gen-ts/gen-js: a const named __NOT_EMITTED__, __DECL_ORDER__ or __STRUCT_ORDER__ makes the artifact redeclare it #6619

Problem

t27c gen-ts and t27c gen-js exit with rc 0 when a spec declares a const named __NOT_EMITTED__, __DECL_ORDER__ or __STRUCT_ORDER__. These are names every artifact exports itself, so the artifact binds the same name twice, and loading it fails:

module c {
pub const __NOT_EMITTED__ : u8 = 1;
}

$ t27c gen-ts in.t27 > a.ts            # rc 0
export const __NOT_EMITTED__ = 1 satisfies number;
...
export const __NOT_EMITTED__ = Object.freeze([] as const);

$ node --experimental-strip-types h.ts  # h.ts: import * as m from "./a.ts"
SyntaxError: Identifier '__NOT_EMITTED__' has already been declared

• __DECL_ORDER__ and __STRUCT_ORDER__: a const with either name fails the same way under gen-ts.
• gen-js: fails the same way.
• gen-python: does not crash. The second assignment __NOT_EMITTED__ = [] silently replaces the spec's own value, so the artifact is wrong without any error.

This is present on master. It is not introduced by #6613, which only refuses these names for the opt-in --fn lowering (#6559).

Expected

Each declaration-level backend should either refuse the clashing declaration with a named reason (listed in __NOT_EMITTED__ or returned as an error), or refuse the whole file. The artifact must never bind its own bookkeeping names twice.

Found in the review of #6613 (#6613 (comment)). Refs #6559.

Boundary

• bootstrap/src/compiler.rs

User Scenarios

• Given a spec file declaring pub const __NOT_EMITTED__ : u8 = 1;, when running t27c gen-ts on it, then the compiler must reject the declaration with a named error reason and exit non-zero.
• Given a spec file declaring pub const __DECL_ORDER__ : u8 = 2;, when running t27c gen-js on it, then the compiler must reject the declaration with a named error reason and exit non-zero.
• Given a spec file declaring pub const __STRUCT_ORDER__ : u8 = 3;, when running t27c gen-ts on it, then the compiler must reject the declaration with a named error reason and exit non-zero.

Requirements

• FR-001: The compiler MUST reject any spec that declares a const named __NOT_EMITTED__, __DECL_ORDER__, or __STRUCT_ORDER__ when targeting gen-ts or gen-js.
• FR-002: The rejection MUST produce a diagnostic that names the reserved identifier and the reason (artifact bookkeeping name clash).
• FR-003: The compiler MUST exit with a non-zero code when such a declaration is present.
• FR-004: The generated artifact MUST never contain duplicate bindings for __NOT_EMITTED__, __DECL_ORDER__, or __STRUCT_ORDER__.

Success Criteria

• t27c gen-ts spec_not_emitted.t27 exits non-zero and prints a diagnostic containing __NOT_EMITTED__ and "reserved" (or equivalent) — bootstrap/src/compiler.rs
• t27c gen-js spec_decl_order.t27 exits non-zero and prints a diagnostic containing __DECL_ORDER__ and "reserved" (or equivalent) — bootstrap/src/compiler.rs
• t27c gen-ts spec_struct_order.t27 exits non-zero and prints a diagnostic containing __STRUCT_ORDER__ and "reserved" (or equivalent) — bootstrap/src/compiler.rs
• Existing valid specs continue to compile: cargo test --package t27c -- gen_ts gen_js passes (5 tests) — bootstrap/src/compiler.rs