[protobuf] Add opt-in enum-name prefixes for enum values #12102

Problem

The @typespec/protobuf emitter writes enum member names verbatim. Protobuf enum values use C++ scoping rules: values are siblings of their enum, so package-level enums cannot share a value name.

For example:

import "@typespec/protobuf";

using Protobuf;

@package({ name: "example.v1" })
namespace Example;

enum OrderState {
  Unspecified: 0,
  Pending: 1,
}

enum PaymentState {
  Unspecified: 0,
  Pending: 1,
}

model Order {
  @field(1) state: OrderState;
  @field(2) paymentState: PaymentState;
}

With the current emitter, protoc rejects the generated file because Unspecified and Pending each occur twice in example.v1. Avoiding this requires repeating the enum name in every TypeSpec member.

Both AIP-126 and the Protobuf style guide recommend prefixing all package-level enum values with the enum name. The style guide explicitly recommends:

Prefix Every Value With the Enum Name (Converted to UPPER_SNAKE_CASE)

Proposal

Add an opt-in emitter option:

options:
  "@typespec/protobuf":
    enum-value-prefix: enum-name

With this option, the example emits:

enum OrderState {
  ORDER_STATE_UNSPECIFIED = 0;
  ORDER_STATE_PENDING = 1;
}

enum PaymentState {
  PAYMENT_STATE_UNSPECIFIED = 0;
  PAYMENT_STATE_PENDING = 1;
}

• Convert enum and member names to UPPER_SNAKE_CASE, treating acronyms as words.
• Preserve already-prefixed names and avoid adding a second prefix after case conversion.
• Report collisions in the resulting Protobuf package scope.
• Default to none, preserving existing output.
• Preserve enum type names, numbers, order, documentation, and aliases.
• Keep members and numbers explicit: do not synthesize an Unspecified member or infer numeric values.

This is a naming change only. Although binary numeric values remain unchanged, opting in changes generated identifiers and Protobuf JSON enum strings.

Related

microsoft/typespec#1857 tracks collision diagnostics more generally. This proposal adds an opt-in naming convention that prevents common enum value collisions without requiring verbose TypeSpec members.