fix(rust): handle enums whose first or only tag is a default tag

[renz011tzar]

Fixes #182.

Two related panics in the Rust backend for enums that declare a default (= ..) tag.

1. enum_is_complete — empty ranges

enum_is_complete filter-maps the tags down to Tag::Value / Tag::Range, then calls ranges.first().unwrap(). An enum whose only tag is a default tag produces an empty vector, so the unwrap panics (mod.rs:997). An enum with no value or range tags cannot cover the backing integer range, so it is not complete — guarded with !ranges.is_empty().

2. default_value — first tag is a default tag

default_value matched on &tags[0], with ast::Tag::Other(_) => todo!() (mod.rs:1075). That is reached whenever the first declared tag is a default tag, even when the enum also declares value tags:

enum Foo : 8 {
  RESERVED = ..,
  A = 0,
}

Per the discussion on #182, this now selects the first tag that is not a default tag, and falls back to the default tag when it is the only tag declared:

enum OpenWithDefaultTagFirst : 8 {   ->  default() == OpenWithDefaultTagFirst::A
  UNKNOWN = ..,
  A = 0,
}

enum OpenWithOnlyDefaultTag : 8 {    ->  default() == OpenWithOnlyDefaultTag::Unknown(Private(0x0))
  UNKNOWN = ..,
}

Testing

• Both cases added to the existing enum_declaration snapshot test; snapshots regenerated with UPDATE_SNAPSHOTS=1.
• The new cases fail on the current main (panicked at mod.rs:1075:31) and pass with this change.
• cargo test -p pdl-compiler: 171 passed, 0 failed.
• Generated output for both enums compiles against pdl-runtime.

One note: the final match arm is unreachable for any parsed input, since the parser rejects an enum with no tags — I used expect rather than leaving a todo!(), but happy to restructure it if you would prefer the arm be total.

---

_{Found and checked while evaluating Rust verification tooling — midas-lex for the specification step, and a small Verus model of both obligations (the empty-ranges index, and that tag selection always yields a value) to confirm the patched forms are total.}

[hchataing]

Looks good! Thank you for the patch :)