Auto merge of #144081 - RalfJung:const-ptr-fragments, r=oli-obk

const-eval: full support for pointer fragments

This fixes https://github.com/rust-lang/const-eval/issues/72 and makes `swap_nonoverlapping` fully work in const-eval by enhancing per-byte provenance tracking with tracking of *which* of the bytes of the pointer this one is. Later, if we see all the same bytes in the exact same order, we can treat it like a whole pointer again without ever risking a leak of the data bytes (that encode the offset into the allocation). This lifts the limitation that was discussed quite a bit in https://github.com/rust-lang/rust/pull/137280.

For a concrete piece of code that used to fail and now works properly consider this example doing a byte-for-byte memcpy in const without using intrinsics:
```rust
use std::{mem::{self, MaybeUninit}, ptr};

type Byte = MaybeUninit<u8>;

const unsafe fn memcpy(dst: *mut Byte, src: *const Byte, n: usize) {
    let mut i = 0;
    while i < n {
        *dst.add(i) = *src.add(i);
        i += 1;
    }
}

const _MEMCPY: () = unsafe {
    let ptr = &42;
    let mut ptr2 = ptr::null::<i32>();
    // Copy from ptr to ptr2.
    memcpy(&mut ptr2 as *mut _ as *mut _, &ptr as *const _ as *const _, mem::size_of::<&i32>());
    assert!(*ptr2 == 42);
};
```
What makes this code tricky is that pointers are "opaque blobs" in const-eval, we cannot just let people look at the individual bytes since *we don't know what those bytes look like* -- that depends on the absolute address the pointed-to object will be placed at. The code above "breaks apart" a pointer into individual bytes, and then puts them back together in the same order elsewhere. This PR implements the logic to properly track how those individual bytes relate to the original pointer, and to recognize when they are in the right order again.

We still reject constants where the final value contains a not-fully-put-together pointer: I have no idea how one could construct an LLVM global where one byte is defined as "the 3rd byte of a pointer to that other global over there" -- and even if LLVM supports this somehow, we can leave implementing that to a future PR. It seems unlikely to me anyone would even want this, but who knows.^^

This also changes the behavior of Miri, by tracking the order of bytes with provenance and only considering a pointer to have valid provenance if all bytes are in the original order again. This is related to https://github.com/rust-lang/unsafe-code-guidelines/issues/558. It means one cannot implement XOR linked lists with strict provenance any more, which is however only of theoretical interest. Practically I am curious if anyone will show up with any code that Miri now complains about - that would be interesting data. Cc `@rust-lang/opsem`

36 files changed, 542 insertions, 358 deletions

diff --git a/compiler/rustc_const_eval/messages.ftl b/compiler/rustc_const_eval/messages.ftl
index 2985eafb63a..60518dafbf2 100644
--- a/compiler/rustc_const_eval/messages.ftl
+++ b/compiler/rustc_const_eval/messages.ftl
@@ -57,7 +57,7 @@ const_eval_const_context = {$kind ->
 }
 
 const_eval_const_heap_ptr_in_final = encountered `const_allocate` pointer in final value that was not made global
-    .note = use `const_make_global` to make allocated pointers immutable before returning
+    .note = use `const_make_global` to turn allocated pointers into immutable globals before returning
 
 const_eval_const_make_global_ptr_already_made_global = attempting to call `const_make_global` twice on the same allocation {$alloc}
 
@@ -231,6 +231,9 @@ const_eval_mutable_borrow_escaping =
 
 const_eval_mutable_ptr_in_final = encountered mutable pointer in final value of {const_eval_intern_kind}
 
+const_eval_partial_pointer_in_final = encountered partial pointer in final value of {const_eval_intern_kind}
+    .note = while pointers can be broken apart into individual bytes during const-evaluation, only complete pointers (with all their bytes in the right order) are supported in the final value
+
 const_eval_nested_static_in_thread_local = #[thread_local] does not support implicit nested statics, please create explicit static items and refer to them instead
 
 const_eval_non_const_await =
@@ -299,10 +302,8 @@ const_eval_panic = evaluation panicked: {$msg}
 
 const_eval_panic_non_str = argument to `panic!()` in a const context must have type `&str`
 
-const_eval_partial_pointer_copy =
-    unable to copy parts of a pointer from memory at {$ptr}
-const_eval_partial_pointer_overwrite =
-    unable to overwrite parts of a pointer in memory at {$ptr}
+const_eval_partial_pointer_read =
+    unable to read parts of a pointer from memory at {$ptr}
 const_eval_pointer_arithmetic_overflow =
     overflowing pointer arithmetic: the total offset in bytes does not fit in an `isize`
 
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 5835660e1c3..4da2663319c 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -117,6 +117,13 @@ fn eval_body_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
                 ecx.tcx.dcx().emit_err(errors::ConstHeapPtrInFinal { span: ecx.tcx.span }),
             )));
         }
+        Err(InternError::PartialPointer) => {
+            throw_inval!(AlreadyReported(ReportedErrorInfo::non_const_eval_error(
+                ecx.tcx
+                    .dcx()
+                    .emit_err(errors::PartialPtrInFinal { span: ecx.tcx.span, kind: intern_kind }),
+            )));
+        }
     }
 
     interp_ok(R::make_result(ret, ecx))
diff --git a/compiler/rustc_const_eval/src/errors.rs b/compiler/rustc_const_eval/src/errors.rs
index a4148cb145f..2d412ee5ec2 100644
--- a/compiler/rustc_const_eval/src/errors.rs
+++ b/compiler/rustc_const_eval/src/errors.rs
@@ -52,6 +52,15 @@ pub(crate) struct ConstHeapPtrInFinal {
 }
 
 #[derive(Diagnostic)]
+#[diag(const_eval_partial_pointer_in_final)]
+#[note]
+pub(crate) struct PartialPtrInFinal {
+    #[primary_span]
+    pub span: Span,
+    pub kind: InternKind,
+}
+
+#[derive(Diagnostic)]
 #[diag(const_eval_unstable_in_stable_exposed)]
 pub(crate) struct UnstableInStableExposed {
     pub gate: String,
@@ -836,8 +845,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
             UnsupportedOpInfo::Unsupported(s) => s.clone().into(),
             UnsupportedOpInfo::ExternTypeField => const_eval_extern_type_field,
             UnsupportedOpInfo::UnsizedLocal => const_eval_unsized_local,
-            UnsupportedOpInfo::OverwritePartialPointer(_) => const_eval_partial_pointer_overwrite,
-            UnsupportedOpInfo::ReadPartialPointer(_) => const_eval_partial_pointer_copy,
+            UnsupportedOpInfo::ReadPartialPointer(_) => const_eval_partial_pointer_read,
             UnsupportedOpInfo::ReadPointerAsInt(_) => const_eval_read_pointer_as_int,
             UnsupportedOpInfo::ThreadLocalStatic(_) => const_eval_thread_local_static,
             UnsupportedOpInfo::ExternStatic(_) => const_eval_extern_static,
@@ -848,7 +856,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
         use UnsupportedOpInfo::*;
 
         use crate::fluent_generated::*;
-        if let ReadPointerAsInt(_) | OverwritePartialPointer(_) | ReadPartialPointer(_) = self {
+        if let ReadPointerAsInt(_) | ReadPartialPointer(_) = self {
             diag.help(const_eval_ptr_as_bytes_1);
             diag.help(const_eval_ptr_as_bytes_2);
         }
@@ -860,7 +868,7 @@ impl ReportErrorExt for UnsupportedOpInfo {
             | UnsupportedOpInfo::ExternTypeField
             | Unsupported(_)
             | ReadPointerAsInt(_) => {}
-            OverwritePartialPointer(ptr) | ReadPartialPointer(ptr) => {
+            ReadPartialPointer(ptr) => {
                 diag.arg("ptr", ptr);
             }
             ThreadLocalStatic(did) | ExternStatic(did) => rustc_middle::ty::tls::with(|tcx| {
diff --git a/compiler/rustc_const_eval/src/interpret/intern.rs b/compiler/rustc_const_eval/src/interpret/intern.rs
index bb59b9f5418..5d510a98352 100644
--- a/compiler/rustc_const_eval/src/interpret/intern.rs
+++ b/compiler/rustc_const_eval/src/interpret/intern.rs
@@ -19,9 +19,12 @@ use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_hir as hir;
 use rustc_hir::definitions::{DefPathData, DisambiguatorState};
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
-use rustc_middle::mir::interpret::{ConstAllocation, CtfeProvenance, InterpResult};
+use rustc_middle::mir::interpret::{
+    AllocBytes, ConstAllocation, CtfeProvenance, InterpResult, Provenance,
+};
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::span_bug;
+use rustc_middle::ty::TyCtxt;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_span::def_id::LocalDefId;
 use tracing::{instrument, trace};
@@ -52,39 +55,30 @@ impl HasStaticRootDefId for const_eval::CompileTimeMachine<'_> {
     }
 }
 
-/// Intern an allocation. Returns `Err` if the allocation does not exist in the local memory.
-///
-/// `mutability` can be used to force immutable interning: if it is `Mutability::Not`, the
-/// allocation is interned immutably; if it is `Mutability::Mut`, then the allocation *must be*
-/// already mutable (as a sanity check).
-///
-/// Returns an iterator over all relocations referred to by this allocation.
-fn intern_shallow<'tcx, M: CompileTimeMachine<'tcx>>(
-    ecx: &mut InterpCx<'tcx, M>,
-    alloc_id: AllocId,
+fn prepare_alloc<'tcx, Prov: Provenance, Extra, Bytes: AllocBytes>(
+    tcx: TyCtxt<'tcx>,
+    kind: MemoryKind<const_eval::MemoryKind>,
+    alloc: &mut Allocation<Prov, Extra, Bytes>,
     mutability: Mutability,
-    disambiguator: Option<&mut DisambiguatorState>,
-) -> Result<impl Iterator<Item = CtfeProvenance> + 'tcx, InternError> {
-    trace!("intern_shallow {:?}", alloc_id);
-    // remove allocation
-    // FIXME(#120456) - is `swap_remove` correct?
-    let Some((kind, mut alloc)) = ecx.memory.alloc_map.swap_remove(&alloc_id) else {
-        return Err(InternError::DanglingPointer);
-    };
-
+) -> Result<(), InternError> {
     match kind {
         MemoryKind::Machine(const_eval::MemoryKind::Heap { was_made_global }) => {
             if !was_made_global {
                 // Attempting to intern a `const_allocate`d pointer that was not made global via
-                // `const_make_global`. We want to error here, but we have to first put the
-                // allocation back into the `alloc_map` to keep things in a consistent state.
-                ecx.memory.alloc_map.insert(alloc_id, (kind, alloc));
+                // `const_make_global`.
+                tcx.dcx().delayed_bug("non-global heap allocation in const value");
                 return Err(InternError::ConstAllocNotGlobal);
             }
         }
         MemoryKind::Stack | MemoryKind::CallerLocation => {}
     }
 
