Constant CAPTURE_PANIC_DETAILS 

const CAPTURE_PANIC_DETAILS: LocalKey<RefCell<usize>>;

Available on crate feature panic and non-target_family=wasm only.

Keeps track of how many catch_unwind_with_details calls we are inside.

When non-zero, the enhanced panic handler records details (the panic location and a backtrace) about caught panics into CAUGHT_PANIC_DETAILS before letting the unwind proceed.