all: make handling of zero-value composites more consistent
We occasionally need to work with immutable, empty lists, maps, and
messages. Notably, Message.Get on an empty repeated field will return a
"frozen" empty value.
Move handling of these immutable, zero-length composites into Converter,
to unify the behavior of regular and extension fields.
Add a Zero method to Converter, MessageType, and ExtensionType, to
provide a consistent way to get an empty, frozen value of a composite
type. Adding this method to the public {Message,Extension}Type
interfaces does increase our API surface, but lets us (for example)
cleanly represent an empty map as a nil map rather than a non-nil
one wrapped in a frozenMap type.
Drop the frozen{List,Map,Message} types as no longer necessary.
(These types did have support for creating a read-only view of a
non-empty value, but we are not currently using that feature.)
Change-Id: Ia76f149d591da07b40ce75b7404a7ab8a60cb9d8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189339
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/legacy_extension.go b/internal/impl/legacy_extension.go
index e6e86fb..aaf8fcc 100644
--- a/internal/impl/legacy_extension.go
+++ b/internal/impl/legacy_extension.go
@@ -232,6 +232,7 @@
func (x *legacyExtensionType) GoType() reflect.Type { return x.typ }
func (x *legacyExtensionType) New() pref.Value { return x.conv.New() }
+func (x *legacyExtensionType) Zero() pref.Value { return x.conv.Zero() }
func (x *legacyExtensionType) ValueOf(v interface{}) pref.Value {
return x.conv.PBValueOf(reflect.ValueOf(v))
}