+    if !alloc.provenance_merge_bytes(&tcx) {
+        // Per-byte provenance is not supported by backends, so we cannot accept it here.
+        tcx.dcx().delayed_bug("partial pointer in const value");
+        return Err(InternError::PartialPointer);
+    }
+
     // Set allocation mutability as appropriate. This is used by LLVM to put things into
     // read-only memory, and also by Miri when evaluating other globals that
     // access this one.
@@ -97,6 +91,36 @@ fn intern_shallow<'tcx, M: CompileTimeMachine<'tcx>>(
             assert_eq!(alloc.mutability, Mutability::Mut);
         }
     }
+    Ok(())
+}
+
+/// Intern an allocation. Returns `Err` if the allocation does not exist in the local memory.
+///
+/// `mutability` can be used to force immutable interning: if it is `Mutability::Not`, the
+/// allocation is interned immutably; if it is `Mutability::Mut`, then the allocation *must be*
+/// already mutable (as a sanity check).
+///
+/// Returns an iterator over all relocations referred to by this allocation.
+fn intern_shallow<'tcx, M: CompileTimeMachine<'tcx>>(
+    ecx: &mut InterpCx<'tcx, M>,
+    alloc_id: AllocId,
+    mutability: Mutability,
+    disambiguator: Option<&mut DisambiguatorState>,
+) -> Result<impl Iterator<Item = CtfeProvenance> + 'tcx, InternError> {
+    trace!("intern_shallow {:?}", alloc_id);
+    // remove allocation
+    // FIXME(#120456) - is `swap_remove` correct?
+    let Some((kind, mut alloc)) = ecx.memory.alloc_map.swap_remove(&alloc_id) else {
+        return Err(InternError::DanglingPointer);
+    };
+
+    if let Err(err) = prepare_alloc(*ecx.tcx, kind, &mut alloc, mutability) {
+        // We want to error here, but we have to first put the
+        // allocation back into the `alloc_map` to keep things in a consistent state.
+        ecx.memory.alloc_map.insert(alloc_id, (kind, alloc));
+        return Err(err);
+    }
+
     // link the alloc id to the actual allocation
     let alloc = ecx.tcx.mk_const_alloc(alloc);
     if let Some(static_id) = ecx.machine.static_def_id() {
@@ -166,6 +190,7 @@ pub enum InternError {
     BadMutablePointer,
     DanglingPointer,
     ConstAllocNotGlobal,
+    PartialPointer,
 }
 
 /// Intern `ret` and everything it references.
@@ -221,13 +246,11 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx>>(
     let mut todo: Vec<_> = if is_static {
         // Do not steal the root allocation, we need it later to create the return value of `eval_static_initializer`.
         // But still change its mutability to match the requested one.
-        let alloc = ecx.memory.alloc_map.get_mut(&base_alloc_id).unwrap();
-        alloc.1.mutability = base_mutability;
-        alloc.1.provenance().ptrs().iter().map(|&(_, prov)| prov).collect()
+        let (kind, alloc) = ecx.memory.alloc_map.get_mut(&base_alloc_id).unwrap();
+        prepare_alloc(*ecx.tcx, *kind, alloc, base_mutability)?;
+        alloc.provenance().ptrs().iter().map(|&(_, prov)| prov).collect()
     } else {
-        intern_shallow(ecx, base_alloc_id, base_mutability, Some(&mut disambiguator))
-            .unwrap()
-            .collect()
+        intern_shallow(ecx, base_alloc_id, base_mutability, Some(&mut disambiguator))?.collect()
     };
     // We need to distinguish "has just been interned" from "was already in `tcx`",
     // so we track this in a separate set.
@@ -235,7 +258,6 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx>>(
     // Whether we encountered a bad mutable pointer.
     // We want to first report "dangling" and then "mutable", so we need to delay reporting these
     // errors.
-    let mut result = Ok(());
     let mut found_bad_mutable_ptr = false;
 
     // Keep interning as long as there are things to intern.
@@ -310,20 +332,15 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx>>(
         // okay with losing some potential for immutability here. This can anyway only affect
         // `static mut`.
         just_interned.insert(alloc_id);
-        match intern_shallow(ecx, alloc_id, inner_mutability, Some(&mut disambiguator)) {
-            Ok(nested) => todo.extend(nested),
-            Err(err) => {
-                ecx.tcx.dcx().delayed_bug("error during const interning");
-                result = Err(err);
-            }
-        }
+        let next = intern_shallow(ecx, alloc_id, inner_mutability, Some(&mut disambiguator))?;
+        todo.extend(next);
     }
-    if found_bad_mutable_ptr && result.is_ok() {
+    if found_bad_mutable_ptr {
         // We found a mutable pointer inside a const where inner allocations should be immutable,
         // and there was no other error. This should usually never happen! However, this can happen
         // in unleash-miri mode, so report it as a normal error then.
         if ecx.tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you {
-            result = Err(InternError::BadMutablePointer);
+            return Err(InternError::BadMutablePointer);
         } else {
             span_bug!(
                 ecx.tcx.span,
@@ -331,7 +348,7 @@ pub fn intern_const_alloc_recursive<'tcx, M: CompileTimeMachine<'tcx>>(
             );
         }
     }
-    result
+    Ok(())
 }
 
 /// Intern `ret`. This function assumes that `ret` references no other allocation.
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 23b40894f16..2c1e5087e1c 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -1314,29 +1314,20 @@ impl<'a, 'tcx, Prov: Provenance, Extra, Bytes: AllocBytes>
     }
 
     /// Mark the given sub-range (relative to this allocation reference) as uninitialized.
-    pub fn write_uninit(&mut self, range: AllocRange) -> InterpResult<'tcx> {
+    pub fn write_uninit(&mut self, range: AllocRange) {
         let range = self.range.subrange(range);
 
-        self.alloc
-            .write_uninit(&self.tcx, range)
-            .map_err(|e| e.to_interp_error(self.alloc_id))
-            .into()
+        self.alloc.write_uninit(&self.tcx, range);
     }
 
     /// Mark the entire referenced range as uninitialized
-    pub fn write_uninit_full(&mut self) -> InterpResult<'tcx> {
-        self.alloc
-            .write_uninit(&self.tcx, self.range)
-            .map_err(|e| e.to_interp_error(self.alloc_id))
-            .into()
+    pub fn write_uninit_full(&mut self) {
+        self.alloc.write_uninit(&self.tcx, self.range);
     }
 
     /// Remove all provenance in the reference range.
-    pub fn clear_provenance(&mut self) -> InterpResult<'tcx> {
-        self.alloc
-            .clear_provenance(&self.tcx, self.range)
-            .map_err(|e| e.to_interp_error(self.alloc_id))
-            .into()
+    pub fn clear_provenance(&mut self) {
+        self.alloc.clear_provenance(&self.tcx, self.range);
     }
 }
 
@@ -1427,11 +1418,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
         // Side-step AllocRef and directly access the underlying bytes more efficiently.
         // (We are staying inside the bounds here and all bytes do get overwritten so all is good.)
-        let alloc_id = alloc_ref.alloc_id;
-        let bytes = alloc_ref
-            .alloc
-            .get_bytes_unchecked_for_overwrite(&alloc_ref.tcx, alloc_ref.range)
-            .map_err(move |e| e.to_interp_error(alloc_id))?;
+        let bytes =
+            alloc_ref.alloc.get_bytes_unchecked_for_overwrite(&alloc_ref.tcx, alloc_ref.range);
         // `zip` would stop when the first iterator ends; we want to definitely
         // cover all of `bytes`.
         for dest in bytes {
@@ -1513,10 +1501,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // `get_bytes_mut` will clear the provenance, which is correct,
         // since we don't want to keep any provenance at the target.
         // This will also error if copying partial provenance is not supported.
-        let provenance = src_alloc
-            .provenance()
-            .prepare_copy(src_range, dest_offset, num_copies, self)
-            .map_err(|e| e.to_interp_error(src_alloc_id))?;
+        let provenance =
+            src_alloc.provenance().prepare_copy(src_range, dest_offset, num_copies, self);
         // Prepare a copy of the initialization mask.
         let init = src_alloc.init_mask().prepare_copy(src_range);
 
@@ -1534,10 +1520,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             dest_range,
         )?;
         // Yes we do overwrite all bytes in `dest_bytes`.
-        let dest_bytes = dest_alloc
-            .get_bytes_unchecked_for_overwrite_ptr(&tcx, dest_range)
-            .map_err(|e| e.to_interp_error(dest_alloc_id))?
-            .as_mut_ptr();
+        let dest_bytes =
+            dest_alloc.get_bytes_unchecked_for_overwrite_ptr(&tcx, dest_range).as_mut_ptr();
 
         if init.no_bytes_init() {
             // Fast path: If all bytes are `uninit` then there is nothing to copy. The target range
@@ -1546,9 +1530,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             // This also avoids writing to the target bytes so that the backing allocation is never
             // touched if the bytes stay uninitialized for the whole interpreter execution. On contemporary
             // operating system this can avoid physically allocating the page.
-            dest_alloc
-                .write_uninit(&tcx, dest_range)
-                .map_err(|e| e.to_interp_error(dest_alloc_id))?;
+            dest_alloc.write_uninit(&tcx, dest_range);
             // `write_uninit` also resets the provenance, so we are done.
             return interp_ok(());
         }
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 6ff50dc700f..3255ffa54aa 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -705,7 +705,7 @@ where
 
         match value {
             Immediate::Scalar(scalar) => {
-                alloc.write_scalar(alloc_range(Size::ZERO, scalar.size()), scalar)
+                alloc.write_scalar(alloc_range(Size::ZERO, scalar.size()), scalar)?;
             }
             Immediate::ScalarPair(a_val, b_val) => {
                 let BackendRepr::ScalarPair(a, b) = layout.backend_repr else {
@@ -725,10 +725,10 @@ where
                 alloc.write_scalar(alloc_range(Size::ZERO, a_val.size()), a_val)?;
                 alloc.write_scalar(alloc_range(b_offset, b_val.size()), b_val)?;
                 // We don't have to reset padding here, `write_immediate` will anyway do a validation run.
-                interp_ok(())
             }
             Immediate::Uninit => alloc.write_uninit_full(),
         }
+        interp_ok(())
     }
 
