add method to get the mutability of an AllocId

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -520,6 +520,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
     /// Gives raw access to the `Allocation`, without bounds or alignment checks.
     /// The caller is responsible for calling the access hooks!
+    ///
+    /// You almost certainly want to use `get_ptr_alloc`/`get_ptr_alloc_mut` instead.
     fn get_alloc_raw(
         &self,
         id: AllocId,
@@ -589,6 +591,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         Ok(&self.get_alloc_raw(id)?.extra)
     }
 
+    /// Return the `mutability` field of the given allocation.
+    pub fn get_alloc_mutability<'a>(&'a self, id: AllocId) -> InterpResult<'tcx, Mutability> {
+        Ok(self.get_alloc_raw(id)?.mutability)
+    }
+
     /// Gives raw mutable access to the `Allocation`, without bounds or alignment checks.
     /// The caller is responsible for calling the access hooks!
     ///