Implementation of GC is not sound with resource limiters

[alexcrichton]

This line of code:

wasmtime/crates/wasmtime/src/runtime/store/gc.rs

Line 124 in b500820

.grow(delta_pages_for_alloc, Some(self.traitobj().as_mut()))?

expands a borrow of &mut StoreOpaque (self in this case) into a &mut dyn VMStore which is effectively a &mut StoreInner<T>. This is a "widening" operation which makes the original mutable borrow wider by encompassing more fields. This is not a sound operation because a caller might simultaneously be borrowing &mut StoreOpaque and &mut T or similarly.

[alexcrichton]

This is also arguably a problem during instance allocation and allocation of the gc heap