     pub fn write_uninit(
@@ -748,7 +748,7 @@ where
                     // Zero-sized access
                     return interp_ok(());
                 };
-                alloc.write_uninit_full()?;
+                alloc.write_uninit_full();
             }
         }
         interp_ok(())
@@ -772,7 +772,7 @@ where
                     // Zero-sized access
                     return interp_ok(());
                 };
-                alloc.clear_provenance()?;
+                alloc.clear_provenance();
             }
         }
         interp_ok(())
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index ab0c0665d51..5e8bee65706 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -949,7 +949,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                 let padding_size = offset - padding_cleared_until;
                 let range = alloc_range(padding_start, padding_size);
                 trace!("reset_padding on {}: resetting padding range {range:?}", mplace.layout.ty);
-                alloc.write_uninit(range)?;
+                alloc.write_uninit(range);
             }
             padding_cleared_until = offset + size;
         }
@@ -1239,7 +1239,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
                 if self.reset_provenance_and_padding {
                     // We can't share this with above as above, we might be looking at read-only memory.
                     let mut alloc = self.ecx.get_ptr_alloc_mut(mplace.ptr(), size)?.expect("we already excluded size 0");
-                    alloc.clear_provenance()?;
+                    alloc.clear_provenance();
                     // Also, mark this as containing data, not padding.
                     self.add_data_range(mplace.ptr(), size);
                 }
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
index 27ead514531..2ea92a39d48 100644
--- a/compiler/rustc_middle/src/mir/interpret/allocation.rs
+++ b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -306,8 +306,6 @@ pub enum AllocError {
     ScalarSizeMismatch(ScalarSizeMismatch),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsInt(Option<BadBytesAccess>),
-    /// Partially overwriting a pointer.
-    OverwritePartialPointer(Size),
     /// Partially copying a pointer.
     ReadPartialPointer(Size),
     /// Using uninitialized data where it is not allowed.
@@ -331,9 +329,6 @@ impl AllocError {
             ReadPointerAsInt(info) => InterpErrorKind::Unsupported(
                 UnsupportedOpInfo::ReadPointerAsInt(info.map(|b| (alloc_id, b))),
             ),
-            OverwritePartialPointer(offset) => InterpErrorKind::Unsupported(
-                UnsupportedOpInfo::OverwritePartialPointer(Pointer::new(alloc_id, offset)),
-            ),
             ReadPartialPointer(offset) => InterpErrorKind::Unsupported(
                 UnsupportedOpInfo::ReadPartialPointer(Pointer::new(alloc_id, offset)),
             ),
@@ -633,11 +628,11 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
         &mut self,
         cx: &impl HasDataLayout,
         range: AllocRange,
-    ) -> AllocResult<&mut [u8]> {
+    ) -> &mut [u8] {
         self.mark_init(range, true);
-        self.provenance.clear(range, cx)?;
+        self.provenance.clear(range, cx);
 
-        Ok(&mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
+        &mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
     }
 
     /// A raw pointer variant of `get_bytes_unchecked_for_overwrite` that avoids invalidating existing immutable aliases
@@ -646,15 +641,15 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
         &mut self,
         cx: &impl HasDataLayout,
         range: AllocRange,
-    ) -> AllocResult<*mut [u8]> {
+    ) -> *mut [u8] {
         self.mark_init(range, true);
-        self.provenance.clear(range, cx)?;
+        self.provenance.clear(range, cx);
 
         assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
         // Crucially, we go via `AllocBytes::as_mut_ptr`, not `AllocBytes::deref_mut`.
         let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
         let len = range.end().bytes_usize() - range.start.bytes_usize();
-        Ok(ptr::slice_from_raw_parts_mut(begin_ptr, len))
+        ptr::slice_from_raw_parts_mut(begin_ptr, len)
     }
 
     /// This gives direct mutable access to the entire buffer, just exposing their internal state
@@ -723,26 +718,45 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
                 let ptr = Pointer::new(prov, Size::from_bytes(bits));
                 return Ok(Scalar::from_pointer(ptr, cx));
             }
-
-            // If we can work on pointers byte-wise, join the byte-wise provenances.
-            if Prov::OFFSET_IS_ADDR {
-                let mut prov = self.provenance.get(range.start, cx);
+            // The other easy case is total absence of provenance.
+            if self.provenance.range_empty(range, cx) {
+                return Ok(Scalar::from_uint(bits, range.size));
+            }
+            // If we get here, we have to check per-byte provenance, and join them together.
+            let prov = 'prov: {
+                // Initialize with first fragment. Must have index 0.
+                let Some((mut joint_prov, 0)) = self.provenance.get_byte(range.start, cx) else {
+                    break 'prov None;
+                };
+                // Update with the remaining fragments.
                 for offset in Size::from_bytes(1)..range.size {
-                    let this_prov = self.provenance.get(range.start + offset, cx);
-                    prov = Prov::join(prov, this_prov);
+                    // Ensure there is provenance here and it has the right index.
+                    let Some((frag_prov, frag_idx)) =
+                        self.provenance.get_byte(range.start + offset, cx)
+                    else {
+                        break 'prov None;
+                    };
+                    // Wildcard provenance is allowed to come with any index (this is needed
+                    // for Miri's native-lib mode to work).
+                    if u64::from(frag_idx) != offset.bytes() && Some(frag_prov) != Prov::WILDCARD {
+                        break 'prov None;
+                    }
+                    // Merge this byte's provenance with the previous ones.
+                    joint_prov = match Prov::join(joint_prov, frag_prov) {
+                        Some(prov) => prov,
+                        None => break 'prov None,
+                    };
                 }
-                // Now use this provenance.
-                let ptr = Pointer::new(prov, Size::from_bytes(bits));
-                return Ok(Scalar::from_maybe_pointer(ptr, cx));
-            } else {
-                // Without OFFSET_IS_ADDR, the only remaining case we can handle is total absence of
-                // provenance.
-                if self.provenance.range_empty(range, cx) {
-                    return Ok(Scalar::from_uint(bits, range.size));
-                }
-                // Else we have mixed provenance, that doesn't work.
+                break 'prov Some(joint_prov);
+            };
+            if prov.is_none() && !Prov::OFFSET_IS_ADDR {
+                // There are some bytes with provenance here but overall the provenance does not add up.
+                // We need `OFFSET_IS_ADDR` to fall back to no-provenance here; without that option, we must error.
                 return Err(AllocError::ReadPartialPointer(range.start));
             }
+            // We can use this provenance.
+            let ptr = Pointer::new(prov, Size::from_bytes(bits));
+            return Ok(Scalar::from_maybe_pointer(ptr, cx));
         } else {
             // We are *not* reading a pointer.
             // If we can just ignore provenance or there is none, that's easy.
@@ -782,7 +796,7 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
 
         let endian = cx.data_layout().endian;
         // Yes we do overwrite all the bytes in `dst`.
-        let dst = self.get_bytes_unchecked_for_overwrite(cx, range)?;
+        let dst = self.get_bytes_unchecked_for_overwrite(cx, range);
         write_target_uint(endian, dst, bytes).unwrap();
 
         // See if we have to also store some provenance.
@@ -795,10 +809,9 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
     }
 
     /// Write "uninit" to the given memory range.
-    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) {
         self.mark_init(range, false);
-        self.provenance.clear(range, cx)?;
-        Ok(())
+        self.provenance.clear(range, cx);
     }
 
     /// Mark all bytes in the given range as initialised and reset the provenance
@@ -817,9 +830,12 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
     }
 
     /// Remove all provenance in the given memory range.
-    pub fn clear_provenance(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
-        self.provenance.clear(range, cx)?;
-        return Ok(());
+    pub fn clear_provenance(&mut self, cx: &impl HasDataLayout, range: AllocRange) {
+        self.provenance.clear(range, cx);
+    }
+
+    pub fn provenance_merge_bytes(&mut self, cx: &impl HasDataLayout) -> bool {
+        self.provenance.merge_bytes(cx)
     }
 
     /// Applies a previously prepared provenance copy.
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs b/compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs
index 119d4be64e6..ea8596ea286 100644
--- a/compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs
+++ b/compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs
@@ -10,7 +10,7 @@ use rustc_macros::HashStable;
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use tracing::trace;
 
-use super::{AllocError, AllocRange, AllocResult, CtfeProvenance, Provenance, alloc_range};
+use super::{AllocRange, CtfeProvenance, Provenance, alloc_range};
 
 /// Stores the provenance information of pointers stored in memory.
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
@@ -19,25 +19,25 @@ pub struct ProvenanceMap<Prov = CtfeProvenance> {
     /// `Provenance` in this map applies from the given offset for an entire pointer-size worth of
     /// bytes. Two entries in this map are always at least a pointer size apart.
     ptrs: SortedMap<Size, Prov>,
-    /// Provenance in this map only applies to the given single byte.
-    /// This map is disjoint from the previous. It will always be empty when
-    /// `Prov::OFFSET_IS_ADDR` is false.
-    bytes: Option<Box<SortedMap<Size, Prov>>>,
+    /// This stores byte-sized provenance fragments.
+    /// The `u8` indicates the position of this byte inside its original pointer.
+    /// If the bytes are re-assembled in their original order, the pointer can be used again.
+    /// Wildcard provenance is allowed to have index 0 everywhere.
+    bytes: Option<Box<SortedMap<Size, (Prov, u8)>>>,
 }
 
