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_middle/src/lib.rs

@@ -59,6 +59,7 @@
 #![feature(result_option_inspect)]
 #![feature(const_option)]
 #![feature(trait_alias)]
+#![feature(ptr_alignment_type)]
 #![recursion_limit = "512"]
 #![allow(rustc::potential_query_instability)]
 

compiler/rustc_middle/src/ty/list.rs

@@ -1,4 +1,5 @@
 use crate::arena::Arena;
+use rustc_data_structures::aligned::{align_of, Aligned};
 use rustc_serialize::{Encodable, Encoder};
 use std::alloc::Layout;
 use std::cmp::Ordering;
@@ -198,22 +199,17 @@ impl<'a, T: Copy> IntoIterator for &'a List<T> {
 
 unsafe impl<T: Sync> Sync for List<T> {}
 
-unsafe impl<'a, T: 'a> rustc_data_structures::tagged_ptr::Pointer for &'a List<T> {
-    const BITS: usize = std::mem::align_of::<usize>().trailing_zeros() as usize;
+// Safety:
+// Layouts of `Equivalent<T>` and `List<T>` are the same, modulo opaque tail,
+// thus aligns of `Equivalent<T>` and `List<T>` must be the same.
+unsafe impl<T> Aligned for List<T> {
+    const ALIGN: ptr::Alignment = {
+        #[repr(C)]
+        struct Equivalent<T> {
+            _len: usize,
+            _data: [T; 0],
+        }
 
-    #[inline]
-    fn into_usize(self) -> usize {
-        self as *const List<T> as usize
-    }
-
-    #[inline]
-    unsafe fn from_usize(ptr: usize) -> &'a List<T> {
-        &*(ptr as *const List<T>)
-    }
-
-    unsafe fn with_ref<R, F: FnOnce(&Self) -> R>(ptr: usize, f: F) -> R {
-        // `Self` is `&'a List<T>` which impls `Copy`, so this is fine.
-        let ptr = Self::from_usize(ptr);
-        f(&ptr)
-    }
+        align_of::<Equivalent<T>>()
+    };
 }

compiler/rustc_middle/src/ty/mod.rs

@@ -1627,7 +1627,8 @@ struct ParamTag {
 }
 
 unsafe impl rustc_data_structures::tagged_ptr::Tag for ParamTag {
-    const BITS: usize = 2;
+    const BITS: u32 = 2;
+
     #[inline]
     fn into_usize(self) -> usize {
         match self {
@@ -1637,6 +1638,7 @@ unsafe impl rustc_data_structures::tagged_ptr::Tag for ParamTag {
             Self { reveal: traits::Reveal::All, constness: hir::Constness::Const } => 3,
         }
     }
+
     #[inline]
     unsafe fn from_usize(ptr: usize) -> Self {
         match ptr {