Adds "Edition Zero: Converged Semantics" to the GitHub code repository.

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 *   [Editions: Life of a Featureset](editions-life-of-a-featureset.md)
 *   [Protobuf Editions for Schema Producers](protobuf-editions-for-schema-producers.md)
 *   [Stricter Schemas with Editions](stricter-schemas-with-editions.md)
 *   [Edition Zero: Converged Semantics](edition-zero-converged-semantics.md)
 *   [Edition Naming](edition-naming.md)
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 # Edition Zero: Converged Semantics
 **Authors:** [@perezd](https://github.com/perezd),
 [@haberman](https://github.com/haberman)
 **Approved:** 2021-10-07
 ## Background
 The Protobuf Team has been exploring potential facilities for introducing
 breaking API and semantic changes. This document is an attempt to make use of
 these facilities to unify proto semantics from this point forward, while giving
 customers the ability to more-granularly manage their project's specific needs.
 ## Objective
 We want to reduce complications of API semantics that are coarsely managed
 through the syntax keyword, and instead default to converged proto2/proto3 when
 opting into editions. Where needed, customers will be able to opt out of
 specific semantics that are incompatible with their existing usages, at a
 fine-grained level, using the new capabilities provided by editions + features.
 ### Why Now?
 As we introduce new facilities for managing breaking changes, we have an
 additional opportunity to cutover and rectify a long-standing vision of
 converging proto 2/3 semantics as a natural extension of this.
 Doing this in lockstep with the introduction of editions provides the protobuf
 team with a few valuable outcomes:
 *   Editions provides us with more granular specification of intent than the
    existing coarse knob of "proto2" or "proto3." By opting into our first
    edition, customers are upgrading to what we've referred to in the past as
    "converged semantics," and if needed can reversibly downgrade back to proto2
    or proto3 semantics respectively by opt-ing out of the specific features
    that are incompatible with their existing needs.
 *   The protobuf team can avoid the n^2 complexity of considering how an
    edition/feature will interplay with an explicit syntax designation of
    "proto2" vs "proto3" for all impacted runtimes. This allows us to transition
    our thinking/support model to be explicitly feature-centric.
 *   The introduction of editions will almost certainly cause a major version
    bump and gives us ample justification to make breaking changes as we
    transition to this granular specification.
 ## Introduction of the `edition` keyword to proto IDL
 The `edition` keyword is used to define which semantic version a particular file
 and all of its contents will adhere to as a baseline. Whenever a proto file
 declares an `edition` keyword, it automatically defaults to converged proto2/3
 semantics.
 An edition's value is represented as a string, encoded by convention as a year.
 ## Introduction of `features` option to `descriptor.proto`
 This option will be uniformly defined as a repeated set of strings which can be
 used to encode the ability to opt-out of a specific feature (eg:
 `"-string_view"`), or to potentially opt-in to a future/experimental feature
 (eg: `"string_view"`). The `features` option will be added to `descriptor.proto`
 for the following descriptor options:
 *   File
 *   Message
 *   Field
 *   Enum
 *   Enum Value
 *   Oneof
 *   Service
 *   Method
 *   Stream (internal repositories only)
 Features are only respected when used in conjunction with the `edition` keyword.
 They are not validated for correctness to ensure they are forward/backward
 compatible with releases.
 Features may be declared at any descriptor level, however, a feature definition
 may influence descendant types at the discretion of the protobuf team. (e.g., a
 file-level feature opt-out could impact all fields within the file, if it was
 desired).
 ## A taxonomy of features
 Features can be broken down into two main categories: language-specific and
 semantic.
 ### Language-specific features
 Language-specific features pertain to the generated API for a given language.
 Referring to the protobuf breaking changes backlog we can see some examples:
 *   (C++) Changing string fields to return `string_view`.
 *   (Java) Removing the confusing `Enum#valueOf(int)` API.
 *   (Java) Rename oneof enums to do appropriate camel casing.
 Language-specific features have no meaning for any other language: they can be
 ignored entirely. They are, in essence, a private (tunneled) interface between
 protobuf IDL and the respective code generator. Each language's code generator
 can independently decide what the "base" set of features is for any given
 edition. Each language defines the migration path between editions
 independently.
