It is an index into a thread local vector. ```rust #![feature(rustc_private)] extern crate syntax_pos; extern crate crossbeam; use syntax_pos::symbol::Symbol; fn assert_sync<T: Sync>() {} fn main() { assert_sync::<Symbol>(); let sym = Symbol::intern("share"); crossbeam::scope(|scope| { scope.spawn(|| { println!("{}", sym); }); }); } ``` ``` thread '<unnamed>' panicked at 'index out of bounds: the len is 0 but the index is 4294967237', /checkout/src/libcollections/vec.rs:1552 ```