add unsafe_precondition to imports

Author: KGrewal1

library/core/src/str/mod.rs

@@ -17,6 +17,7 @@ use self::pattern::{DoubleEndedSearcher, Pattern, ReverseSearcher, Searcher};
 use crate::char::{self, EscapeDebugExtArgs};
 use crate::ops::Range;
 use crate::slice::{self, SliceIndex};
+use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ascii, mem};
 
 pub mod pattern;