Stacked Borrows: alignment does not matter

rust-version

@@ -1 +1 @@
-9b2f8dbba39dd4167f22a7026674a585c3d907d8
+b2e36e6c2d229126b59e892c9147fbb68115d292

src/eval.rs

@@ -98,6 +98,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
  start_id,
  tcx.mk_substs(::std::iter::once(ty::subst::GenericArg::from(main_ret_ty))),
  )
+ .unwrap()
  .unwrap();
 
  // First argument: pointer to `main()`.

src/stacked_borrows.rs

@@ -11,7 +11,7 @@ use log::trace;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_middle::mir::RetagKind;
 use rustc_middle::ty;
-use rustc_target::abi::{LayoutOf, Size};
+use rustc_target::abi::{Align, LayoutOf, Size};
 use rustc_hir::Mutability;
 
 use crate::*;
@@ -577,11 +577,13 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  .size_and_align_of_mplace(place)?
  .map(|(size, _)| size)
  .unwrap_or_else(|| place.layout.size);
+ // `reborrow` relies on getting a `Pointer` and everything being in-bounds,
+ // so let's ensure that. However, we do not care about alignment.
  // We can see dangling ptrs in here e.g. after a Box's `Unique` was
- // updated using "self.0 = ..." (can happen in Box::from_raw); see miri#1050.
- let place = this.mplace_access_checked(place)?;
+ // updated using "self.0 = ..." (can happen in Box::from_raw) so we cannot ICE; see miri#1050.
+ let place = this.mplace_access_checked(place, Some(Align::from_bytes(1).unwrap()))?;
+ // Nothing to do for ZSTs.
  if size == Size::ZERO {
- // Nothing to do for ZSTs.
  return Ok(val);
  }
 

tests/run-pass/packed_struct.rs

@@ -1,11 +1,12 @@
-#![feature(unsize, coerce_unsized)]
+#![feature(unsize, coerce_unsized, raw_ref_op)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::Hash;
 
 fn test_basic() {
  #[repr(packed)]
  struct S {
+ fill: u8,
  a: i32,
  b: i64,
  }
@@ -30,16 +31,21 @@ fn test_basic() {
  }
 
  let mut x = S {
+ fill: 0,
  a: 42,
  b: 99,
  };
  let a = x.a;
  let b = x.b;
  assert_eq!(a, 42);
  assert_eq!(b, 99);
+ assert_eq!(&x.fill, &0); // `fill` just requirs 1-byte-align, so this is fine
  // can't do `assert_eq!(x.a, 42)`, because `assert_eq!` takes a reference
  assert_eq!({x.a}, 42);
  assert_eq!({x.b}, 99);
+ // but we *can* take a raw pointer!
+ assert_eq!(unsafe { (&raw const x.a).read_unaligned() }, 42);
+ assert_eq!(unsafe { (&raw const x.b).read_unaligned() }, 99);
 
  x.b = 77;
  assert_eq!({x.b}, 77);