Rollup merge of #50393 - oli-obk:packed_const_panic, r=eddyb

Allow unaligned reads in constants

fixes #50356

introduced in https://github.com/rust-lang/rust/pull/49513

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() {
+}