 // These impls are generic over `Prov` since `CtfeProvenance` is only decodable/encodable
 // for some particular `D`/`S`.
 impl<D: Decoder, Prov: Provenance + Decodable<D>> Decodable<D> for ProvenanceMap<Prov> {
     fn decode(d: &mut D) -> Self {
-        assert!(!Prov::OFFSET_IS_ADDR); // only `CtfeProvenance` is ever serialized
+        // `bytes` is not in the serialized format
         Self { ptrs: Decodable::decode(d), bytes: None }
     }
 }
 impl<S: Encoder, Prov: Provenance + Encodable<S>> Encodable<S> for ProvenanceMap<Prov> {
     fn encode(&self, s: &mut S) {
         let Self { ptrs, bytes } = self;
-        assert!(!Prov::OFFSET_IS_ADDR); // only `CtfeProvenance` is ever serialized
-        debug_assert!(bytes.is_none()); // without `OFFSET_IS_ADDR`, this is always empty
+        assert!(bytes.is_none()); // interning refuses allocations with pointer fragments
         ptrs.encode(s)
     }
 }
@@ -58,10 +58,10 @@ impl ProvenanceMap {
     /// Give access to the ptr-sized provenances (which can also be thought of as relocations, and
     /// indeed that is how codegen treats them).
     ///
-    /// Only exposed with `CtfeProvenance` provenance, since it panics if there is bytewise provenance.
+    /// Only use on interned allocations, as other allocations may have per-byte provenance!
     #[inline]
     pub fn ptrs(&self) -> &SortedMap<Size, CtfeProvenance> {
-        debug_assert!(self.bytes.is_none()); // `CtfeProvenance::OFFSET_IS_ADDR` is false so this cannot fail
+        assert!(self.bytes.is_none(), "`ptrs()` called on non-interned allocation");
         &self.ptrs
     }
 }
@@ -88,12 +88,12 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
     }
 
     /// `pm.range_ptrs_is_empty(r, cx)` == `pm.range_ptrs_get(r, cx).is_empty()`, but is faster.
-    pub(super) fn range_ptrs_is_empty(&self, range: AllocRange, cx: &impl HasDataLayout) -> bool {
+    fn range_ptrs_is_empty(&self, range: AllocRange, cx: &impl HasDataLayout) -> bool {
         self.ptrs.range_is_empty(Self::adjusted_range_ptrs(range, cx))
     }
 
     /// Returns all byte-wise provenance in the given range.
-    fn range_bytes_get(&self, range: AllocRange) -> &[(Size, Prov)] {
+    fn range_bytes_get(&self, range: AllocRange) -> &[(Size, (Prov, u8))] {
         if let Some(bytes) = self.bytes.as_ref() {
             bytes.range(range.start..range.end())
         } else {
@@ -107,19 +107,47 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
     }
 
     /// Get the provenance of a single byte.
-    pub fn get(&self, offset: Size, cx: &impl HasDataLayout) -> Option<Prov> {
+    pub fn get_byte(&self, offset: Size, cx: &impl HasDataLayout) -> Option<(Prov, u8)> {
         let prov = self.range_ptrs_get(alloc_range(offset, Size::from_bytes(1)), cx);
         debug_assert!(prov.len() <= 1);
         if let Some(entry) = prov.first() {
             // If it overlaps with this byte, it is on this byte.
             debug_assert!(self.bytes.as_ref().is_none_or(|b| !b.contains_key(&offset)));
-            Some(entry.1)
+            Some((entry.1, (offset - entry.0).bytes() as u8))
         } else {
             // Look up per-byte provenance.
             self.bytes.as_ref().and_then(|b| b.get(&offset).copied())
         }
     }
 
+    /// Attempt to merge per-byte provenance back into ptr chunks, if the right fragments
+    /// sit next to each other. Return `false` is that is not possible due to partial pointers.
+    pub fn merge_bytes(&mut self, cx: &impl HasDataLayout) -> bool {
+        let Some(bytes) = self.bytes.as_deref_mut() else {
+            return true;
+        };
+        let ptr_size = cx.data_layout().pointer_size();
+        while let Some((offset, (prov, _))) = bytes.iter().next().copied() {
+            // Check if this fragment starts a pointer.
+            let range = offset..offset + ptr_size;
+            let frags = bytes.range(range.clone());
+            if frags.len() != ptr_size.bytes_usize() {
+                return false;
+            }
+            for (idx, (_offset, (frag_prov, frag_idx))) in frags.iter().copied().enumerate() {
+                if frag_prov != prov || frag_idx != idx as u8 {
+                    return false;
+                }
+            }
+            // Looks like a pointer! Move it over to the ptr provenance map.
+            bytes.remove_range(range);
+            self.ptrs.insert(offset, prov);
+        }
+        // We managed to convert everything into whole pointers.
+        self.bytes = None;
+        true
+    }
+
     /// Check if there is ptr-sized provenance at the given index.
     /// Does not mean anything for bytewise provenance! But can be useful as an optimization.
     pub fn get_ptr(&self, offset: Size) -> Option<Prov> {
@@ -137,7 +165,7 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
 
     /// Yields all the provenances stored in this map.
     pub fn provenances(&self) -> impl Iterator<Item = Prov> {
-        let bytes = self.bytes.iter().flat_map(|b| b.values());
+        let bytes = self.bytes.iter().flat_map(|b| b.values().map(|(p, _i)| p));
         self.ptrs.values().chain(bytes).copied()
     }
 
@@ -148,16 +176,12 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
 
     /// Removes all provenance inside the given range.
     /// If there is provenance overlapping with the edges, might result in an error.
-    pub fn clear(&mut self, range: AllocRange, cx: &impl HasDataLayout) -> AllocResult {
+    pub fn clear(&mut self, range: AllocRange, cx: &impl HasDataLayout) {
         let start = range.start;
         let end = range.end();
         // Clear the bytewise part -- this is easy.
-        if Prov::OFFSET_IS_ADDR {
-            if let Some(bytes) = self.bytes.as_mut() {
-                bytes.remove_range(start..end);
-            }
-        } else {
-            debug_assert!(self.bytes.is_none());
+        if let Some(bytes) = self.bytes.as_mut() {
+            bytes.remove_range(start..end);
         }
 
         let pointer_size = cx.data_layout().pointer_size();
@@ -168,7 +192,7 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
             // Find all provenance overlapping the given range.
             if self.range_ptrs_is_empty(range, cx) {
                 // No provenance in this range, we are done. This is the common case.
-                return Ok(());
+                return;
             }
 
             // This redoes some of the work of `range_get_ptrs_is_empty`, but this path is much
@@ -179,28 +203,20 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
 
         // We need to handle clearing the provenance from parts of a pointer.
         if first < start {
-            if !Prov::OFFSET_IS_ADDR {
-                // We can't split up the provenance into less than a pointer.
-                return Err(AllocError::OverwritePartialPointer(first));
-            }
             // Insert the remaining part in the bytewise provenance.
             let prov = self.ptrs[&first];
             let bytes = self.bytes.get_or_insert_with(Box::default);
             for offset in first..start {
-                bytes.insert(offset, prov);
+                bytes.insert(offset, (prov, (offset - first).bytes() as u8));
             }
         }
         if last > end {
             let begin_of_last = last - pointer_size;
-            if !Prov::OFFSET_IS_ADDR {
-                // We can't split up the provenance into less than a pointer.
-                return Err(AllocError::OverwritePartialPointer(begin_of_last));
-            }
             // Insert the remaining part in the bytewise provenance.
             let prov = self.ptrs[&begin_of_last];
             let bytes = self.bytes.get_or_insert_with(Box::default);
             for offset in end..last {
-                bytes.insert(offset, prov);
+                bytes.insert(offset, (prov, (offset - begin_of_last).bytes() as u8));
             }
         }
 
@@ -208,8 +224,6 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
         // Since provenance do not overlap, we know that removing until `last` (exclusive) is fine,
         // i.e., this will not remove any other provenance just after the ones we care about.
         self.ptrs.remove_range(first..last);
-
-        Ok(())
     }
 
     /// Overwrites all provenance in the given range with wildcard provenance.
@@ -218,10 +232,6 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
     ///
     /// Provided for usage in Miri and panics otherwise.
     pub fn write_wildcards(&mut self, cx: &impl HasDataLayout, range: AllocRange) {
-        assert!(
-            Prov::OFFSET_IS_ADDR,
-            "writing wildcard provenance is not supported when `OFFSET_IS_ADDR` is false"
-        );
         let wildcard = Prov::WILDCARD.unwrap();
 
         let bytes = self.bytes.get_or_insert_with(Box::default);
@@ -229,21 +239,22 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
         // Remove pointer provenances that overlap with the range, then readd the edge ones bytewise.
         let ptr_range = Self::adjusted_range_ptrs(range, cx);
         let ptrs = self.ptrs.range(ptr_range.clone());
-        if let Some((offset, prov)) = ptrs.first() {
-            for byte_ofs in *offset..range.start {
-                bytes.insert(byte_ofs, *prov);
+        if let Some((offset, prov)) = ptrs.first().copied() {
+            for byte_ofs in offset..range.start {
+                bytes.insert(byte_ofs, (prov, (byte_ofs - offset).bytes() as u8));
             }
         }
-        if let Some((offset, prov)) = ptrs.last() {
-            for byte_ofs in range.end()..*offset + cx.data_layout().pointer_size() {
-                bytes.insert(byte_ofs, *prov);
+        if let Some((offset, prov)) = ptrs.last().copied() {
+            for byte_ofs in range.end()..offset + cx.data_layout().pointer_size() {
+                bytes.insert(byte_ofs, (prov, (byte_ofs - offset).bytes() as u8));
             }
         }
         self.ptrs.remove_range(ptr_range);
 
