We don't forward this variable for storage in any structs, so there's no reason
to go through an Arc instead of returning the `&'static EnvStack` directly.
NB: This particular change was safe, and passes all tests on its own.
We don't forward this variable for storage in any structs, so there's no reason
to go through an Arc instead of returning the `&'static EnvStack` directly.
`Parser` is a single-threaded `!Send`, `!Sync` type and does not need to use
`Arc` for anything. We were using it because that's all we had for the parser's
`EnvStack`, but though that is *technically* protected internally by a mutex
(shared with global EnvStack), there's nothing to say that other parsers with a
narrower scope/lifetime on other threads will be necessarily using the same
backing mutex.
We can safely marshal the existing `Arc<EnvStack>` we get from
`environment::principal()` into an `Rc<EnvStack>` since the underlying reference
is always valid. To prove this point, we could have PRINCIPAL_STACK be a static
`EnvStack` and have `environment::principal()` use `Arc::from_raw()` to turn
that into an `Arc<EnvStack>`, but there's no need to factorize this process.
