`Arc` vs `&` in `AddressSpace`

[rileyshahar]

The methods of AddressSpace take &dyn DataSource, e.g.:

cs393_vm_api/src/address_space.rs

Lines 43 to 50 in 2d1299b

	pub fn add_mapping(
	&self,
	source: &dyn DataSource,
	offset: usize,
	span: usize,
	) -> Result<VirtualAddress, &str> {
	todo!()
	}

But the struct itself stores Arc<dyn DataSource>:

cs393_vm_api/src/address_space.rs

Line 9 in 2d1299b

source: Arc<dyn DataSource>,

My instinct is that add_mapping should just take an Arc. The broader issue is that the methods of DataSource only have shared access to self, and there's no way to safely upcast &self to Arc<self>. So as I see it, either:

1. The DataSource methods to have a stronger reference to self.
2. MapType needs to store &dyn DataSource (maybe fine! The DataSource API is immutable anyway).
3. There's some clever invariant that will make it ok to unsafely mutate &dyn DataSource into Arc<dyn DataSource>. This seems pretty unlikely to me.

[dylanmc]

Dyn trait objects in Rust are new to me - so these are great insights.