit imposed",
}, {
label: label + "/LargeStringInInterface",
Expand Down Expand Up @@ -1354,26 +1327,14 @@ using the AllowUnexported option.`, "\n"),
}`,
reason: "short multiline JSON should prefer triple-quoted string diff as it is more readable",
}, {
label: label + "/PointerToStringOrAny",
x: func() *string {
var v string = "hello"
return &v
}(),
y: func() *interface{} {
var v interface{} = "hello"
return &v
}(),
label: label + "/PointerToStringOrAny",
x: ptr("hello"),
y: ptr[interface{}]("hello"),
reason: "mismatched types between any and *any should print differently",
}, {
label: label + "/NamedPointer",
x: func() *string {
v := "hello"
return &v
}(),
y: func() PointerString {
v := "hello"
return &v
}(),
label: label + "/NamedPointer",
x: ptr("hello"),
y: PointerString(ptr("hello")),
reason: "mismatched pointer types should print differently",
}, {
label: label + "/MapStringAny",
Expand Down Expand Up @@ -2594,7 +2555,7 @@ func project1Tests() []test {
Target: "corporation",
Immutable: &ts.GoatImmutable{
ID: "southbay",
State: (*pb.Goat_States)(newInt(5)),
State: (*pb.Goat_States)(ptr(5)),
Started: now,
},
},
Expand Down Expand Up @@ -2622,7 +2583,7 @@ func project1Tests() []test {
Immutable: &ts.EagleImmutable{
ID: "eagleID",
Birthday: now,
MissingCall: (*pb.Eagle_MissingCalls)(newInt(55)),
MissingCall: (*pb.Eagle_MissingCalls)(ptr(55)),
},
}
}
Expand Down Expand Up @@ -2671,7 +2632,7 @@ func project1Tests() []test {
x: func() ts.Eagle {
eg := createEagle()
eg.Dreamers[1].Animal[0].(ts.Goat).Immutable.ID = "southbay2"
eg.Dreamers[1].Animal[0].(ts.Goat).Immutable.State = (*pb.Goat_States)(newInt(6))
eg.Dreamers[1].Animal[0].(ts.Goat).Immutable.State = (*pb.Goat_States)(ptr(6))
eg.Slaps[0].Immutable.MildSlap = false
return eg
}(),
Expand Down