Rollup merge of rust-lang#158651 - tshepang:restrict-use-of-unsafe, r=LawnGnome

ptr doc: reduce use of unsafe block to where needed

Author: JonathanBrouwer

library/core/src/ptr/mod.rs

@@ -300,27 +300,25 @@
 //! represent the tagged pointer as an actual pointer and not a `usize`*. For instance:
 //!
 //! ```
-//! unsafe {
-//!     // A flag we want to pack into our pointer
-//!     static HAS_DATA: usize = 0x1;
-//!     static FLAG_MASK: usize = !HAS_DATA;
-//!
-//!     // Our value, which must have enough alignment to have spare least-significant-bits.
-//!     let my_precious_data: u32 = 17;
-//!     assert!(align_of::<u32>() > 1);
-//!
-//!     // Create a tagged pointer
-//!     let ptr = &my_precious_data as *const u32;
-//!     let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
-//!
-//!     // Check the flag:
-//!     if tagged.addr() & HAS_DATA != 0 {
-//!         // Untag and read the pointer
-//!         let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
-//!         assert_eq!(data, 17);
-//!     } else {
-//!         unreachable!()
-//!     }
+//! // A flag we want to pack into our pointer
+//! static HAS_DATA: usize = 0x1;
+//! static FLAG_MASK: usize = !HAS_DATA;
+//!
+//! // Our value, which must have enough alignment to have spare least-significant-bits.
+//! let my_precious_data: u32 = 17;
+//! assert!(align_of::<u32>() > 1);
+//!
+//! // Create a tagged pointer
+//! let ptr = &my_precious_data as *const u32;
+//! let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
+//!
+//! // Check the flag:
+//! if tagged.addr() & HAS_DATA != 0 {
+//!     // Untag and read the pointer
+//!     let data = unsafe { *tagged.map_addr(|addr| addr & FLAG_MASK) };
+//!     assert_eq!(data, 17);
+//! } else {
+//!     unreachable!()
 //! }
 //! ```
 //!