CSRUST

Approach D from issue #25: rewrite a C# subset into the existing C++ subset (tools/cs2cpp.py), then run tools/cpprust.py unchanged. CLI: tools/csrust.py. Includes of .cs files go through the same out-of-process protocol as .cpp (shivyc/preproc.py).

Line numbers are preserved across the rewrite so a diagnostic in the generated C++ still points at the user's .cs line.

What a class means (§1)

Crust has no GC. Default class is single ownership: the lowered object is a value with one owner; destruction is at scope exit.

class Node { }              // single owner
[Shared] class Node { }     // shared_ptr; assignment aliases; cycles may leak

Pinned in tests/test_csrust.py (TestSemantics).

Phase 1 surface

In (see tools/cs2cpp.py / tests/test_csrust.py):

Area	Lowering
class / struct / interface / enum	C++ types; interface → pure virtual
fields, methods, constructors, ~T	cpprust method shape T_method(T *this, …)
single inheritance, virtual / abstract	vtables
class Box<T>	template<typename T> class Box → monomorphise
List<T> / Dictionary<K,V>	std::vector / std::map
T[], jagged T[][], .Length	vector, .size()
var, foreach, this., null	auto, range-for, this->, NULL
auto-properties { get; set; }	field + get_ / set_
delegate	typedef function pointer
x => … lambdas	C++ lambdas
throw / catch	raise / except (checked model)
using X = Y;	kept; using System; dropped
--emit-decls	same class digest as C++ (CPPRPY.md)

Out (refused in C# terms before conversion): async/await, LINQ, yield, dynamic, event, multidimensional arrays, ref/out/in parameters, $"…", file-scoped namespaces, ?? / ?., char, lock, decimal, partial, goto, params, stackalloc, checked/unchecked.

Layout

file	role
tools/cs2cpp.py	refusals + C# → C++ subset (translate)
tools/csrust.py	CLI; cs2cpp.translate then cpprust.translate
tests/test_csrust.py	semantics, lowering, Shared, generics, except, digest

A Unity scene plus these scripts is a different job: see UNITY_PACK.md. That packer emits engine.c / data.c with packed instance arrays (_Coin_inst_array[i]) instead of Unity object headers.

Deliberately later

Extracting cpprust_core.py (approach C) — issue §5 milestone 9 — waits until shared seams are known from real use. Full four-language TU demo (C + Rust + C++ + C# in one file) builds on the digest C# already emits; C++ cannot yet inherit from a foreign digest entry (include the header instead).