[v0ropaev]

A proto2 extension of an enum that doesn't start at zero reads as 0 when it's absent and has no [default = ...].

enum Level { LOW = 1; HIGH = 2; }
extend Carrier { optional Level level = 100; }

On main Carrier::default().extension_or_default(&LEVEL) returns 0, which isn't a Level. An editions enum with option features.enum_type = CLOSED and values 3 and 4 does the same and gives 0 instead of 3. Everywhere else buffa already reads an absent closed enum as its first value: the generated Default, reflection since #535, and the map codec. That's also the default the descriptor gives the extension.

The cause is in buffa-codegen/src/extension.rs, which only builds a default fn when default_value is non-empty. These extensions get Extension::new, so extension_or_default falls through to i32::default().

Now a singular extension whose enum resolves as closed gets Extension::with_default with a fn returning Enumeration::to_i32(&<Level as Default>::default()). That's the expression reflection already uses (reflect_view::enum_default_number). Going through Default keeps it working for an extern_path enum whose values codegen can't see, which is also why it isn't a const fn. Open enums are left alone because their first value has to be 0, and repeated extensions have no default. A closed enum that does start at 0, like Color in ext_json.proto, gets a fn that returns 0. That's harmless, and I didn't special-case it because codegen only records first values when enum_type overrides are configured.

enum_extension_doc_mentions_type now uses the Color from gen_in_pkg, since it pointed at an enum missing from its context and the closed-enum path now resolves it.

There are two new codegen unit tests and two buffa-test cases on real generated code (proto2 in custom_options.proto, editions in group_ext.proto with an open enum next to it as a control). With only the tests applied on main, the closed-enum codegen test fails and the buffa-test pair fails with left: 0, right: 1 and left: 0, right: 3.

fmt, clippy with -D warnings and the buffa-codegen, buffa and buffa-test suites pass on 1.95.0. No checked-in generated code changes, since none of the bundled protos declares an enum extension.

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