Skip no-op adjustments in trans

That allows us to keep using trans_into() in case of adjustments that
may actually be ignored in trans because they are a plain deref/ref pair
with no overloaded deref or unsizing.

Unoptimized(!) benchmarks from servo/servo#7638

Before
```
test goser::bench_clone                          ... bench:      17,701 ns/iter (+/- 58) = 30 MB/s
test goser::bincode::bench_decoder               ... bench:      33,715 ns/iter (+/- 300) = 11 MB/s
test goser::bincode::bench_deserialize           ... bench:      36,804 ns/iter (+/- 329) = 9 MB/s
test goser::bincode::bench_encoder               ... bench:      34,695 ns/iter (+/- 149) = 11 MB/s
test goser::bincode::bench_populate              ... bench:      18,879 ns/iter (+/- 88)
test goser::bincode::bench_serialize             ... bench:      31,668 ns/iter (+/- 156) = 11 MB/s
test goser::capnp::bench_deserialize             ... bench:       2,049 ns/iter (+/- 87) = 218 MB/s
test goser::capnp::bench_deserialize_packed      ... bench:      10,707 ns/iter (+/- 258) = 31 MB/s
test goser::capnp::bench_populate                ... bench:         635 ns/iter (+/- 5)
test goser::capnp::bench_serialize               ... bench:      35,657 ns/iter (+/- 155) = 12 MB/s
test goser::capnp::bench_serialize_packed        ... bench:      37,881 ns/iter (+/- 146) = 8 MB/s
test goser::msgpack::bench_decoder               ... bench:      50,634 ns/iter (+/- 307) = 5 MB/s
test goser::msgpack::bench_encoder               ... bench:      25,738 ns/iter (+/- 90) = 11 MB/s
test goser::msgpack::bench_populate              ... bench:      18,900 ns/iter (+/- 138)
test goser::protobuf::bench_decoder              ... bench:       2,791 ns/iter (+/- 29) = 102 MB/s
test goser::protobuf::bench_encoder              ... bench:      75,414 ns/iter (+/- 358) = 3 MB/s
test goser::protobuf::bench_populate             ... bench:      19,248 ns/iter (+/- 92)
test goser::rustc_serialize_json::bench_decoder  ... bench:     109,999 ns/iter (+/- 797) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder  ... bench:      58,777 ns/iter (+/- 418) = 10 MB/s
test goser::rustc_serialize_json::bench_populate ... bench:      18,887 ns/iter (+/- 76)
test goser::serde_json::bench_deserializer       ... bench:     104,803 ns/iter (+/- 770) = 5 MB/s
test goser::serde_json::bench_populate           ... bench:      18,890 ns/iter (+/- 69)
test goser::serde_json::bench_serializer         ... bench:      75,046 ns/iter (+/- 435) = 8 MB/s
```

