Rollup merge of rust-lang#103694 - WaffleLapkin:mask_doc_example, r=scottmcm

Add documentation examples for `pointer::mask`

The examples are somewhat convoluted, but I don't know how to make this better :(

Author: Dylan-DPC

library/core/src/ptr/const_ptr.rs

@@ -568,6 +568,31 @@ impl<T: ?Sized> *const T {
     ///
     /// For non-`Sized` pointees this operation changes only the data pointer,
     /// leaving the metadata untouched.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// #![feature(ptr_mask, strict_provenance)]
+    /// let v = 17_u32;
+    /// let ptr: *const u32 = &v;
+    ///
+    /// // `u32` is 4 bytes aligned,
+    /// // which means that lower 2 bits are always 0.
+    /// let tag_mask = 0b11;
+    /// let ptr_mask = !tag_mask;
+    ///
+    /// // We can store something in these lower bits
+    /// let tagged_ptr = ptr.map_addr(|a| a | 0b10);
+    ///
+    /// // Get the "tag" back
+    /// let tag = tagged_ptr.addr() & tag_mask;
+    /// assert_eq!(tag, 0b10);
+    ///
+    /// // Note that `tagged_ptr` is unaligned, it's UB to read from it.
+    /// // To get original pointer `mask` can be used:
+    /// let masked_ptr = tagged_ptr.mask(ptr_mask);
+    /// assert_eq!(unsafe { *masked_ptr }, 17);
+    /// ```
     #[unstable(feature = "ptr_mask", issue = "98290")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline(always)]

library/core/src/ptr/mut_ptr.rs

@@ -588,6 +588,34 @@ impl<T: ?Sized> *mut T {
     ///
     /// For non-`Sized` pointees this operation changes only the data pointer,
     /// leaving the metadata untouched.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// #![feature(ptr_mask, strict_provenance)]
+    /// let mut v = 17_u32;
+    /// let ptr: *mut u32 = &mut v;
+    ///
+    /// // `u32` is 4 bytes aligned,
+    /// // which means that lower 2 bits are always 0.
+    /// let tag_mask = 0b11;
+    /// let ptr_mask = !tag_mask;
+    ///
+    /// // We can store something in these lower bits
+    /// let tagged_ptr = ptr.map_addr(|a| a | 0b10);
+    ///
+    /// // Get the "tag" back
+    /// let tag = tagged_ptr.addr() & tag_mask;
+    /// assert_eq!(tag, 0b10);
+    ///
+    /// // Note that `tagged_ptr` is unaligned, it's UB to read from/write to it.
+    /// // To get original pointer `mask` can be used:
+    /// let masked_ptr = tagged_ptr.mask(ptr_mask);
+    /// assert_eq!(unsafe { *masked_ptr }, 17);
+    ///
+    /// unsafe { *masked_ptr = 0 };
+    /// assert_eq!(v, 0);
+    /// ```
     #[unstable(feature = "ptr_mask", issue = "98290")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline(always)]