Constant `ALREADY_RUNNING_SAME_THREAD`

pub(in ::bridge::server) const ALREADY_RUNNING_SAME_THREAD: LocalKey<Cell<bool>>;

While running a proc-macro with the same-thread executor, this flag will be set, forcing nested proc-macro invocations (e.g. due to TokenStream::expand_expr) to be run using a cross-thread executor.

This is required as the thread-local state in the proc_macro client does not handle being re-entered, and will invalidate all Symbols when entering a nested macro.