Code shared from the Rust Playground

playground.rs

struct DefaultValueMarker;
struct DefaultFunctionMarker;

trait GcResourceMutability {}


// Immutable GC resources can only be immutably borrowed, but both in sync and
// async ops.
struct GcResourceImmutable;
impl GcResourceMutability for GcResourceImmutable {}

// Mutable GC resources can both be immutably and mutably borrowed, but only in
// sync ops. They can not be used in async ops at all.
struct GcResourceSyncMutable;
impl GcResourceMutability for GcResourceSyncMutable {}


trait GcResource {
    type Mutability: GcResourceMutability;
}

// -- AN IMMUTABLE STRUCT --

struct DataResource {}
impl GcResource for DataResource {
    type Mutability = GcResourceImmutable;
}

fn op_data_read(#[gc] data: &DataResource) {} // this is OK
fn op_data_write(#[gc] data: &mut DataResource) {} // this is NOT OK
async fn op_data_read_async(#[gc] data: &DataResource) {} // this is OK
async fn op_data_write_async(#[gc] data: &mut DataResource) {} // this is NOT OK

// -- A MUTABLE STRUCT --

struct SocketResource {}
impl GcResource for SocketResource {
    type Mutability = GcResourceSyncMutable;
}

fn op_data_read(#[gc] socket: &SocketResource) {} // this is OK
fn op_data_write(#[gc] socket: &mut SocketResource) {} // this is NOT OK
async fn op_data_read_async(#[gc] socket: &SocketResource) {} // this is NOT OK
async fn op_data_write_async(#[gc] socket: &mut SocketResource) {} // this is NOT OK