Auto merge of #25773 - dotdash:deref_dst, r=eddyb

Fat pointers aren't immediate, so in a datum, they're not actually
ByValue but ByRef.

Fixes #24589

src/test/run-pass/issue-24589.rs

@@ -0,0 +1,26 @@
+// Copyright 2015 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.
+
+pub struct _X([u8]);
+
+impl std::ops::Deref for _X {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        &self.0
+    }
+}
+
+pub fn _g(x: &_X) -> &[u8] {
+    x
+}
+
+fn main() {
+}