Rust: a record holding a struct through an auto-property fails to compile (E0277), because the held struct never derives Clone/Debug/PartialEq #353

What's wrong

The #302 fix (PR #315) makes a struct that a record holds derive Clone, Debug, PartialEq along with the record. It only follows fields, though, not auto-properties:

• The record: it is lowered with Separated(classDecl) (Coder/Languages/RustGenerator.cs:~501). Its own derive check therefore sees the auto-property as a field, and the record derives.
• The held struct: when it is written, Derives (:~739, loop at :~756) walks the original records from GenerationRoot.
• Why the walk misses it: UnderivableReason collects held types only from VariableDeclaration and non-associated FieldDeclaration members (:812-818). A bodiless PropertyDeclaration, which Rust emits as a field, is never walked, so the struct behind it is never marked to derive.

Repro

A SourceFile with:

• struct Inner { int x } (plain struct)
• a record struct Outer whose only member is new PropertyDeclaration("Value", "Inner")

The output is:

pub struct Inner { pub x: i32 }

#[derive(Clone, Debug, PartialEq)]
pub struct Outer { pub Value: Inner }

rustc rejects it: error[E0277]: the trait bound Inner: Clone is not satisfied (and the same for Debug and PartialEq).

The same document with VariableDeclaration("value", "Inner") in place of the property generates #[derive(...)] on Inner and compiles. Reproduced at HEAD 1173b65.

Suggested fix

Make the record walk in UnderivableReason / Derives see what Rust actually emits. Either walk Separated(record), or add a PropertyDeclaration arm for auto-properties (no getter or setter body) that yields its Type.

Acceptance criteria

• A RustGeneratedSourceCompilesTests case (record → auto-property → plain struct) compiles under rustc.
• A record → auto-property → struct → field → struct chain derives transitively, as fields already do.
• Existing Rust: a record with a field of a plain struct type doesn't compile (E0277/E0369) because only the record derives Clone/Debug/PartialEq #302 tests still pass.