 ### Semantic Features
 Semantic features define behavior changes that apply to the protobuf data model,
 independent of language. These can also have API implications, but their meaning
 goes deeper than just a surface-level API. Some examples of semantic features:
 *   Open enums (enums are placed directly into the field instead of the
    `UnknownFieldSet`).
 *   Packed (whether repeated fields are packed on the wire)
 Semantic features have significantly broader scope, since they must be respected
 across languages, and each language must implement the semantic correctly. This
 also implies that every language must either (1) know the canonical set of "base
 features" for each edition, or (2) that the set of "default" features for the
 edition must be resolved in protoc itself and propagated explicitly into the
 descriptor.
 ## Rev'ing the protobuf IDL vs. descriptor.proto
 Changing `descriptor.proto` to reflect editions is a much more intrusive change
 than changing just the protobuf IDL. The protobuf IDL is parsed and resolved in
 protoc, and we have only a single implementation of that parser. Any change that
 can be resolved in the parser alone is relatively unintrusive (though there are
 build horizon issues since GCL parses protos in prod).
 Rev'ing `descriptor.proto` is a far more intrusive change that affects many
 downstream systems. Many systems access descriptors through either a descriptor
 API (for example, `proto2::Descriptor` in C++) or by directly accessing a proto
 from `descriptor.proto` (eg. `google.protobuf.DescriptorProto`). Any changes
 here need to be managed much more delicately.
 ## Deprecation of the `syntax` keyword from proto IDL
 The `syntax` keyword shall no longer be required/observed when an `edition`
 keyword is present, as it is now considered redundant. If `edition` and `syntax`
 are both present, `edition` takes precedence and `syntax` is ignored.
 ## Migrating from `proto2` and `proto3` to Editions + Features
 Today's usage of syntax opaquely bundles a collection of implied feature flags
 that are set based on the presence of `proto2` or `proto3`. This is often a
 source of confusion for customers (eg: what am I gaining by moving to proto3?
 What am I losing?).
 By deciding that editions/features exist in a state of proto2/3 convergence,
 this enables customers to decide for themselves what features are important to
 their usage of protos.
 Migrating existing users of proto2 and proto3 to editions w/converged semantics
 would mean we'd need to execute a large-scale change to make their
 implicit/implied behavior explicit. Here are examples of implied behavior.
 today:
 <table>
  <tr>
   <td><strong>Feature</strong>
   </td>
   <td><strong><code>proto2</code> implied behavior</strong>
   </td>
   <td><strong><code>proto3</code> implied behavior</strong>
   </td>
  </tr>
  <tr>
   <td>packed_repeated_primitives
   </td>
   <td>🚫
   </td>
   <td>✅
   </td>
  </tr>
  <tr>
   <td>extensions
   </td>
   <td>✅
   </td>
   <td>🚫
   </td>
  </tr>
  <tr>
   <td>required
   </td>
   <td>✅
   </td>
   <td>🚫
   </td>
  </tr>
  <tr>
   <td>groups
   </td>
   <td>✅
   </td>
   <td>🚫
   </td>
  </tr>
  <tr>
   <td>cpp_string_view
   </td>
   <td>🚫
   </td>
   <td>🚫
   </td>
  </tr>
  <tr>
   <td>java_enum_no_value_of
   </td>
   <td>🚫
   </td>
   <td>🚫
   </td>
  </tr>
  <tr>
   <td>open_enums
   </td>
   <td>🚫
   </td>
   <td>✅
   </td>
  </tr>
  <tr>
   <td>MORE STUFF ...
   </td>
   <td>
   </td>
   <td>
   </td>
  </tr>
 </table>
 ### Managing Complexity of `features` for Large Deployments
 A separate concept has been established to help mitigate the complexity of
 editions and progressive feature rollouts and synchronizations for larger proto
 projects.
 This facility could be used to migrate existing usages of the `syntax` keyword
 to use Editions + Features across google3, for example.
 ## Prior Work
 *   proto{2,3} Convergence Vision (not available externally)
 *   Epochs for descriptor.proto (not available externally)
 *   Rust editions: https://doc.rust-lang.org/edition-guide/editions/index.html