message: permit locking constant memory (60)
diff: @@ -562,7 +562,7 @@ impl<'a, 'tcx> Memory<'a, 'tcx> {
  self.check_bounds(ptr.offset(len, self.layout)?, true)?; // if ptr.offset is in bounds, then so is ptr (because offset checks for overflow)
  self.check_locks(ptr, len, kind)?; // make sure we have the access we are acquiring
  let lifetime = DynamicLifetime { frame: self.cur_frame, region };
  - let alloc = self.get_mut(ptr.alloc_id)?;
  + let alloc = self.get_mut_unchecked(ptr.alloc_id)?;
  alloc.locks.entry(MemoryRange::new(ptr.offset, len)).or_insert_with(|| Vec::new()).push(LockInfo { lifetime, kind, status: LockStatus::Held });
  Ok(())
  }
  @@ -571,7 +571,7 @@ impl<'a, 'tcx> Memory<'a, 'tcx> {
  pub(crate) fn release_write_lock_until(&mut self, ptr: MemoryPointer, len: u64, release_until: Option<CodeExtent>) -> EvalResult<'tcx> {
  assert!(len > 0);
  let cur_frame = self.cur_frame;
  - let alloc = self.get_mut(ptr.alloc_id)?;
  + let alloc = self.get_mut_unchecked(ptr.alloc_id)?;
  for (range, locks) in alloc.iter_lock_vecs_mut(ptr.offset, len) {
  if !range.contains(ptr.offset, len) {
  @@ -649,13 +649,9 @@ impl<'a, 'tcx> Memory<'a, 'tcx> {
  }
  }
  - pub fn get_mut(&mut self, id: AllocId) -> EvalResult<'tcx, &mut Allocation> {
  + fn get_mut_unchecked(&mut self, id: AllocId) -> EvalResult<'tcx, &mut Allocation> {
  match self.alloc_map.get_mut(&id) {
  - Some(alloc) => if alloc.mutable == Mutability::Mutable {
  - Ok(alloc)
  - } else {
  - Err(EvalError::ModifiedConstantMemory)
  - },
  + Some(alloc) => Ok(alloc),
  None => match self.functions.get(&id) {
  Some(_) => Err(EvalError::DerefFunctionPointer),
  None => Err(EvalError::DanglingPointerDeref),
  @@ -663,6 +659,15 @@ impl<'a, 'tcx> Memory<'a, 'tcx> {
  }
  }
  + pub fn get_mut(&mut self, id: AllocId) -> EvalResult<'tcx, &mut Allocation> {
  + let alloc = self.get_mut_unchecked(id)?;
  + if alloc.mutable == Mutability::Mutable {
  + Ok(alloc)
  + } else {
  + Err(EvalError::ModifiedConstantMemory)
  + }
  + }
  +
  pub fn get_fn(&self, ptr: MemoryPointer) -> EvalResult<'tcx, ty::Instance<'tcx>> {
  if ptr.offset != 0 {
  return Err(EvalError::InvalidFunctionPointer);
  