Auto merge of #58351 - oli-obk:double_check_const_eval, r=RalfJung

Refactor interning to properly mark memory as mutable or immutable

r? @RalfJung

This implementation is incomplete out of multiple reasons

* [ ] add `-Zunleash_the_miri_inside_of_you` tests
* [ ] report an error if there's an `UnsafeCell` behind a reference in a constant
* [ ] make validity checks actually test whether the mutability of their allocations match what they see in the type

2 files changed, 8 insertions, 3 deletions

diff --git a/src/librustc/mir/interpret/value.rs b/src/librustc/mir/interpret/value.rs
index f7b3385668f..40e81119973 100644
--- a/src/librustc/mir/interpret/value.rs
+++ b/src/librustc/mir/interpret/value.rs
@@ -2,7 +2,7 @@ use std::fmt;
 use rustc_macros::HashStable;
 use rustc_apfloat::{Float, ieee::{Double, Single}};
 
-use crate::ty::{Ty, InferConst, ParamConst, layout::{HasDataLayout, Size}, subst::SubstsRef};
+use crate::ty::{Ty, InferConst, ParamConst, layout::{HasDataLayout, Size, Align}, subst::SubstsRef};
 use crate::ty::PlaceholderConst;
 use crate::hir::def_id::DefId;
 
@@ -45,7 +45,12 @@ pub enum ConstValue<'tcx> {
 
     /// An allocation together with a pointer into the allocation.
     /// Invariant: the pointer's `AllocId` resolves to the allocation.
-    ByRef(Pointer, &'tcx Allocation),
+    /// The alignment exists to allow `const_field` to have `ByRef` access to nonprimitive fields
+    /// of `repr(packed)` structs. The alignment may be lower than the type of this constant.
+    /// This permits reads with lower alignment than what the type would normally require.
+    /// FIXME(RalfJ,oli-obk): The alignment checks are part of miri, but const eval doesn't really
+    /// need them. Disabling them may be too hard though.
+    ByRef(Pointer, Align, &'tcx Allocation),
 
     /// Used in the HIR by using `Unevaluated` everywhere and later normalizing to one of the other
     /// variants when the code is monomorphic enough for that.
diff --git a/src/librustc/ty/structural_impls.rs b/src/librustc/ty/structural_impls.rs
index a4efb566e13..4cd0fd3e824 100644
--- a/src/librustc/ty/structural_impls.rs
+++ b/src/librustc/ty/structural_impls.rs
@@ -1335,7 +1335,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::Const<'tcx> {
 impl<'tcx> TypeFoldable<'tcx> for ConstValue<'tcx> {
     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
         match *self {
-            ConstValue::ByRef(ptr, alloc) => ConstValue::ByRef(ptr, alloc),
+            ConstValue::ByRef(ptr, align, alloc) => ConstValue::ByRef(ptr, align, alloc),
             ConstValue::Infer(ic) => ConstValue::Infer(ic.fold_with(folder)),
             ConstValue::Param(p) => ConstValue::Param(p.fold_with(folder)),
             ConstValue::Placeholder(p) => ConstValue::Placeholder(p),