Auto merge of #110243 - WaffleLapkin:bless_tagged_pointers🙏, r=Nilstrieb

Tagged pointers, now with strict provenance!

This is a big refactor of tagged pointers in rustc, with three main goals:
1. Porting the code to the strict provenance
2. Cleanup the code
3. Document the code (and safety invariants) better

This PR has grown quite a bit (almost a complete rewrite at this point...), so I'm not sure what's the best way to review this, but reviewing commit-by-commit should be fine.

r? `@Nilstrieb`

compiler/rustc_data_structures/src/aligned.rs

@@ -0,0 +1,33 @@
+use std::ptr::Alignment;
+
+/// Returns the ABI-required minimum alignment of a type in bytes.
+///
+/// This is equivalent to [`mem::align_of`], but also works for some unsized
+/// types (e.g. slices or rustc's `List`s).
+///
+/// [`mem::align_of`]: std::mem::align_of
+pub const fn align_of<T: ?Sized + Aligned>() -> Alignment {
+    T::ALIGN
+}
+
+/// A type with a statically known alignment.
+///
+/// # Safety
+///
+/// `Self::ALIGN` must be equal to the alignment of `Self`. For sized types it
+/// is [`mem::align_of<Self>()`], for unsized types it depends on the type, for
+/// example `[T]` has alignment of `T`.
+///
+/// [`mem::align_of<Self>()`]: std::mem::align_of
+pub unsafe trait Aligned {
+    /// Alignment of `Self`.
+    const ALIGN: Alignment;
+}
+
+unsafe impl<T> Aligned for T {
+    const ALIGN: Alignment = Alignment::of::<Self>();
+}
+
+unsafe impl<T> Aligned for [T] {
+    const ALIGN: Alignment = Alignment::of::<T>();
+}

compiler/rustc_data_structures/src/lib.rs

@@ -29,6 +29,8 @@
 #![feature(get_mut_unchecked)]
 #![feature(lint_reasons)]
 #![feature(unwrap_infallible)]
+#![feature(strict_provenance)]
+#![feature(ptr_alignment_type)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
@@ -82,6 +84,7 @@ pub mod transitive_relation;
 pub mod vec_linked_list;
 pub mod work_queue;
 pub use atomic_ref::AtomicRef;
+pub mod aligned;
 pub mod frozen;
 pub mod owned_slice;
 pub mod sso;