         // Overwrite bytewise provenance.
         for offset in range.start..range.end() {
-            bytes.insert(offset, wildcard);
+            // The fragment index does not matter for wildcard provenance.
+            bytes.insert(offset, (wildcard, 0));
         }
     }
 }
@@ -253,7 +264,7 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
 /// Offsets are already adjusted to the destination allocation.
 pub struct ProvenanceCopy<Prov> {
     dest_ptrs: Option<Box<[(Size, Prov)]>>,
-    dest_bytes: Option<Box<[(Size, Prov)]>>,
+    dest_bytes: Option<Box<[(Size, (Prov, u8))]>>,
 }
 
 impl<Prov: Provenance> ProvenanceMap<Prov> {
@@ -263,7 +274,7 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
         dest: Size,
         count: u64,
         cx: &impl HasDataLayout,
-    ) -> AllocResult<ProvenanceCopy<Prov>> {
+    ) -> ProvenanceCopy<Prov> {
         let shift_offset = move |idx, offset| {
             // compute offset for current repetition
             let dest_offset = dest + src.size * idx; // `Size` operations
@@ -301,24 +312,16 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
         let mut dest_bytes_box = None;
         let begin_overlap = self.range_ptrs_get(alloc_range(src.start, Size::ZERO), cx).first();
         let end_overlap = self.range_ptrs_get(alloc_range(src.end(), Size::ZERO), cx).first();
-        if !Prov::OFFSET_IS_ADDR {
-            // There can't be any bytewise provenance, and we cannot split up the begin/end overlap.
-            if let Some(entry) = begin_overlap {
-                return Err(AllocError::ReadPartialPointer(entry.0));
-            }
-            if let Some(entry) = end_overlap {
-                return Err(AllocError::ReadPartialPointer(entry.0));
-            }
-            debug_assert!(self.bytes.is_none());
-        } else {
-            let mut bytes = Vec::new();
+        // We only need to go here if there is some overlap or some bytewise provenance.
+        if begin_overlap.is_some() || end_overlap.is_some() || self.bytes.is_some() {
+            let mut bytes: Vec<(Size, (Prov, u8))> = Vec::new();
             // First, if there is a part of a pointer at the start, add that.
             if let Some(entry) = begin_overlap {
                 trace!("start overlapping entry: {entry:?}");
                 // For really small copies, make sure we don't run off the end of the `src` range.
                 let entry_end = cmp::min(entry.0 + ptr_size, src.end());
                 for offset in src.start..entry_end {
-                    bytes.push((offset, entry.1));
+                    bytes.push((offset, (entry.1, (offset - entry.0).bytes() as u8)));
                 }
             } else {
                 trace!("no start overlapping entry");
@@ -334,8 +337,9 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
                 let entry_start = cmp::max(entry.0, src.start);
                 for offset in entry_start..src.end() {
                     if bytes.last().is_none_or(|bytes_entry| bytes_entry.0 < offset) {
-                        // The last entry, if it exists, has a lower offset than us.
-                        bytes.push((offset, entry.1));
+                        // The last entry, if it exists, has a lower offset than us, so we
+                        // can add it at the end and remain sorted.
+                        bytes.push((offset, (entry.1, (offset - entry.0).bytes() as u8)));
                     } else {
                         // There already is an entry for this offset in there! This can happen when the
                         // start and end range checks actually end up hitting the same pointer, so we
@@ -358,7 +362,7 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
             dest_bytes_box = Some(dest_bytes.into_boxed_slice());
         }
 
-        Ok(ProvenanceCopy { dest_ptrs: dest_ptrs_box, dest_bytes: dest_bytes_box })
+        ProvenanceCopy { dest_ptrs: dest_ptrs_box, dest_bytes: dest_bytes_box }
     }
 
     /// Applies a provenance copy.
@@ -368,14 +372,10 @@ impl<Prov: Provenance> ProvenanceMap<Prov> {
         if let Some(dest_ptrs) = copy.dest_ptrs {
             self.ptrs.insert_presorted(dest_ptrs.into());
         }
-        if Prov::OFFSET_IS_ADDR {
-            if let Some(dest_bytes) = copy.dest_bytes
-                && !dest_bytes.is_empty()
-            {
-                self.bytes.get_or_insert_with(Box::default).insert_presorted(dest_bytes.into());
-            }
-        } else {
-            debug_assert!(copy.dest_bytes.is_none());
+        if let Some(dest_bytes) = copy.dest_bytes
+            && !dest_bytes.is_empty()
+        {
+            self.bytes.get_or_insert_with(Box::default).insert_presorted(dest_bytes.into());
         }
     }
 }
diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
index 77427a12d11..7c72a8ec243 100644
--- a/compiler/rustc_middle/src/mir/interpret/error.rs
+++ b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -582,9 +582,6 @@ pub enum UnsupportedOpInfo {
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     //
-    /// Overwriting parts of a pointer; without knowing absolute addresses, the resulting state
-    /// cannot be represented by the CTFE interpreter.
-    OverwritePartialPointer(Pointer<AllocId>),
     /// Attempting to read or copy parts of a pointer to somewhere else; without knowing absolute
     /// addresses, the resulting state cannot be represented by the CTFE interpreter.
     ReadPartialPointer(Pointer<AllocId>),
diff --git a/compiler/rustc_middle/src/mir/interpret/pointer.rs b/compiler/rustc_middle/src/mir/interpret/pointer.rs
index e25ff7651f6..c581b2ebf09 100644
--- a/compiler/rustc_middle/src/mir/interpret/pointer.rs
+++ b/compiler/rustc_middle/src/mir/interpret/pointer.rs
@@ -56,7 +56,7 @@ impl<T: HasDataLayout> PointerArithmetic for T {}
 /// mostly opaque; the `Machine` trait extends it with some more operations that also have access to
 /// some global state.
 /// The `Debug` rendering is used to display bare provenance, and for the default impl of `fmt`.
-pub trait Provenance: Copy + fmt::Debug + 'static {
+pub trait Provenance: Copy + PartialEq + fmt::Debug + 'static {
     /// Says whether the `offset` field of `Pointer`s with this provenance is the actual physical address.
     /// - If `false`, the offset *must* be relative. This means the bytes representing a pointer are
     ///   different from what the Abstract Machine prescribes, so the interpreter must prevent any
@@ -79,7 +79,7 @@ pub trait Provenance: Copy + fmt::Debug + 'static {
     fn get_alloc_id(self) -> Option<AllocId>;
 
     /// Defines the 'join' of provenance: what happens when doing a pointer load and different bytes have different provenance.
-    fn join(left: Option<Self>, right: Option<Self>) -> Option<Self>;
+    fn join(left: Self, right: Self) -> Option<Self>;
 }
 
 /// The type of provenance in the compile-time interpreter.
@@ -192,8 +192,8 @@ impl Provenance for CtfeProvenance {
         Some(self.alloc_id())
     }
 
-    fn join(_left: Option<Self>, _right: Option<Self>) -> Option<Self> {
-        panic!("merging provenance is not supported when `OFFSET_IS_ADDR` is false")
+    fn join(left: Self, right: Self) -> Option<Self> {
+        if left == right { Some(left) } else { None }
     }
 }
 
@@ -224,8 +224,8 @@ impl Provenance for AllocId {
         Some(self)
     }
 
-    fn join(_left: Option<Self>, _right: Option<Self>) -> Option<Self> {
-        panic!("merging provenance is not supported when `OFFSET_IS_ADDR` is false")
+    fn join(_left: Self, _right: Self) -> Option<Self> {
+        unreachable!()
     }
 }
 
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index d4d925d2057..f2d5b13cbc8 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -1789,7 +1789,7 @@ pub fn write_allocation_bytes<'tcx, Prov: Provenance, Extra, Bytes: AllocBytes>(
                 ascii.push('╼');
                 i += ptr_size;
             }
-        } else if let Some(prov) = alloc.provenance().get(i, &tcx) {
+        } else if let Some((prov, idx)) = alloc.provenance().get_byte(i, &tcx) {
             // Memory with provenance must be defined
             assert!(
                 alloc.init_mask().is_range_initialized(alloc_range(i, Size::from_bytes(1))).is_ok()
@@ -1799,7 +1799,7 @@ pub fn write_allocation_bytes<'tcx, Prov: Provenance, Extra, Bytes: AllocBytes>(
             // Format is similar to "oversized" above.
             let j = i.bytes_usize();
             let c = alloc.inspect_with_uninit_and_ptr_outside_interpreter(j..j + 1)[0];
-            write!(w, "╾{c:02x}{prov:#?} (1 ptr byte)╼")?;
+            write!(w, "╾{c:02x}{prov:#?} (ptr fragment {idx})╼")?;
             i += Size::from_bytes(1);
         } else if alloc
             .init_mask()
diff --git a/library/core/src/ptr/mod.rs b/library/core/src/ptr/mod.rs
index be26e714e67..f5c490ca7ce 100644
--- a/library/core/src/ptr/mod.rs
+++ b/library/core/src/ptr/mod.rs
@@ -1347,40 +1347,6 @@ pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
 /// assert_eq!(x, [7, 8, 3, 4]);
 /// assert_eq!(y, [1, 2, 9]);
 /// ```
-///
-/// # Const evaluation limitations
-///
-/// If this function is invoked during const-evaluation, the current implementation has a small (and
-/// rarely relevant) limitation: if `count` is at least 2 and the data pointed to by `x` or `y`
-/// contains a pointer that crosses the boundary of two `T`-sized chunks of memory, the function may
-/// fail to evaluate (similar to a panic during const-evaluation). This behavior may change in the
-/// future.
-///
-/// The limitation is illustrated by the following example:
-///
-/// ```
-/// use std::mem::size_of;
-/// use std::ptr;
-///
-/// const { unsafe {
-///     const PTR_SIZE: usize = size_of::<*const i32>();
-///     let mut data1 = [0u8; PTR_SIZE];
-///     let mut data2 = [0u8; PTR_SIZE];
-///     // Store a pointer in `data1`.
-///     data1.as_mut_ptr().cast::<*const i32>().write_unaligned(&42);
-///     // Swap the contents of `data1` and `data2` by swapping `PTR_SIZE` many `u8`-sized chunks.
-///     // This call will fail, because the pointer in `data1` crosses the boundary
-///     // between several of the 1-byte chunks that are being swapped here.
-///     //ptr::swap_nonoverlapping(data1.as_mut_ptr(), data2.as_mut_ptr(), PTR_SIZE);
-///     // Swap the contents of `data1` and `data2` by swapping a single chunk of size
-///     // `[u8; PTR_SIZE]`. That works, as there is no pointer crossing the boundary between
-///     // two chunks.
-///     ptr::swap_nonoverlapping(&mut data1, &mut data2, 1);
-///     // Read the pointer from `data2` and dereference it.
-///     let ptr = data2.as_ptr().cast::<*const i32>().read_unaligned();
-///     assert!(*ptr == 42);
-/// } }
-/// ```
 #[inline]
 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
 #[rustc_const_stable(feature = "const_swap_nonoverlapping", since = "1.88.0")]
@@ -1409,9 +1375,7 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
     const_eval_select!(
         @capture[T] { x: *mut T, y: *mut T, count: usize }:
         if const {
-            // At compile-time we want to always copy this in chunks of `T`, to ensure that if there
-            // are pointers inside `T` we will copy them in one go rather than trying to copy a part
-            // of a pointer (which would not work).
+            // At compile-time we don't need all the special code below.
             // SAFETY: Same preconditions as this function
             unsafe { swap_nonoverlapping_const(x, y, count) }
         } else {
diff --git a/library/coretests/tests/ptr.rs b/library/coretests/tests/ptr.rs
index 197a14423b5..c13fb96a67f 100644
--- a/library/coretests/tests/ptr.rs
+++ b/library/coretests/tests/ptr.rs
@@ -936,22 +936,18 @@ fn test_const_swap_ptr() {
         assert!(*s1.0.ptr == 666);
         assert!(*s2.0.ptr == 1);
 
