Allow unaligned reads in constants

src/librustc_mir/hair/pattern/mod.rs

@@ -792,7 +792,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
                 ConstVal::Value(miri) => const_val_field(
                     self.tcx, self.param_env, instance,
                     variant_opt, field, miri, cv.ty,
-                ).unwrap(),
+                ).expect("field access failed"),
                 _ => bug!("{:#?} is not a valid adt", cv),
             };
             self.const_to_pat(instance, val, id, span)

src/librustc_mir/interpret/eval_context.rs

@@ -1340,9 +1340,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
         use syntax::ast::FloatTy;
 
         let layout = self.layout_of(ty)?;
-        // do the strongest layout check of the two
-        let align = layout.align.max(ptr_align);
-        self.memory.check_align(ptr, align)?;
+        self.memory.check_align(ptr, ptr_align)?;
 
         if layout.size.bytes() == 0 {
             return Ok(Some(Value::ByVal(PrimVal::Undef)));

src/test/ui/const-eval/ice-packed.rs

@@ -0,0 +1,28 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+#[derive(Copy, Clone, PartialEq, Eq)]
+#[repr(packed)]
+pub struct Num(u64);
+
+impl Num {
+    pub const ZERO: Self = Num(0);
+}
+
+pub fn decrement(a: Num) -> Num {
+    match a {
+        Num::ZERO => Num::ZERO,
+        a => Num(a.0 - 1)
+    }
+}
+
+fn main() {
+}