Affected versions allow arbitrary caller-provided code to execute before the lifetime of main. If the caller-provided code accesses particular pieces of the standard library that require an initialized Rust runtime, such as std::io or std::thread, these may not behave as documented. Panics are likely; UB is possible. The flaw was corrected by enforcing that only code written within the inventory crate, which is guaranteed not to access runtime-dependent parts of the standard library, runs before main. Caller-provided code is restricted to running at compile time.