Next Steps

This is a loose, prioritized TODO list of features we plan to add next. It is not meant to be comprehensive — a lot of coverage will come as we try more complex examples.

Interpreter features

Roughly in priority order (top first):

• PassMode::Cast in the native ABI. Passing/returning small structs and arrays by value across the C ABI. The single largest bucket of ignored tests. Needs CastTarget exposed from rustc_public (see below).
• Atomic intrinsics (atomic_load/atomic_store/atomic_xsub, …). Needed by anything with an atomic refcount or an interior-mutable static: Arc coerces and dispatches correctly but cannot be dropped, and AtomicU32::store is unsupported.
• Float comparison (Eq/ordering on f32/f64).
• Calling a fn pointer through the Fn traits. The FnOnce/FnMut impls for fn pointers are shims without bodies, so passing a function (rather than a closure) where a closure is expected fails with an argument count mismatch — e.g. iter().max(), which calls Ord::cmp through one.
• Other collections (String growth beyond the basics, maps, sets). Vec works; the rest are untested.
• Crate loading.
• Unwinding. UnwindAction is ignored, so a panic ends interpretation without running cleanup and destructors do not run on the panic path.
• Assert messages. AssertMessage::description drops the operands, so a failed bounds check reports index out of bounds where rustc reports index out of bounds: the len is 2 but the index is 5.
• Rendering formatted panic messages. panic!("{x}"), assert_eq! and libcore’s UB checks build a core::fmt::Arguments, and Result::unwrap formats the Err through dyn Debug. Rendering them needs dyn Debug dispatch through vtables whose slots hold our synthetic reified addresses.
• Threads. Needs rustc_public support, then making the Statics/Code segments shareable across threads (they are currently single-threaded).

Native calls

• PassMode::Cast support (requires CastTarget in rustc_public) — see the PassMode::Cast interpreter item above.
• Symbol caching (avoid repeated dlsym lookups).
• Trampoline caching (reuse compiled trampolines for identical signatures).
• Function-pointer callbacks (native → interpreter via JIT stubs). A reified fn pointer maps to an Instance we interpret, but its synthetic address is not real machine code, so native code cannot call it until we JIT re-entry stubs.
• Handle uninitialized padding bytes written in native code. Possible changes:
  • Track all memory reachable across the interpreter/native boundary.
  • Change Value to use MaybeUninit and treat padding as uninitialized.
  • Sanitize values from native code by zeroing their padding bytes.

RustC Public

Improvements we would like in rustc_public:

• Add PlaceRef to make processing a Place more efficient (would let the interpreter avoid recreating places).
• Add a way to retrieve all mono items.
• Add a way to retrieve the source for a span (for annotate_snippets).
• Expose CastTarget details from PassMode::Cast (needed for struct pass/return in the C ABI) — see the PassMode::Cast interpreter item.
• Build the caller location from the call terminator’s fn_span, as rustc_codegen_ssa does, rather than from source_info.span. The two differ for a method call: rustc and Miri report x.unwrap() at the unwrap segment, while Body::caller_location reports the start of the receiver, so a panic’s column does not match. TerminatorKind::Call would also need to expose fn_span.