-        // Swap them back, again as an array.
+        // Swap them back, byte-for-byte
         unsafe {
             ptr::swap_nonoverlapping(
-                ptr::from_mut(&mut s1).cast::<T>(),
-                ptr::from_mut(&mut s2).cast::<T>(),
-                1,
+                ptr::from_mut(&mut s1).cast::<u8>(),
+                ptr::from_mut(&mut s2).cast::<u8>(),
+                size_of::<A>(),
             );
         }
 
         // Make sure they still work.
         assert!(*s1.0.ptr == 1);
         assert!(*s2.0.ptr == 666);
-
-        // This is where we'd swap again using a `u8` type and a `count` of `size_of::<T>()` if it
-        // were not for the limitation of `swap_nonoverlapping` around pointers crossing multiple
-        // elements.
     };
 }
 
diff --git a/src/tools/miri/src/machine.rs b/src/tools/miri/src/machine.rs
index 00c3373bb0f..49021b1c4a2 100644
--- a/src/tools/miri/src/machine.rs
+++ b/src/tools/miri/src/machine.rs
@@ -280,16 +280,16 @@ impl interpret::Provenance for Provenance {
         Ok(())
     }
 
-    fn join(left: Option<Self>, right: Option<Self>) -> Option<Self> {
+    fn join(left: Self, right: Self) -> Option<Self> {
         match (left, right) {
             // If both are the *same* concrete tag, that is the result.
             (
-                Some(Provenance::Concrete { alloc_id: left_alloc, tag: left_tag }),
-                Some(Provenance::Concrete { alloc_id: right_alloc, tag: right_tag }),
-            ) if left_alloc == right_alloc && left_tag == right_tag => left,
+                Provenance::Concrete { alloc_id: left_alloc, tag: left_tag },
+                Provenance::Concrete { alloc_id: right_alloc, tag: right_tag },
+            ) if left_alloc == right_alloc && left_tag == right_tag => Some(left),
             // If one side is a wildcard, the best possible outcome is that it is equal to the other
             // one, and we use that.
-            (Some(Provenance::Wildcard), o) | (o, Some(Provenance::Wildcard)) => o,
+            (Provenance::Wildcard, o) | (o, Provenance::Wildcard) => Some(o),
             // Otherwise, fall back to `None`.
             _ => None,
         }
diff --git a/src/tools/miri/src/shims/native_lib/mod.rs b/src/tools/miri/src/shims/native_lib/mod.rs
index 2827ed997a7..36c18379cb8 100644
--- a/src/tools/miri/src/shims/native_lib/mod.rs
+++ b/src/tools/miri/src/shims/native_lib/mod.rs
@@ -246,7 +246,7 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                         let p_map = alloc.provenance();
                         for idx in overlap {
                             // If a provenance was read by the foreign code, expose it.
-                            if let Some(prov) = p_map.get(Size::from_bytes(idx), this) {
+                            if let Some((prov, _idx)) = p_map.get_byte(Size::from_bytes(idx), this) {
                                 this.expose_provenance(prov)?;
                             }
                         }
diff --git a/src/tools/miri/tests/fail/provenance/mix-ptrs1.rs b/src/tools/miri/tests/fail/provenance/mix-ptrs1.rs
new file mode 100644
index 00000000000..0b7266a4da6
--- /dev/null
+++ b/src/tools/miri/tests/fail/provenance/mix-ptrs1.rs
@@ -0,0 +1,21 @@
+use std::{mem, ptr};
+
+const PTR_SIZE: usize = mem::size_of::<&i32>();
+
+fn main() {
+    unsafe {
+        let ptr = &0 as *const i32;
+        let arr = [ptr; 2];
+        // We want to do a scalar read of a pointer at offset PTR_SIZE/2 into this array. But we
+        // cannot use a packed struct or `read_unaligned`, as those use the memcpy code path in
+        // Miri. So instead we shift the entire array by a bit and then the actual read we want to
+        // do is perfectly aligned.
+        let mut target_arr = [ptr::null::<i32>(); 3];
+        let target = target_arr.as_mut_ptr().cast::<u8>();
+        target.add(PTR_SIZE / 2).cast::<[*const i32; 2]>().write_unaligned(arr);
+        // Now target_arr[1] is a mix of the two `ptr` we had stored in `arr`.
+        // They all have the same provenance, but not in the right order, so we reject this.
+        let strange_ptr = target_arr[1];
+        assert_eq!(*strange_ptr.with_addr(ptr.addr()), 0); //~ERROR: no provenance
+    }
+}
diff --git a/src/tools/miri/tests/fail/provenance/mix-ptrs1.stderr b/src/tools/miri/tests/fail/provenance/mix-ptrs1.stderr
new file mode 100644
index 00000000000..88d75135a76
--- /dev/null
+++ b/src/tools/miri/tests/fail/provenance/mix-ptrs1.stderr
@@ -0,0 +1,16 @@
+error: Undefined Behavior: pointer not dereferenceable: pointer must be dereferenceable for 4 bytes, but got $HEX[noalloc] which is a dangling pointer (it has no provenance)
+  --> tests/fail/provenance/mix-ptrs1.rs:LL:CC
+   |
+LL |         assert_eq!(*strange_ptr.with_addr(ptr.addr()), 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at RUSTLIB/core/src/macros/mod.rs:LL:CC
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+
diff --git a/src/tools/miri/tests/fail/provenance/mix-ptrs2.rs b/src/tools/miri/tests/fail/provenance/mix-ptrs2.rs
new file mode 100644
index 00000000000..9ef40c194ab
--- /dev/null
+++ b/src/tools/miri/tests/fail/provenance/mix-ptrs2.rs
@@ -0,0 +1,37 @@
+use std::mem;
+
+const PTR_SIZE: usize = mem::size_of::<&i32>();
+
+/// Overwrite one byte of a pointer, then restore it.
+fn main() {
+    unsafe fn ptr_bytes<'x>(ptr: &'x mut *const i32) -> &'x mut [mem::MaybeUninit<u8>; PTR_SIZE] {
+        mem::transmute(ptr)
+    }
+
+    // Returns a value with the same provenance as `x` but 0 for the integer value.
+    // `x` must be initialized.
+    unsafe fn zero_with_provenance(x: mem::MaybeUninit<u8>) -> mem::MaybeUninit<u8> {
+        let ptr = [x; PTR_SIZE];
+        let ptr: *const i32 = mem::transmute(ptr);
+        let mut ptr = ptr.with_addr(0);
+        ptr_bytes(&mut ptr)[0]
+    }
+
+    unsafe {
+        let ptr = &42;
+        let mut ptr = ptr as *const i32;
+        // Get a bytewise view of the pointer.
+        let ptr_bytes = ptr_bytes(&mut ptr);
+
+        // The highest bytes must be 0 for this to work.
+        let hi = if cfg!(target_endian = "little") { ptr_bytes.len() - 1 } else { 0 };
+        assert_eq!(*ptr_bytes[hi].as_ptr().cast::<u8>(), 0);
+        // Overwrite provenance on the last byte.
+        ptr_bytes[hi] = mem::MaybeUninit::new(0);
+        // Restore it from the another byte.
+        ptr_bytes[hi] = zero_with_provenance(ptr_bytes[1]);
+
+        // Now ptr is almost good, except the provenance fragment indices do not work out...
+        assert_eq!(*ptr, 42); //~ERROR: no provenance
+    }
+}
diff --git a/src/tools/miri/tests/fail/provenance/mix-ptrs2.stderr b/src/tools/miri/tests/fail/provenance/mix-ptrs2.stderr
new file mode 100644
index 00000000000..636378952ba
--- /dev/null
+++ b/src/tools/miri/tests/fail/provenance/mix-ptrs2.stderr
@@ -0,0 +1,16 @@
+error: Undefined Behavior: pointer not dereferenceable: pointer must be dereferenceable for 4 bytes, but got $HEX[noalloc] which is a dangling pointer (it has no provenance)
+  --> tests/fail/provenance/mix-ptrs2.rs:LL:CC
+   |
+LL |         assert_eq!(*ptr, 42);
+   |         ^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at RUSTLIB/core/src/macros/mod.rs:LL:CC
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+
diff --git a/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.rs b/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.rs
index 1cf70d9d9a3..78a5098d21a 100644
--- a/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.rs
+++ b/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.rs
@@ -9,11 +9,10 @@ use std::alloc::{Layout, alloc, dealloc};
 use std::mem::{self, MaybeUninit};
 use std::slice::from_raw_parts;
 
-fn byte_with_provenance<T>(val: u8, prov: *const T) -> MaybeUninit<u8> {
-    let ptr = prov.with_addr(val as usize);
+fn byte_with_provenance<T>(val: u8, prov: *const T, frag_idx: usize) -> MaybeUninit<u8> {
+    let ptr = prov.with_addr(usize::from_ne_bytes([val; _]));
     let bytes: [MaybeUninit<u8>; mem::size_of::<*const ()>()] = unsafe { mem::transmute(ptr) };
-    let lsb = if cfg!(target_endian = "little") { 0 } else { bytes.len() - 1 };
-    bytes[lsb]
+    bytes[frag_idx]
 }
 
 fn main() {
@@ -21,10 +20,10 @@ fn main() {
     unsafe {
         let ptr = alloc(layout);
         let ptr_raw = ptr.cast::<MaybeUninit<u8>>();
-        *ptr_raw.add(0) = byte_with_provenance(0x42, &42u8);
+        *ptr_raw.add(0) = byte_with_provenance(0x42, &42u8, 0);
         *ptr.add(1) = 0x12;
         *ptr.add(2) = 0x13;
-        *ptr_raw.add(3) = byte_with_provenance(0x43, &0u8);
+        *ptr_raw.add(3) = byte_with_provenance(0x43, &0u8, 1);
         let slice1 = from_raw_parts(ptr, 8);
         let slice2 = from_raw_parts(ptr.add(8), 8);
         drop(slice1.cmp(slice2));
diff --git a/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.stderr b/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.stderr
index d3253317fae..305d8d4fbed 100644
--- a/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.stderr
+++ b/src/tools/miri/tests/fail/uninit/uninit_alloc_diagnostic_with_provenance.stderr
@@ -17,7 +17,7 @@ LL |         drop(slice1.cmp(slice2));
 
