2 files changed, 99 insertions, 4 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index df79f058075..40ac6a95fa8 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -336,6 +336,47 @@ func TestSetValue(t *testing.T) {
 	}
 }
 
+func TestMapIterSet(t *testing.T) {
+	m := make(map[string]interface{}, len(valueTests))
+	for _, tt := range valueTests {
+		m[tt.s] = tt.i
+	}
+	v := ValueOf(m)
+
+	k := New(v.Type().Key()).Elem()
+	e := New(v.Type().Elem()).Elem()
+
+	iter := v.MapRange()
+	for iter.Next() {
+		iter.SetKey(k)
+		iter.SetValue(e)
+		want := m[k.String()]
+		got := e.Interface()
+		if got != want {
+			t.Errorf("%q: want (%T) %v, got (%T) %v", k.String(), want, want, got, got)
+		}
+		if setkey, key := valueToString(k), valueToString(iter.Key()); setkey != key {
+			t.Errorf("MapIter.Key() = %q, MapIter.SetKey() = %q", key, setkey)
+		}
+		if setval, val := valueToString(e), valueToString(iter.Value()); setval != val {
+			t.Errorf("MapIter.Value() = %q, MapIter.SetValue() = %q", val, setval)
+		}
+	}
+
+	got := int(testing.AllocsPerRun(10, func() {
+		iter := v.MapRange()
+		for iter.Next() {
+			iter.SetKey(k)
+			iter.SetValue(e)
+		}
+	}))
+	// Making a *MapIter and making an hiter both allocate.
+	// Those should be the only two allocations.
+	if got != 2 {
+		t.Errorf("wanted 2 allocs, got %d", got)
+	}
+}
+
 func TestCanSetField(t *testing.T) {
 	type embed struct{ x, X int }
 	type Embed struct{ x, X int }
diff --git a/src/reflect/value.go b/src/reflect/value.go
index de01f13825f..a8274cc871f 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1578,13 +1578,40 @@ func (it *MapIter) Key() Value {
 	if it.it == nil {
 		panic("MapIter.Key called before Next")
 	}
-	if mapiterkey(it.it) == nil {
+	iterkey := mapiterkey(it.it)
+	if iterkey == nil {
 		panic("MapIter.Key called on exhausted iterator")
 	}
 
 	t := (*mapType)(unsafe.Pointer(it.m.typ))
 	ktype := t.key
-	return copyVal(ktype, it.m.flag.ro()|flag(ktype.Kind()), mapiterkey(it.it))
+	return copyVal(ktype, it.m.flag.ro()|flag(ktype.Kind()), iterkey)
+}
+
+// SetKey assigns dst to the key of the iterator's current map entry.
+// It is equivalent to dst.Set(it.Key()), but it avoids allocating a new Value.
+// As in Go, the key must be assignable to dst's type.
+func (it *MapIter) SetKey(dst Value) {
+	if it.it == nil {
+		panic("MapIter.SetKey called before Next")
+	}
+	iterkey := mapiterkey(it.it)
+	if iterkey == nil {
+		panic("MapIter.SetKey called on exhausted iterator")
+	}
+
+	dst.mustBeAssignable()
+	var target unsafe.Pointer
+	if dst.kind() == Interface {
+		target = dst.ptr
+	}
+
+	t := (*mapType)(unsafe.Pointer(it.m.typ))
+	ktype := t.key
+
+	key := Value{ktype, iterkey, it.m.flag | flag(ktype.Kind())}
+	key = key.assignTo("reflect.MapIter.SetKey", dst.typ, target)
+	typedmemmove(dst.typ, dst.ptr, key.ptr)
 }
 
 // Value returns the value of the iterator's current map entry.
@@ -1592,13 +1619,40 @@ func (it *MapIter) Value() Value {
 	if it.it == nil {
 		panic("MapIter.Value called before Next")
 	}
-	if mapiterkey(it.it) == nil {
+	iterelem := mapiterelem(it.it)
+	if iterelem == nil {
 		panic("MapIter.Value called on exhausted iterator")
 	}
 
 	t := (*mapType)(unsafe.Pointer(it.m.typ))
 	vtype := t.elem
-	return copyVal(vtype, it.m.flag.ro()|flag(vtype.Kind()), mapiterelem(it.it))
+	return copyVal(vtype, it.m.flag.ro()|flag(vtype.Kind()), iterelem)
+}
+
+// SetValue assigns dst to the value of the iterator's current map entry.
+// It is equivalent to dst.Set(it.Value()), but it avoids allocating a new Value.
+// As in Go, the value must be assignable to dst's type.
+func (it *MapIter) SetValue(dst Value) {
+	if it.it == nil {
+		panic("MapIter.SetValue called before Next")
+	}
+	iterelem := mapiterelem(it.it)
+	if iterelem == nil {
+		panic("MapIter.SetValue called on exhausted iterator")
+	}
+
+	dst.mustBeAssignable()
+	var target unsafe.Pointer
+	if dst.kind() == Interface {
+		target = dst.ptr
+	}
+
+	t := (*mapType)(unsafe.Pointer(it.m.typ))
+	vtype := t.elem
+
+	elem := Value{vtype, iterelem, it.m.flag | flag(vtype.Kind())}
+	elem = elem.assignTo("reflect.MapIter.SetValue", dst.typ, target)
+	typedmemmove(dst.typ, dst.ptr, elem.ptr)
 }
 
 // Next advances the map iterator and reports whether there is another