Function std::intrinsics::ub_checks

source ·
pub const fn ub_checks() -> bool
🔬This is a nightly-only experimental API. (core_intrinsics)
Expand description

Returns whether we should perform some UB-checking at runtime. This eventually evaluates to cfg!(ub_checks), but behaves different from cfg! when mixing crates built with different flags: if the crate has UB checks enabled or carries the #[rustc_preserve_ub_checks] attribute, evaluation is delayed until monomorphization (or until the call gets inlined into a crate that does not delay evaluation further); otherwise it can happen any time.

The common case here is a user program built with ub_checks linked against the distributed sysroot which is built without ub_checks but with #[rustc_preserve_ub_checks]. For code that gets monomorphized in the user crate (i.e., generic functions and functions with #[inline]), gating assertions on ub_checks() rather than cfg!(ub_checks) means that assertions are enabled whenever the user crate has UB checks enabled. However if the user has UB checks disabled, the checks will still get optimized out. This intrinsic is primarily used by ub_checks::assert_unsafe_precondition.