add safety sections for three functions in core::mem

library/core/src/mem/mod.rs

@@ -706,16 +706,18 @@ pub const fn needs_drop<T: ?Sized>() -> bool {
 /// This means that, for example, the padding byte in `(u8, u16)` is not
 /// necessarily zeroed.
 ///
-/// There is no guarantee that an all-zero byte-pattern represents a valid value
-/// of some type `T`. For example, the all-zero byte-pattern is not a valid value
-/// for reference types (`&T`, `&mut T`) and function pointers. Using `zeroed`
-/// on such types causes immediate [undefined behavior][ub] because [the Rust
-/// compiler assumes][inv] that there always is a valid value in a variable it
-/// considers initialized.
-///
 /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
 /// It is useful for FFI sometimes, but should generally be avoided.
 ///
+///
+/// # Safety
+///
+/// The all-zero byte-pattern must represent a valid value of type `T`.
+/// For example, it is not valid for reference types (`&T`, `&mut T`) or function
+/// pointers. Using `zeroed` on such types causes immediate [undefined behavior][ub]
+/// because [the Rust compiler assumes][inv] that there always is a valid value in a
+/// variable it considers initialized.
+///
 /// [zeroed]: MaybeUninit::zeroed
 /// [ub]: ../../reference/behavior-considered-undefined.html
 /// [inv]: MaybeUninit#initialization-invariant