Function collect_enum_variants 

pub fn collect_enum_variants(
    modules: &[(&AIRModule, &Path)],
) -> EnumVariantRegistry

Pre-scan every reached module and build the EnumVariantRegistry.

Walks each module’s top-level EnumDecls (the only place enum variants are declared) and records every variant. A pre-scan — rather than recording variants as their decls are emitted — is required because a use site may precede its enum’s declaration in source order (forward reference), and because a used enum’s decl can live in a different module than its construction site (cross-module use). This mirrors the Go backend’s existing collect_methods / collect_optional_returns pre-scans.

The built-in Optional/Result constructors are pre-seeded (B1) so the same registry describes built-in ADTs; backends treat these as a fallback behind their bespoke Optional/Result lowering (see the module comment).