fix(rust): type union base properties on variants that have no own properties

[wiebren]

Description

Linear ticket: n/a — found while running one OpenAPI document through fern's SDK generators
and comparing what each one produces.

A discriminated union with base properties generates constructors and discriminant-switching
getters that reference those properties on every variant — but the variant struct itself
is emitted as an empty braced variant whenever the variant has no properties of its own. The
enum then cannot compile:

error[E0559]: variant `SrsError::ProcessError` has no field named `message`
error[E0026]: variant `SrsError::ProcessError` does not have a field named `message`

An error taxonomy is the natural way to hit this: a union of error subtypes shares a base
message, and for many subtypes the type name is the information — they add no fields of
their own. One such union with nine message-only subtypes produced 18 compile errors in the
generated crate.

The repository's own seed output already exhibits the bug: the committed
seed/rust-sdk/property-access crate does not build —

Empty {},
...
pub fn get_normal(&self) -> &str {
    match self {
        ...
        Self::Empty { normal, .. } => normal,   // error[E0026]

$ cargo build   # on seed/rust-sdk/property-access as committed on main
error[E0026]: variant `UserOrAdminDiscriminated::Empty` does not have a field named `normal`
error[E0026]: variant `UserOrAdminDiscriminated::Empty` does not have a field named `foo`

Root cause

In generators/rust/model/src/union/UnionGenerator.ts two variant-emission paths skip
generateBaseProperties():

• the noProperties variant shape, and
• an inlined samePropertiesAsObject variant whose referenced object has an empty property
  list.

Both write Variant {},. Everything else already treats base properties as present on every
variant: generateImplementationBlock emits Self::Variant { field, .. } match arms for each
base-property getter across all variants, and the constructor for the empty inlined case
already assigns the base fields into the variant it just declared without them (that is the
E0559 above).

Changes Made

• Emit the union's base properties on both empty-variant paths, keeping the plain Variant {}
  form when the union declares no base properties.
• Let the noProperties constructor accept the base properties, mirroring what the empty
  inlined-object constructor already does.
• Add the rust-union-base-properties test definition (language-prefixed, so only the rust
  generators run it): a union with a base message whose variants include an inlined object
  with an empty property list, an inlined object with its own property, and a noProperties
  variant.
• Regenerate the two affected seed outputs (property-access, rust-union-base-properties).
  No other fixture output changes.

Testing

• pnpm seed test --generator rust-sdk — 149/149 pass with the change.
• cargo build + cargo test on both affected fixtures:

fixture	before	after
property-access	error[E0026] ×2, does not build	builds, 86 tests pass
rust-union-base-properties	error[E0559], error[E0026] ×2, does not build	builds, 86 tests pass

• pnpm turbo run test --filter @fern-api/rust-model --filter @fern-api/rust-sdk — all unit
  tests pass.

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Generated with Claude Code

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[nitpickybot]

AI Review Summary

Straightforward generator fix: empty union variants now carry the union's base properties, matching what the getters/constructors already assume. Seed fixtures regenerated and a new test definition added. One thing worth double-checking: the generated ValidationError variant in the new fixture emits #[serde(default)] field: String before message, which suggests base-property ordering/serde attributes may still be inconsistent, but that's pre-existing behavior outside these lines.

• 🔵 1 suggestion(s)

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[nitpickybot]

🔵 suggestion

This block is now identical to the noProperties branch above (lines 307-317). Worth extracting a small helper, e.g. writeEmptyVariant(writer, variantName), so the two paths can't drift.

[devin-ai-integration]

🔍 Devin Review: 1 flag

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