Commits on Nov 8, 2024

1. Allow types containing references to have finalizers …

   Previously FSA prevented `Gc<T>`s from having a finalizer if `T`
   (directly or indirectly) contained a reference. This made retro-fitting
   GC to large libraries difficult. It also, somewhat surprisingly,
   completely prevents finalizable nested `Gc<dyn Trait>`s (due to the
   underlying vptr being a `&'static` reference).
   
   This commit loosens this restriction, allowing FSA to interrogate drop
   method bodies (much the same as for `Send + Sync + FinalizerSafe`)
   looking for unsound uses of references.
   
   This change is based on the following observation: a reference inside of
   `T`'s drop method is safe to dereference provided it never originates
   from `T` or one of `T`'s fields. For example:
   
   ```rust
   fn drop(&mut self) {
       let a = 123;
   
       let r1 = &1; // const with 'static lifetime
       let r2 = &a; // stack-local
       let r3 = &*(unsafe { ... }); // unsafe code, all bets are off.
   
       let r4: &u64 = self.a;
       let r5: &u64 = self.b.a;
       let r6: &u64 = self.c[0]; // c: [&u64; 2]
   }
   ```
   
   In this example, `r1`, `r2`, and `r3` are allowed by FSA: `r1` and `r2`
   are sound because their referent will always be valid when the finalizer
   is called. `r3` uses unsafe code, so FSA gets out of the way here, as it
   is up to the user to ensure that this is sound themselves.
   
   `r4`, `r5`, and `r6` on the other hand are not allowed by FSA. This is
   because they are references which are obtained through field or array
   index projections into `T`. Such projections are nearly always unsound,
   so we always prevent these.
   
   This change means that we can remove the `ReferenceFree` trait, since we
   now always check for references inside a drop method. As with our
   relaxed FSA rules for `FinalizerSafe` and friends -- if for any reason
   we can't see into a type, we err on the side of caution and emit an
   error.

   

   jacob-hughes committed Nov 8, 2024
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