Don't ICE when pattern matching packed structs

Author: oli-obk

src/librustc_mir/const_eval.rs

@@ -482,7 +482,12 @@ pub fn const_field<'tcx>(
     trace!("const_field: {:?}, {:?}", field, value);
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
     // get the operand again
-    let op = ecx.eval_const_to_op(value, None).unwrap();
+    let mut op = ecx.eval_const_to_op(value, None).unwrap();
+    // adjust the alignment of `op` to the one of the allocation, since it may be a field of a
+    // packed struct and thus end up causing an alignment error if we read from it.
+    if let ConstValue::ByRef(_, alloc) = value.val {
+        op.force_alignment(alloc.align);
+    }
     // downcast
     let down = match variant {
         None => op,

src/librustc_mir/interpret/operand.rs

@@ -4,7 +4,9 @@
 use std::convert::TryInto;
 
 use rustc::{mir, ty};
-use rustc::ty::layout::{self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, VariantIdx};
+use rustc::ty::layout::{
+    self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, VariantIdx, Align,
+};
 
 use rustc::mir::interpret::{
     GlobalId, AllocId, CheckInAllocMsg,
@@ -177,6 +179,21 @@ impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
     }
 }
 
+impl<'tcx, Tag> OpTy<'tcx, Tag> {
+    /// This function exists solely for pattern matching. If we pattern match a packed struct with
+    /// an ADT field, the constant representing that field will have lost the information about the
+    /// packedness. We could clone the allocation and adjust the alignment, but that seems wasteful,
+    /// since the alignment is already encoded in the allocation. We know it is alright, because
+    /// validation checked everything before the initial constant entered match checking.
+    pub(crate) fn force_alignment(&mut self, align: Align) {
+        if let Operand::Indirect(mplace) = &mut self.op {
+            if align < mplace.align {
+                mplace.align = align;
+            }
+        }
+    }
+}
+
 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag>
 {
     #[inline]