Implement Sync for yksom VM types

This is needed because their trait objects are passed to a GC so must be
FSA-safe.

src/lib/vm/objects/block.rs

@@ -74,10 +74,15 @@ pub struct Block {
     pub method_stack_base: usize,
 }
 
-// By default, `Block` is not `FinalizerSafe` because it has fields to other
-// `Gc`s. These fields would be unsound to access inside a `drop` method, so
-// this explicit impl is needed to tell the compiler we are not doing that.
+// The following impls are needed to allow the GC to finalise `Block`. By
+// default, `Block` is not `FinalizerSafe` because it has fields to other `Gc`s.
+// These fields would be unsound to access inside a `drop` method, so this
+// explicit impl is needed to tell the compiler we are not doing that. The only
+// time a `Block` is shared between threads is inside a finaliser, which is
+// Sync-safe because we guarantee it will not be accessed at the same time as
+// the main-thread.
 unsafe impl FinalizerSafe for Block {}
+unsafe impl Sync for Block {}
 
 impl Obj for Block {
     fn dyn_objtype(self: Gc<Self>) -> ObjType {

src/lib/vm/objects/class.rs

@@ -42,6 +42,7 @@ pub struct Class {
 // thread, it is guaranteed that the finalizer thread will be the only thread
 // accessing its data.
 unsafe impl FinalizerSafe for Class {}
+unsafe impl Sync for Class {}
 
 impl Obj for Class {
     fn dyn_objtype(self: Gc<Self>) -> ObjType {

src/lib/vm/objects/method.rs

@@ -22,6 +22,7 @@ pub struct Method {
 // method. This explicit impl is needed to tell the compiler we are not doing
 // that.
 unsafe impl FinalizerSafe for Method {}
+unsafe impl Sync for Method {}
 
 impl Obj for Method {
     fn dyn_objtype(self: Gc<Self>) -> ObjType {

src/lib/vm/objects/mod.rs

@@ -40,6 +40,7 @@ pub use string_::String_;
 
 use natrob::narrowable_alloy;
 use std::gc::Gc;
+use std::gc::ReferenceFree;
 
 use crate::vm::{
     core::VM,
@@ -80,7 +81,7 @@ impl ObjType {
 /// The main SOM Object trait. Notice that code should almost never call these functions directly:
 /// you should instead call the equivalent function in the `Val` struct.
 #[narrowable_alloy(ThinObj)]
-pub trait Obj: std::fmt::Debug + FinalizerSafe {
+pub trait Obj: std::fmt::Debug + Send + Sync + FinalizerSafe + ReferenceFree {
     /// What `ObjType` does this `Val` represent?
     fn dyn_objtype(self: Gc<Self>) -> ObjType;
 

src/lib/vm/objects/string_.rs

@@ -34,6 +34,7 @@ pub struct String_ {
 // thread, it is guaranteed that the finalizer thread will be the only thread
 // accessing its data.
 unsafe impl FinalizerSafe for String_ {}
+unsafe impl Sync for String_ {}
 
 impl Obj for String_ {
     fn dyn_objtype(self: Gc<Self>) -> ObjType {

src/lib/vm/val.rs

@@ -9,8 +9,6 @@ use num_enum::{IntoPrimitive, UnsafeFromPrimitive};
 use num_traits::{FromPrimitive, ToPrimitive};
 use std::gc::Gc;
 
-use std::gc::NoTrace;
-
 use super::{
     core::VM,
     error::{VMError, VMErrorKind},
@@ -62,8 +60,6 @@ pub struct Val {
     pub val: usize,
 }
 
-impl !NoTrace for Val {}
-
 impl Val {
     /// Create a new `Val` from an object that should be allocated on the heap.
     pub fn from_obj<T: Obj + 'static>(obj: T) -> Val {