fix(rust): index unit structs — bodiless is a definition, not a forward decl (#1800)

Land upstream PR #1514 for issue #1513 by cherry-picking
ctype_lab's a94c9dc94eee348bf63c7e2dd567c0b43577678a.

Keep allowBodilessStruct as a Rust-only opt-in, with matching behavior in
the wasm/TypeScript walker and native kernel. Create the node before
checking for a body and walk members only when present. Resolve against
main's shared struct/union walker while retaining its stack guard, kinds,
fields, and existing impl-receiver fixes.

Verified FAIL to PASS on Linux x64 with Node 22.19.0 after rebuilding via
tsc, copy-assets, and build:kernel. The identical fixture on main 7b339373
had two structs and two implements edges; fresh wasm (CODEGRAPH_KERNEL=0)
and native indexes now have three of each. UnitStruct and its Greet
implements edge are recovered; tuple and brace structs remain intact.
The native run loaded the rebuilt kernel and completed without fallback.

Focused extraction.test.ts and kernel-rustlang-parity.test.ts runs:
645 tests passed with the kernel disabled, and 645 with it enabled;
all three parity tests ran in each configuration, with no skips.

(cherry picked from commit a94c9dc94eee348bf63c7e2dd567c0b43577678a)

Co-authored-by: ctype_lab <cksgud1226@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

src/extraction/languages/rust.ts

@@ -81,6 +81,9 @@ export const rustExtractor: LanguageExtractor = {
   methodTypes: ['function_item', 'function_signature_item'],
   interfaceTypes: ['trait_item'],
   structTypes: ['struct_item'],
+  // `struct Unit;` is a unit struct — a complete definition with no body
+  // field, not a forward declaration. Rust has no forward declarations.
+  allowBodilessStruct: true,
   // Unions share struct member syntax and impl attachment, but retain their
   // distinct semantic kind in the graph.
   unionTypes: ['union_item'],

src/extraction/tree-sitter-types.ts

@@ -187,6 +187,19 @@ export interface LanguageExtractor {
    * bodiless class IS complete (Kotlin `class Empty`, Scala `case object`). (#1093)
    */
   skipBodilessClass?: boolean;
+  /**
+   * Keep a bodiless struct node — it IS a complete definition, not a forward
+   * declaration. Set only for languages where a bodiless `struct` is complete:
+   * Rust's unit struct (`struct Unit;`). Leave unset for C/C++, where
+   * `struct Foo;` is a forward declaration.
+   *
+   * Opposite polarity from `skipBodilessClass` (#1093) because the defaults
+   * differ: a bodiless CLASS is kept unless a language opts into skipping,
+   * a bodiless STRUCT is skipped unless a language opts into keeping. The
+   * hardcoded C# `record_declaration` carve-out (#831) is the same situation
+   * predating this flag.
+   */
+  allowBodilessStruct?: boolean;
   /** NodeKind to use for interface-like declarations (Rust: 'trait'). Default: 'interface' */
   interfaceKind?: NodeKind;
 

src/extraction/tree-sitter.ts

@@ -1974,8 +1974,16 @@ export class TreeSitterExtractor {
     // Skip forward declarations and type references (no body = not a definition)
     // — EXCEPT C# positional records (`record struct M(decimal Amount);`),
     // complete definitions with no body block. (#831)
+    //
+    // `allowBodilessStruct` is the per-language escape hatch for the same
+    // situation: a bodiless struct that IS a complete definition (Rust's unit
+    // struct `struct Unit;`). Opposite polarity from `skipBodilessClass`
+    // (#1093) because the two defaults differ — a bodiless CLASS is kept
+    // unless a language opts into skipping, a bodiless STRUCT is skipped
+    // unless a language opts into keeping.
     const body = getChildByField(node, this.extractor.bodyField);
-    if (!body && node.type !== 'record_declaration') return;
+    if (!body && node.type !== 'record_declaration' && !this.extractor.allowBodilessStruct)
+      return;
 
     const name = extractName(node, this.source, this.extractor);
     const docstring = getPrecedingDocstring(node, this.source);