 Uninitialized memory occurred at ALLOC[0x4..0x8], in this allocation:
 ALLOC (Rust heap, size: 16, align: 8) {
-    ╾42[ALLOC]<TAG> (1 ptr byte)╼ 12 13 ╾43[ALLOC]<TAG> (1 ptr byte)╼ __ __ __ __ __ __ __ __ __ __ __ __ │ ━..━░░░░░░░░░░░░
+    ╾42[ALLOC]<TAG> (ptr fragment 0)╼ 12 13 ╾43[ALLOC]<TAG> (ptr fragment 1)╼ __ __ __ __ __ __ __ __ __ __ __ __ │ ━..━░░░░░░░░░░░░
 }
 ALLOC (global (static or const), size: 1, align: 1) {
     2a                                              │ *
diff --git a/src/tools/miri/tests/native-lib/fail/tracing/partial_init.stderr b/src/tools/miri/tests/native-lib/fail/tracing/partial_init.stderr
index 74a599ede5c..1c0ad2f0dc9 100644
--- a/src/tools/miri/tests/native-lib/fail/tracing/partial_init.stderr
+++ b/src/tools/miri/tests/native-lib/fail/tracing/partial_init.stderr
@@ -35,7 +35,7 @@ LL |     partial_init();
 
 Uninitialized memory occurred at ALLOC[0x2..0x3], in this allocation:
 ALLOC (stack variable, size: 3, align: 1) {
-    ╾00[wildcard] (1 ptr byte)╼ ╾00[wildcard] (1 ptr byte)╼ __                                        │ ━━░
+    ╾00[wildcard] (ptr fragment 0)╼ ╾00[wildcard] (ptr fragment 0)╼ __                                        │ ━━░
 }
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
diff --git a/src/tools/miri/tests/pass/provenance.rs b/src/tools/miri/tests/pass/provenance.rs
index 8c52b703190..46f5bd73d55 100644
--- a/src/tools/miri/tests/pass/provenance.rs
+++ b/src/tools/miri/tests/pass/provenance.rs
@@ -6,7 +6,6 @@ const PTR_SIZE: usize = mem::size_of::<&i32>();
 
 fn main() {
     basic();
-    partial_overwrite_then_restore();
     bytewise_ptr_methods();
     bytewise_custom_memcpy();
     bytewise_custom_memcpy_chunked();
@@ -29,40 +28,6 @@ fn basic() {
     assert_eq!(unsafe { *ptr_back }, 42);
 }
 
-/// Overwrite one byte of a pointer, then restore it.
-fn partial_overwrite_then_restore() {
-    unsafe fn ptr_bytes<'x>(ptr: &'x mut *const i32) -> &'x mut [mem::MaybeUninit<u8>; PTR_SIZE] {
-        mem::transmute(ptr)
-    }
-
-    // Returns a value with the same provenance as `x` but 0 for the integer value.
-    // `x` must be initialized.
-    unsafe fn zero_with_provenance(x: mem::MaybeUninit<u8>) -> mem::MaybeUninit<u8> {
-        let ptr = [x; PTR_SIZE];
-        let ptr: *const i32 = mem::transmute(ptr);
-        let mut ptr = ptr.with_addr(0);
-        ptr_bytes(&mut ptr)[0]
-    }
-
-    unsafe {
-        let ptr = &42;
-        let mut ptr = ptr as *const i32;
-        // Get a bytewise view of the pointer.
-        let ptr_bytes = ptr_bytes(&mut ptr);
-
-        // The highest bytes must be 0 for this to work.
-        let hi = if cfg!(target_endian = "little") { ptr_bytes.len() - 1 } else { 0 };
-        assert_eq!(*ptr_bytes[hi].as_ptr().cast::<u8>(), 0);
-        // Overwrite provenance on the last byte.
-        ptr_bytes[hi] = mem::MaybeUninit::new(0);
-        // Restore it from the another byte.
-        ptr_bytes[hi] = zero_with_provenance(ptr_bytes[1]);
-
-        // Now ptr should be good again.
-        assert_eq!(*ptr, 42);
-    }
-}
-
 fn bytewise_ptr_methods() {
     let mut ptr1 = &1;
     let mut ptr2 = &2;
diff --git a/src/tools/miri/tests/pass/transmute_ptr.rs b/src/tools/miri/tests/pass/transmute_ptr.rs
index 0944781c6de..0a53b778294 100644
--- a/src/tools/miri/tests/pass/transmute_ptr.rs
+++ b/src/tools/miri/tests/pass/transmute_ptr.rs
@@ -1,6 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-use std::{mem, ptr};
+use std::mem;
 
 fn t1() {
     // If we are careful, we can exploit data layout...
@@ -27,27 +27,8 @@ fn ptr_integer_array() {
     let _x: [u8; PTR_SIZE] = unsafe { mem::transmute(&0) };
 }
 
-fn ptr_in_two_halves() {
-    unsafe {
-        let ptr = &0 as *const i32;
-        let arr = [ptr; 2];
-        // We want to do a scalar read of a pointer at offset PTR_SIZE/2 into this array. But we
-        // cannot use a packed struct or `read_unaligned`, as those use the memcpy code path in
-        // Miri. So instead we shift the entire array by a bit and then the actual read we want to
-        // do is perfectly aligned.
-        let mut target_arr = [ptr::null::<i32>(); 3];
-        let target = target_arr.as_mut_ptr().cast::<u8>();
-        target.add(PTR_SIZE / 2).cast::<[*const i32; 2]>().write_unaligned(arr);
-        // Now target_arr[1] is a mix of the two `ptr` we had stored in `arr`.
-        let strange_ptr = target_arr[1];
-        // Check that the provenance works out.
-        assert_eq!(*strange_ptr.with_addr(ptr.addr()), 0);
-    }
-}
-
 fn main() {
     t1();
     t2();
     ptr_integer_array();
-    ptr_in_two_halves();
 }
diff --git a/tests/ui/consts/const-eval/heap/ptr_not_made_global.stderr b/tests/ui/consts/const-eval/heap/ptr_not_made_global.stderr
index cb2bb1e8cd8..e5b108df298 100644
--- a/tests/ui/consts/const-eval/heap/ptr_not_made_global.stderr
+++ b/tests/ui/consts/const-eval/heap/ptr_not_made_global.stderr
@@ -4,7 +4,7 @@ error: encountered `const_allocate` pointer in final value that was not made glo
 LL | const FOO: &i32 = foo();
    | ^^^^^^^^^^^^^^^
    |
-   = note: use `const_make_global` to make allocated pointers immutable before returning
+   = note: use `const_make_global` to turn allocated pointers into immutable globals before returning
 
