Auto merge of #28428 - dotdash:same_adjust, r=nikomatsakis

Currently, we're generating adjustments, for example, to get from &[u8]
to &[u8], which is unneeded and kicks us out of trans_into()
into trans() which means an additional stack slot and copy in the
unoptimized code.

Author: bors

Diff for: src/librustc_typeck/check/coercion.rs

@@ -110,6 +110,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                a,
                b);
 
+        if a == b {
+            return Ok(None);
+        }
+
         // Consider coercing the subtype to a DST
         let unsize = self.unpack_actual_value(a, |a| {
             self.coerce_unsized(a, b)

Diff for: src/test/codegen/adjustments.rs

@@ -0,0 +1,28 @@
+// 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
+
+// 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: @no_op_slice_adjustment
+#[no_mangle]
+pub fn no_op_slice_adjustment(x: &[u8]) -> &[u8] {
+    // We used to generate an extra alloca and memcpy for the block's trailing expression value, so
+    // check that we copy directly to the return value slot
+// CHECK: [[SRC:%[0-9]+]] = bitcast { i8*, [[USIZE]] }* %x to
+// CHECK: [[DST:%[0-9]+]] = bitcast { i8*, [[USIZE]] }* %sret_slot to i8*
+// CHECK: call void @llvm.memcpy.{{.*}}(i8* [[DST]], i8* [[SRC]],
+    { x }
+}