Perform FSA on a GC entry point's return type #143
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Previously, when we encountered a GC FSA entry-point (e.g. `Gc::new`) we
performed FSA on the argument type of that entry point. When `Gc::new`
was our only entry point, this worked fine, because it would always
resolve to the `T` in `Gc<T>`.
However, with the addition the of `Gc::from` conversion trait entry
point, we can end up being overly conservative with FSA for no good
reason. Consider the following:
```rust
let x: Gc<HasRef> = Gc::from(Box::new(HasRef::default()));
```
Where the argument type to this entry-point is `Box<HasRef>`, but the
actual constructed `Gc` type is `Gc<HasRef>`. Using our previous
approach, we would have to perform FSA on `Box<HasRef>`, even though the
`Box` is never used in the context of GC. On large codebases, this lead
to FSA rejecting sound `Gc<T>`s.
This commit therefore tweaks FSA to check the return type of an
entry-point, which should always be some `Gc<T>` (we assert! this,
because if it isn't, something has gone very wrong.)
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Previously, when we encountered a GC FSA entry-point (e.g.
Gc::new) we performed FSA on the argument type of that entry point. WhenGc::newwas our only entry point, this worked fine, because it would always resolve to theTinGc<T>.However, with the addition the of
Gc::fromconversion trait entry point, we can end up being overly conservative with FSA for no good reason. Consider the following:Where the argument type to this entry-point is
Box<HasRef>, but the actual constructedGctype isGc<HasRef>. Using our previous approach, we would have to perform FSA onBox<HasRef>, even though theBoxis never used in the context of GC. On large codebases, this lead to FSA rejecting soundGc<T>s.This commit therefore tweaks FSA to check the return type of an entry-point, which should always be some
Gc<T>(we assert! this, because if it isn't, something has gone very wrong.)