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Rollup merge of #127879 - kornelski:bad-pointer-printf, r=workingjubilee

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Document futility of printing temporary pointers

In the user forum I've seen a few people trying to understand how borrowing and moves are implemented by peppering their code with printing of `{:p}` of references to variables and expressions. This is a bad idea. It gives misleading and confusing results, because of autoderef magic, printing pointers of temporaries on the stack, and/or causes LLVM to optimize code differently when values had their address exposed.
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library/core/src/fmt/mod.rs
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@ -975,9 +975,17 @@ pub trait UpperHex {
/// `p` formatting. /// `p` formatting.
/// ///
/// The `Pointer` trait should format its output as a memory location. This is commonly presented /// The `Pointer` trait should format its output as a memory location. This is commonly presented
/// as hexadecimal. /// as hexadecimal. For more information on formatters, see [the module-level documentation][module].
/// ///
/// For more information on formatters, see [the module-level documentation][module]. /// Printing of pointers is not a reliable way to discover how Rust programs are implemented.
/// The act of reading an address changes the program itself, and may change how the data is represented
/// in memory, and may affect which optimizations are applied to the code.
///
/// The printed pointer values are not guaranteed to be stable nor unique identifiers of objects.
/// Rust allows moving values to different memory locations, and may reuse the same memory locations
/// for different purposes.
///
/// There is no guarantee that the printed value can be converted back to a pointer.
/// ///
/// [module]: ../../std/fmt/index.html /// [module]: ../../std/fmt/index.html
/// ///