Rollup merge of rust-lang#138309 - DiuDiu777:intrinsic-doc-fix, r=thomcc

Add missing doc for intrinsic (Fix PR135334)

The previous [PR135334](rust-lang#135334) mentioned that some of the intrinsic APIs were missing safety descriptions.

Among intrinsic APIs that miss safety specifications, most are related to numerical operations. They might need to be discussed and then seen how to organize.

Apart from them, only a few intrinsics lack safety. So this PR deals with the APIs with non-numerical operations in priority.

Author: jieyouxu

core/src/intrinsics/mod.rs

@@ -1774,10 +1774,14 @@ pub fn ptr_mask<T>(ptr: *const T, mask: usize) -> *const T;
 /// a size of `count` * `size_of::<T>()` and an alignment of
 /// `min_align_of::<T>()`
 ///
-/// The volatile parameter is set to `true`, so it will not be optimized out
-/// unless size is equal to zero.
-///
 /// This intrinsic does not have a stable counterpart.
+/// # Safety
+///
+/// The safety requirements are consistent with [`copy_nonoverlapping`]
+/// while the read and write behaviors are volatile,
+/// which means it will not be optimized out unless `_count` or `size_of::<T>()` is equal to zero.
+///
+/// [`copy_nonoverlapping`]: ptr::copy_nonoverlapping
 #[rustc_intrinsic]
 #[rustc_nounwind]
 pub unsafe fn volatile_copy_nonoverlapping_memory<T>(dst: *mut T, src: *const T, count: usize);
@@ -1796,10 +1800,13 @@ pub unsafe fn volatile_copy_memory<T>(dst: *mut T, src: *const T, count: usize);
 /// size of `count * size_of::<T>()` and an alignment of
 /// `min_align_of::<T>()`.
 ///
-/// The volatile parameter is set to `true`, so it will not be optimized out
-/// unless size is equal to zero.
-///
 /// This intrinsic does not have a stable counterpart.
+/// # Safety
+///
+/// The safety requirements are consistent with [`write_bytes`] while the write behavior is volatile,
+/// which means it will not be optimized out unless `_count` or `size_of::<T>()` is equal to zero.
+///
+/// [`write_bytes`]: ptr::write_bytes
 #[rustc_intrinsic]
 #[rustc_nounwind]
 pub unsafe fn volatile_set_memory<T>(dst: *mut T, val: u8, count: usize);
@@ -3341,8 +3348,18 @@ pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
 /// The stabilized form of this intrinsic is [`crate::mem::swap`].
 ///
 /// # Safety
+/// Behavior is undefined if any of the following conditions are violated:
+///
+/// * Both `x` and `y` must be [valid] for both reads and writes.
 ///
-/// `x` and `y` are readable and writable as `T`, and non-overlapping.
+/// * Both `x` and `y` must be properly aligned.
+///
+/// * The region of memory beginning at `x` must *not* overlap with the region of memory
+///   beginning at `y`.
+///
+/// * The memory pointed by `x` and `y` must both contain values of type `T`.
+///
+/// [valid]: crate::ptr#safety
 #[rustc_nounwind]
 #[inline]
 #[rustc_intrinsic]