Adds "Legacy Syntax Editions" to the GitHub code repository.

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docs/design/editions/README.md

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 *   [Edition Evolution](edition-evolution.md)
 *   [Edition Naming](edition-naming.md)
 *   [Editions Feature Visibility](editions-feature-visibility.md)
+*   [Legacy Syntax Editions](legacy-syntax-editions.md)

docs/design/editions/legacy-syntax-editions.md

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+# Legacy Syntax Editions
+
+**Author:** [@mkruskal-google](https://github.com/mkruskal-google)
+
+**Approved:** 2023-09-08
+
+Should proto2/proto3 be treated as editions?
+
+## Background
+
+[Edition Zero Features](edition-zero-features.md) lays out our plan for edition
+2023, which will unify proto2 and proto3. Since early in the design process,
+we've discussed the possibility of making proto2 and proto3 "special" editions,
+but never laid out what exactly it would look like or determined if it was
+necessary.
+
+We recently redesigned editions to be represented as enums
+([Edition Naming](edition-naming.md)), and also how edition defaults are
+propagated to generators and runtimes
+([Editions: Life of a FeatureSet](editions-life-of-a-featureset.md)). With these
+changes, there could be an opportunity to special-case proto2 and proto3 in a
+beneficial way.
+
+## Problem Description
+
+While the original plan was to keep editions and syntax orthogonal, that naively
+means we'd be supporting two very different codebases. This has some serious
+maintenance costs though, especially when it comes to test coverage. We could
+expect to have sub-optimal test coverage of editions initially, which would
+gradually become poor coverage of syntax later. Since we need to support both
+syntax and editions long-term, this isn't ideal.
+
+In the implementation of editions in C++, we decided to unify a lot of the
+infrastructure to avoid this issue. We define global feature sets for proto2 and
+proto3, and try to use those internally instead of checking syntax directly. By
+pushing the syntax/editions branch earlier in the stack, it gives us a lot of
+indirect test coverage for editions much earlier.
+
+A separate issue is how Prototiller will support the conversion of syntax to
+edition 2023. For features it knows about, we can hardcode defaults into the
+transforms. However, third party feature owners will have no way of signaling
+what the old proto2/proto3 behavior was, so Prototiller won't be able to provide
+any transformations by default. They'd need to provide custom Prototiller
+transforms hardcoding all of their features.
+
+## Recommended Solution
+
+We recommend adding two new special editions to our current set:
+
+```
+enum Edition {
+  EDITION_UNKNOWN = 0;
+  EDITION_PROTO2 = 998;
+  EDITION_PROTO3 = 999;
+  EDITION_2023 = 1000;
+}
+```
+
+These will be treated the same as any other edition, except in our parser which
+will reject `edition = "proto2"` and `edition = "proto3"` in proto files. The
+real benefit here is that this allows features to specify what their
+proto2/proto3 defaults are, making it easier for Prototiller to handle
+migration. It also allows generators and runtimes to unify their internals more
+completely, treating proto2/proto3 files exactly the same as editions.
+
+### Serialized Descriptors
+
+As we now know, there are a lot of serialized `descriptor.proto` descriptor sets
+out there that need to continue working for O(months). In order to avoid
+blocking edition zero for that long, we may need fallbacks in protoc for the
+case where feature resolution *fails*. If the file is proto2/proto3, failure
+should result in a fallback to the existing hardcoded defaults. We can remove
+these later once we're willing to break stale `descriptor.proto` snapshots that
+predate the changes in this doc.
+
+### Bootstrapping
+
+In order to get feature resolution running in proto2 and proto3, we need to be
+able to support bootstrapped protos. For these builds, we can't use any
+reflection without deadlocking, which means feature defaults can't be compiled
+during runtime. We would have had to solve this problem anyway when it came time
+to migrate these protos to editions, but this proposal forces our hand early.
+Luckily, "Editions: Life of a FeatureSet" already set us up for this scenario,
+and we have Blaze rules for embedding these defaults into code. For C++
+specifically, this will need to be checked in alongside the other bootstrapped
+protos. Other languages will be able to do this more dynamically via genrules.
+
+### Feature Inference
+
+While we can calculate defaults using the same logic as in editions, actually
+inferring "features" from proto2/proto3 needs some custom code. For example:
+
+*   The `required` keyword sets `LEGACY_REQUIRED` feature
+*   The `optional` keyword in proto3 sets `EXPLICIT` presence
+*   The `group` keyword implies `DELIMITED` encoding
+*   The `enforce_utf8` options flips between `PACKED` and `EXPANDED` encoding
+
+This logic needs to be written in code, and will need to be duplicated in every
+language we support. Any language-specific feature transformations will also
+need to be included in that language. To make this as portable as possible, we
+will define functions like:
+
+Each type of descriptor will have its own set of transformations that should be
+applied to its features for legacy editions.
+
+#### Pros
+
+*   Makes it clearer that proto2/proto3 are "like" editions
+
+*   Gives Prototiller a little more information in the transformation from
+    proto2/proto3 to editions (not necessarily 2023)
+
+*   Allows proto2/proto3 defaults to be specified in a single location
+
+*   Makes unification of syntax/edition code easier to implement in runtimes
+
+*   Allows cross-language proto2/proto3 testing with the conformance framework
+    mentioned in "Editions: Life of a FeatureSet"
+
+#### Cons
+
+*   Adds special-case legacy editions, which may be somewhat confusing
+
+*   We will need to port feature inference logic across all languages. This is
+    arguably cheaper than maintaining branched proto2/proto3 code in all
+    languages though
+
+## Considered Alternatives
+
+### Do Nothing
+
+If we do nothing, there will be no built-in unification of syntax and editions.
+Runtimes could choose any point to split the logic.
+
+#### Pros
+
+*   Requires no changes to editions code
+
+#### Cons
+
+*   Likely results in lower test coverage
+*   May hide issues until we start rolling out edition 2023
+*   Prototiller would have to hard-code proto2/proto3 defaults of features it
+    knows, and couldn't even try to migrate runtimes it doesn't