Constant GC allocations when sending large messages to threads?

[cc]

On Friday, 31 January 2020 at 15:47:26 UTC, Steven Schveighoffer 
wrote:
> You could use RefCounted to build a struct that then is 
> sendable with the data you need. RefCounted allocates using C 
> malloc, not the GC.

Thanks for the tips.  How exactly would I go about sending a 
RefCounted value?

static struct Foo {
	int a;
	@disable this(this);
}
auto t = RefCounted!Foo(Foo(5));
tid.send(t);

Gives me: phobos\std\concurrency.d(625): Error: static assert:  
"Aliases to mutable thread-local data not allowed."

Whereas trying to declare it as immutable gives me a pile of 
errors including: Error: mutable method 
`std.typecons.RefCounted!(int, 
cast(RefCountedAutoInitialize)1).RefCounted.__postblit` is not 
callable using a `immutable` object