After
```
test goser::bench_clone                          ... bench:      16,052 ns/iter (+/- 188) = 34 MB/s
test goser::bincode::bench_decoder               ... bench:      31,194 ns/iter (+/- 941) = 12 MB/s
test goser::bincode::bench_deserialize           ... bench:      33,934 ns/iter (+/- 352) = 10 MB/s
test goser::bincode::bench_encoder               ... bench:      30,737 ns/iter (+/- 1,969) = 13 MB/s
test goser::bincode::bench_populate              ... bench:      17,234 ns/iter (+/- 176)
test goser::bincode::bench_serialize             ... bench:      28,269 ns/iter (+/- 452) = 12 MB/s
test goser::capnp::bench_deserialize             ... bench:       2,019 ns/iter (+/- 85) = 221 MB/s
test goser::capnp::bench_deserialize_packed      ... bench:      10,662 ns/iter (+/- 527) = 31 MB/s
test goser::capnp::bench_populate                ... bench:         607 ns/iter (+/- 2)
test goser::capnp::bench_serialize               ... bench:      30,488 ns/iter (+/- 219) = 14 MB/s
test goser::capnp::bench_serialize_packed        ... bench:      33,731 ns/iter (+/- 201) = 9 MB/s
test goser::msgpack::bench_decoder               ... bench:      46,921 ns/iter (+/- 461) = 6 MB/s
test goser::msgpack::bench_encoder               ... bench:      22,315 ns/iter (+/- 96) = 12 MB/s
test goser::msgpack::bench_populate              ... bench:      17,268 ns/iter (+/- 73)
test goser::protobuf::bench_decoder              ... bench:       2,658 ns/iter (+/- 44) = 107 MB/s
test goser::protobuf::bench_encoder              ... bench:      71,024 ns/iter (+/- 359) = 4 MB/s
test goser::protobuf::bench_populate             ... bench:      17,704 ns/iter (+/- 104)
test goser::rustc_serialize_json::bench_decoder  ... bench:     107,867 ns/iter (+/- 759) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder  ... bench:      52,327 ns/iter (+/- 479) = 11 MB/s
test goser::rustc_serialize_json::bench_populate ... bench:      17,262 ns/iter (+/- 68)
test goser::serde_json::bench_deserializer       ... bench:      99,156 ns/iter (+/- 657) = 6 MB/s
test goser::serde_json::bench_populate           ... bench:      17,264 ns/iter (+/- 77)
test goser::serde_json::bench_serializer         ... bench:      66,135 ns/iter (+/- 392) = 9 MB/s

```

Author: dotdash

src/librustc_trans/trans/expr.rs

@@ -119,7 +119,7 @@ pub fn trans_into<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 
     debuginfo::set_source_location(bcx.fcx, expr.id, expr.span);
 
-    if bcx.tcx().tables.borrow().adjustments.contains_key(&expr.id) {
+    if adjustment_required(bcx, expr) {
         // use trans, which may be less efficient but
         // which will perform the adjustments:
         let datum = unpack_datum!(bcx, trans(bcx, expr));
@@ -334,6 +334,37 @@ pub fn unsized_info<'ccx, 'tcx>(ccx: &CrateContext<'ccx, 'tcx>,
     }
 }
 
+fn adjustment_required<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
+                                   expr: &hir::Expr) -> bool {
+    let adjustment = match bcx.tcx().tables.borrow().adjustments.get(&expr.id).cloned() {
+        None => { return false; }
+        Some(adj) => adj
+    };
+
+    // Don't skip a conversion from Box<T> to &T, etc.
+    if bcx.tcx().is_overloaded_autoderef(expr.id, 0) {
+        return true;
+    }
+
+    match adjustment {
+        AdjustReifyFnPointer => {
+            // FIXME(#19925) once fn item types are
+            // zero-sized, we'll need to return true here
+            false
+        }
+        AdjustUnsafeFnPointer => {
+            // purely a type-level thing
+            false
+        }
+        AdjustDerefRef(ref adj) => {
+            // We are a bit paranoid about adjustments and thus might have a re-
+            // borrow here which merely derefs and then refs again (it might have
+            // a different region or mutability, but we don't care here).
+            !(adj.autoderefs == 1 && adj.autoref.is_some() && adj.unsize.is_none())
+        }
+    }
+}
+
 /// Helper for trans that apply adjustments from `expr` to `datum`, which should be the unadjusted
 /// translation of `expr`.
 fn apply_adjustments<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,

src/test/codegen/adjustments.rs

@@ -26,3 +26,12 @@ pub fn no_op_slice_adjustment(x: &[u8]) -> &[u8] {
 // CHECK: call void @llvm.memcpy.{{.*}}(i8* [[DST]], i8* [[SRC]],
     { x }
 }
+
+// CHECK-LABEL: @no_op_slice_adjustment2
+#[no_mangle]
+pub fn no_op_slice_adjustment2(x: &[u8]) -> &[u8] {
+    // We used to generate an extra alloca and memcpy for the function's return value, so check
+    // that there's no memcpy (the slice is written to sret_slot element-wise)
+// CHECK-NOT: call void @llvm.memcpy.
+    no_op_slice_adjustment(x)
+}