 error: encountered `const_allocate` pointer in final value that was not made global
   --> $DIR/ptr_not_made_global.rs:12:1
@@ -12,7 +12,7 @@ error: encountered `const_allocate` pointer in final value that was not made glo
 LL | const FOO_RAW: *const i32 = foo();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: use `const_make_global` to make allocated pointers immutable before returning
+   = note: use `const_make_global` to turn allocated pointers into immutable globals before returning
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/consts/const-eval/heap/ptr_not_made_global_mut.stderr b/tests/ui/consts/const-eval/heap/ptr_not_made_global_mut.stderr
index 2445ce633d6..2d1993f96d3 100644
--- a/tests/ui/consts/const-eval/heap/ptr_not_made_global_mut.stderr
+++ b/tests/ui/consts/const-eval/heap/ptr_not_made_global_mut.stderr
@@ -4,7 +4,7 @@ error: encountered `const_allocate` pointer in final value that was not made glo
 LL | const BAR: *mut i32 = unsafe { intrinsics::const_allocate(4, 4) as *mut i32 };
    | ^^^^^^^^^^^^^^^^^^^
    |
-   = note: use `const_make_global` to make allocated pointers immutable before returning
+   = note: use `const_make_global` to turn allocated pointers into immutable globals before returning
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/const-eval/partial_ptr_overwrite.rs b/tests/ui/consts/const-eval/partial_ptr_overwrite.rs
deleted file mode 100644
index bd97bec0f71..00000000000
--- a/tests/ui/consts/const-eval/partial_ptr_overwrite.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-// Test for the behavior described in <https://github.com/rust-lang/rust/issues/87184>.
-
-const PARTIAL_OVERWRITE: () = {
-    let mut p = &42;
-    unsafe {
-        let ptr: *mut _ = &mut p;
-        *(ptr as *mut u8) = 123; //~ ERROR unable to overwrite parts of a pointer
-    }
-    let x = *p;
-};
-
-fn main() {}
diff --git a/tests/ui/consts/const-eval/partial_ptr_overwrite.stderr b/tests/ui/consts/const-eval/partial_ptr_overwrite.stderr
deleted file mode 100644
index 6ef1cfd35c8..00000000000
--- a/tests/ui/consts/const-eval/partial_ptr_overwrite.stderr
+++ /dev/null
@@ -1,12 +0,0 @@
-error[E0080]: unable to overwrite parts of a pointer in memory at ALLOC0
-  --> $DIR/partial_ptr_overwrite.rs:7:9
-   |
-LL |         *(ptr as *mut u8) = 123;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `PARTIAL_OVERWRITE` failed here
-   |
-   = help: this code performed an operation that depends on the underlying bytes representing a pointer
-   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/ptr_fragments.rs b/tests/ui/consts/const-eval/ptr_fragments.rs
new file mode 100644
index 00000000000..04dcbe55590
--- /dev/null
+++ b/tests/ui/consts/const-eval/ptr_fragments.rs
@@ -0,0 +1,63 @@
+//! Test that various operations involving pointer fragments work as expected.
+//@ run-pass
+
+use std::mem::{self, MaybeUninit, transmute};
+use std::ptr;
+
+type Byte = MaybeUninit<u8>;
+
+const unsafe fn memcpy(dst: *mut Byte, src: *const Byte, n: usize) {
+    let mut i = 0;
+    while i < n {
+        *dst.add(i) = *src.add(i);
+        i += 1;
+    }
+}
+
+const _MEMCPY: () = unsafe {
+    let ptr = &42;
+    let mut ptr2 = ptr::null::<i32>();
+    memcpy(&mut ptr2 as *mut _ as *mut _, &ptr as *const _ as *const _, mem::size_of::<&i32>());
+    assert!(*ptr2 == 42);
+};
+const _MEMCPY_OFFSET: () = unsafe {
+    // Same as above, but the pointer has a non-zero offset so not all the data bytes are the same.
+    let ptr = &(42, 18);
+    let ptr = &ptr.1;
+    let mut ptr2 = ptr::null::<i32>();
+    memcpy(&mut ptr2 as *mut _ as *mut _, &ptr as *const _ as *const _, mem::size_of::<&i32>());
+    assert!(*ptr2 == 18);
+};
+
+const MEMCPY_RET: MaybeUninit<*const i32> = unsafe {
+    let ptr = &42;
+    let mut ptr2 = MaybeUninit::new(ptr::null::<i32>());
+    memcpy(&mut ptr2 as *mut _ as *mut _, &ptr as *const _ as *const _, mem::size_of::<&i32>());
+    // Return in a MaybeUninit so it does not get treated as a scalar.
+    ptr2
+};
+
+#[allow(dead_code)]
+fn reassemble_ptr_fragments_in_static() {
+    static DATA: i32 = 1i32;
+
+    #[cfg(target_pointer_width = "64")]
+    struct Thing {
+        x: MaybeUninit<u32>,
+        y: MaybeUninit<u32>,
+    }
+    #[cfg(target_pointer_width = "32")]
+    struct Thing {
+        x: MaybeUninit<u16>,
+        y: MaybeUninit<u16>,
+    }
+
+    static X: Thing = unsafe {
+        let Thing { x, y } = transmute(&raw const DATA);
+        Thing { x, y }
+    };
+}
+
+fn main() {
+    assert_eq!(unsafe { MEMCPY_RET.assume_init().read() }, 42);
+}
diff --git a/tests/ui/consts/const-eval/ptr_fragments_in_final.rs b/tests/ui/consts/const-eval/ptr_fragments_in_final.rs
new file mode 100644
index 00000000000..e2f3f51b086
--- /dev/null
+++ b/tests/ui/consts/const-eval/ptr_fragments_in_final.rs
@@ -0,0 +1,25 @@
+//! Test that we properly error when there is a pointer fragment in the final value.
+
+use std::{mem::{self, MaybeUninit}, ptr};
+
+type Byte = MaybeUninit<u8>;
+
+const unsafe fn memcpy(dst: *mut Byte, src: *const Byte, n: usize) {
+    let mut i = 0;
+    while i < n {
+        dst.add(i).write(src.add(i).read());
+        i += 1;
+    }
+}
+
+const MEMCPY_RET: MaybeUninit<*const i32> = unsafe { //~ERROR: partial pointer in final value
+    let ptr = &42;
+    let mut ptr2 = MaybeUninit::new(ptr::null::<i32>());
+    memcpy(&mut ptr2 as *mut _ as *mut _, &ptr as *const _ as *const _, mem::size_of::<&i32>() / 2);
+    // Return in a MaybeUninit so it does not get treated as a scalar.
+    ptr2
+};
+
+fn main() {
+    assert_eq!(unsafe { MEMCPY_RET.assume_init().read() }, 42);
+}
diff --git a/tests/ui/consts/const-eval/ptr_fragments_in_final.stderr b/tests/ui/consts/const-eval/ptr_fragments_in_final.stderr
new file mode 100644
index 00000000000..628bf2566e5
--- /dev/null
+++ b/tests/ui/consts/const-eval/ptr_fragments_in_final.stderr
@@ -0,0 +1,10 @@
+error: encountered partial pointer in final value of constant
+  --> $DIR/ptr_fragments_in_final.rs:15:1
+   |
+LL | const MEMCPY_RET: MaybeUninit<*const i32> = unsafe {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: while pointers can be broken apart into individual bytes during const-evaluation, only complete pointers (with all their bytes in the right order) are supported in the final value
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/consts/const-eval/read_partial_ptr.rs b/tests/ui/consts/const-eval/read_partial_ptr.rs
new file mode 100644
index 00000000000..bccef9c0bc6
--- /dev/null
+++ b/tests/ui/consts/const-eval/read_partial_ptr.rs
@@ -0,0 +1,49 @@
+//! Ensure we error when trying to load from a pointer whose provenance has been messed with.
+
+const PARTIAL_OVERWRITE: () = {
+    let mut p = &42;
+    // Overwrite one byte with a no-provenance value.
+    unsafe {
+        let ptr: *mut _ = &mut p;
+        *(ptr as *mut u8) = 123;
+    }
+    let x = *p; //~ ERROR: unable to read parts of a pointer
+};
+
+const PTR_BYTES_SWAP: () = {
+    let mut p = &42;
+    // Swap the first two bytes.
+    unsafe {
+        let ptr = &mut p as *mut _ as *mut std::mem::MaybeUninit<u8>;
+        let byte0 = ptr.read();
+        let byte1 = ptr.add(1).read();
+        ptr.write(byte1);
+        ptr.add(1).write(byte0);
+    }
+    let x = *p; //~ ERROR: unable to read parts of a pointer
+};
+
+const PTR_BYTES_REPEAT: () = {
+    let mut p = &42;
+    // Duplicate the first byte over the second.
+    unsafe {
+        let ptr = &mut p as *mut _ as *mut std::mem::MaybeUninit<u8>;
+        let byte0 = ptr.read();
+        ptr.add(1).write(byte0);
+    }
+    let x = *p; //~ ERROR: unable to read parts of a pointer
+};
+
+const PTR_BYTES_MIX: () = {
+    let mut p = &42;
+    let q = &43;
+    // Overwrite the first byte of p with the first byte of q.
+    unsafe {
+        let ptr = &mut p as *mut _ as *mut std::mem::MaybeUninit<u8>;
+        let qtr = &q as *const _ as *const std::mem::MaybeUninit<u8>;
+        ptr.write(qtr.read());
+    }
+    let x = *p; //~ ERROR: unable to read parts of a pointer
+};
+
+fn main() {}
diff --git a/tests/ui/consts/const-eval/read_partial_ptr.stderr b/tests/ui/consts/const-eval/read_partial_ptr.stderr
new file mode 100644
index 00000000000..196606c77a3
--- /dev/null
+++ b/tests/ui/consts/const-eval/read_partial_ptr.stderr
@@ -0,0 +1,39 @@
+error[E0080]: unable to read parts of a pointer from memory at ALLOC0
+  --> $DIR/read_partial_ptr.rs:10:13
+   |
+LL |     let x = *p;
+   |             ^^ evaluation of `PARTIAL_OVERWRITE` failed here
+   |
+   = help: this code performed an operation that depends on the underlying bytes representing a pointer
+   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
+
+error[E0080]: unable to read parts of a pointer from memory at ALLOC1
+  --> $DIR/read_partial_ptr.rs:23:13
+   |
+LL |     let x = *p;
+   |             ^^ evaluation of `PTR_BYTES_SWAP` failed here
+   |
+   = help: this code performed an operation that depends on the underlying bytes representing a pointer
+   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
+
+error[E0080]: unable to read parts of a pointer from memory at ALLOC2
+  --> $DIR/read_partial_ptr.rs:34:13
+   |
+LL |     let x = *p;
+   |             ^^ evaluation of `PTR_BYTES_REPEAT` failed here
+   |
+   = help: this code performed an operation that depends on the underlying bytes representing a pointer
+   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
+
+error[E0080]: unable to read parts of a pointer from memory at ALLOC3
+  --> $DIR/read_partial_ptr.rs:46:13
+   |
+LL |     let x = *p;
+   |             ^^ evaluation of `PTR_BYTES_MIX` failed here
+   |
+   = help: this code performed an operation that depends on the underlying bytes representing a pointer
+   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/missing_span_in_backtrace.rs b/tests/ui/consts/missing_span_in_backtrace.rs
index 4f3f9aa6ada..09a11c75fd7 100644
--- a/tests/ui/consts/missing_span_in_backtrace.rs
+++ b/tests/ui/consts/missing_span_in_backtrace.rs
@@ -1,3 +1,4 @@
+//! Check what happens when the error occurs inside a std function that we can't print the span of.
 //@ ignore-backends: gcc
 //@ compile-flags: -Z ui-testing=no
 
@@ -7,15 +8,15 @@ use std::{
 };
 
 const X: () = {
-    let mut ptr1 = &1;
-    let mut ptr2 = &2;
+    let mut x1 = 1;
+    let mut x2 = 2;
 
     // Swap them, bytewise.
     unsafe {
-        ptr::swap_nonoverlapping( //~ ERROR unable to copy parts of a pointer
-            &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
-            &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
-            mem::size_of::<&i32>(),
+        ptr::swap_nonoverlapping( //~ ERROR beyond the end of the allocation
+            &mut x1 as *mut _ as *mut MaybeUninit<u8>,
+            &mut x2 as *mut _ as *mut MaybeUninit<u8>,
+            10,
         );
     }
 };
diff --git a/tests/ui/consts/missing_span_in_backtrace.stderr b/tests/ui/consts/missing_span_in_backtrace.stderr
index 0ac1e281107..55dde635828 100644
--- a/tests/ui/consts/missing_span_in_backtrace.stderr
+++ b/tests/ui/consts/missing_span_in_backtrace.stderr
@@ -1,15 +1,13 @@
-error[E0080]: unable to copy parts of a pointer from memory at ALLOC0
-  --> $DIR/missing_span_in_backtrace.rs:15:9
+error[E0080]: memory access failed: attempting to access 1 byte, but got ALLOC0+0x4 which is at or beyond the end of the allocation of size 4 bytes
+  --> $DIR/missing_span_in_backtrace.rs:16:9
    |
-15 | /         ptr::swap_nonoverlapping(
-16 | |             &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
-17 | |             &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
-18 | |             mem::size_of::<&i32>(),
-19 | |         );
+16 | /         ptr::swap_nonoverlapping(
+17 | |             &mut x1 as *mut _ as *mut MaybeUninit<u8>,
+18 | |             &mut x2 as *mut _ as *mut MaybeUninit<u8>,
+19 | |             10,
+20 | |         );
    | |_________^ evaluation of `X` failed inside this call
    |
-   = help: this code performed an operation that depends on the underlying bytes representing a pointer
-   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 note: inside `swap_nonoverlapping::compiletime::<MaybeUninit<u8>>`
   --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
 note: inside `std::ptr::swap_nonoverlapping_const::<MaybeUninit<u8>>`