Fix panic in subslice patterns of arrays (fixes #276)

src/librustc_mir/interpret/lvalue.rs

@@ -476,7 +476,12 @@ impl<'a, 'tcx, M: Machine<'tcx>> EvalContext<'a, 'tcx, M> {
                 );
                 assert!(u64::from(from) <= n - u64::from(to));
                 let ptr = base_ptr.offset(u64::from(from) * elem_size, &self)?;
-                let extra = LvalueExtra::Length(n - u64::from(to) - u64::from(from));
+                // sublicing arrays produces arrays
+                let extra = if self.type_is_sized(base_ty) {
+                    LvalueExtra::None
+                } else {
+                    LvalueExtra::Length(n - u64::from(to) - u64::from(from))
+                };
                 (ptr, extra)
             }
         };

tests/run-pass/subslice_array.rs

@@ -0,0 +1,14 @@
+#![feature(advanced_slice_patterns)]
+#![feature(slice_patterns)]
+
+fn bar(a: &'static str, b: &'static str) -> [&'static str; 4] {
+    [a, b, b, a]
+}
+
+fn main() {
+    let out = bar("baz", "foo");
+    let [a, xs.., d] = out;
+    assert_eq!(a, "baz");
+    assert_eq!(xs, ["foo", "foo"]);
+    assert_eq!(d, "baz");
+}