Auto merge of #28787 - dotdash:no_more___fat_ptr, r=eddyb

A DST value and a fat pointer to it have the same representation, all we
have to do is to adjust the type of the datum holding the pointer.

src/test/codegen/refs.rs

@@ -0,0 +1,30 @@
+// 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+// Hack to get the correct size for the length part in slices
+// CHECK: @helper([[USIZE:i[0-9]+]])
+#[no_mangle]
+fn helper(_: usize) {
+}
+
+// CHECK-LABEL: @ref_dst
+#[no_mangle]
+pub fn ref_dst(s: &[u8]) {
+    // We used to generate an extra alloca and memcpy to ref the dst, so check that we copy
+    // directly to the alloca for "x"
+// CHECK: [[SRC:%[0-9]+]] = bitcast { i8*, [[USIZE]] }* %s to i8*
+// CHECK: [[DST:%[0-9]+]] = bitcast { i8*, [[USIZE]] }* %x to i8*
+// CHECK: call void @llvm.memcpy.{{.*}}(i8* [[DST]], i8* [[SRC]],
+    let x = &*s;
+}