Read discriminant as a signed integer if specified by layout.

This ensures it gets sign extended correctly.

Fixes #78

src/interpreter/terminator/mod.rs

@@ -252,11 +252,16 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
         let adt_layout = self.type_layout(adt_ty);
 
         let discr_val = match *adt_layout {
-            General { discr, .. } | CEnum { discr, .. } => {
+            General { discr, .. } | CEnum { discr, signed: false, .. } => {
                 let discr_size = discr.size().bytes();
                 self.memory.read_uint(adt_ptr, discr_size as usize)?
             }
 
+            CEnum { discr, signed: true, .. } => {
+                let discr_size = discr.size().bytes();
+                self.memory.read_int(adt_ptr, discr_size as usize)? as u64
+            }
+
             RawNullablePointer { nndiscr, .. } => {
                 self.read_nonnull_discriminant_value(adt_ptr, nndiscr)?
             }

tests/run-pass/negative_discriminant.rs

@@ -0,0 +1,13 @@
+enum AB { A = -1, B = 1 }
+
+fn main() {
+    match AB::A {
+        AB::A => (),
+        AB::B => panic!(),
+    }
+
+    match AB::B {
+        AB::A => panic!(),
+        AB::B => (),
+    }
+}