Adds "C++ APIs for Edition Zero" to the GitHub code repository.

PiperOrigin-RevId: 563818554

docs/design/editions/README.md

@@ -28,5 +28,6 @@ The following topics are in this repository:
 *   [Edition Zero: JSON Handling](edition-zero-json-handling.md)
 *   [Edition Zero: Converged Semantics](edition-zero-converged-semantics.md)
 *   [Edition Zero Feature: Enum Field Closedness](edition-zero-feature-enum-field-closedness.md)
+*   [C++ APIs for Edition Zero](cpp-apis-for-edition-zero.md)
 *   [Edition Evolution](edition-evolution.md)
 *   [Edition Naming](edition-naming.md)

docs/design/editions/cpp-apis-for-edition-zero.md

@@ -0,0 +1,79 @@
+# C++ APIs for Edition Zero
+
+**Author:** [@mcy](https://github.com/mcy)
+
+**Approved:** 2022-06-27
+
+## Overview
+
+As recorded in *FileDescriptor::syntaxAudit Report* (not available externally),
+there are significant uses of `FileDescriptor::syntax()` in internal Google
+repositories that Edition Zero will break; for example, existing usage checks
+`syntax() == PROTO3` to determine if enums are open.
+
+Because we expect to make everything that callers are querying be controlled by
+individual edition-gated features, the cleanest way forward is to enrich the
+`Descriptor` types with focused APIs that serve current usages, followed by a
+migration to them.
+
+Tier 1 proposal:
+
+## Proposed API
+
+This proposal adds the following code to `descriptor.h:`
+
+```
+class FileDescriptor {
+  // Copies package, syntax, edition, dependencies, and file-level options.
+  void CopyHeadingTo(FileDescriptorProto*) const;
+};
+class FieldDescriptor {
+  // Returns whether this field has a proto3-like zero default value.
+  bool has_zero_default_value() const;
+  // Returns whether this is a string field that enforces UTF-8 in the codec.
+  bool enforces_utf8() const;
+};
+class EnumDescriptor {
+  // Returns whether this enum is a proto2-style closed enum.
+  bool is_closed() const;
+};
+```
+
+This covers everything not already covered by existing accessors.
+`CopyHeadingTo` is intended to simplify the common, easy-to-get-wrong pattern of
+copying the heading of a proto file before doing custom manipulation of
+descriptors within.
+
+Additionally, we propose generating `unknown_fields()` and
+`mutable_unknown_fields()` for all protos unconditionally.
+
+## Migration
+
+In addition to the above functions, we'll use `FieldDescriptor::has_presence()`
+and `FieldDescriptor::is_packed()` to migrate callers performing comparisons to
+`syntax()`. Users can migrate to the new functions by:
+
+1.  Searching for calls to `syntax()`.
+2.  Identifying what proto2/proto3 distinction is being picked up on, among the
+    following:
+    1.  UTF-8 verification on parse (use new API).
+    2.  Closed/open enum (use new API).
+    3.  Packed-ness (use existing API instead of guessing from `syntax`).
+    4.  Whether fields have hasbits (use `has_presence()`).
+3.  Migrate to the new API.
+
+The volume of such changes in google3 is small enough that it's probably easiest
+to create a giant CL that fixes every instance of a particular misuse and then
+hand it to Rosie for splitting up.
+
+Once all the "easy" usages are migrated, we will mark `syntax()` as
+`ABSL_DEPRECATED`. `syntax()` will return a new special value,
+`Syntax::EDITIONS`, intended to explicitly break caller expectations about what
+this function returns. Virtually all cases not covered by this proposal are
+using `syntax()` to reject one of the two existing syntaxes, or produce an error
+when encountering an unknown `Syntax` value, so they will continue to work as
+expected (i.e. fail on `editions` protos).
+
+A number of existing tools are hostile to proto2 or proto3. Once the migration
+plan is approved, we should reach out to them individually to coordinate either
+updating or deprecating their tool.