Avoid unnecessary temporaries when ref'ing a DST value

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/librustc_trans/trans/expr.rs

@@ -1691,29 +1691,20 @@ fn trans_uniq_expr<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
     immediate_rvalue_bcx(bcx, val, box_ty).to_expr_datumblock()
 }
 
-fn ref_fat_ptr<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
-                           lval: Datum<'tcx, Lvalue>)
-                           -> DatumBlock<'blk, 'tcx, Expr> {
-    let dest_ty = bcx.tcx().mk_imm_ref(bcx.tcx().mk_region(ty::ReStatic), lval.ty);
-    let scratch = rvalue_scratch_datum(bcx, dest_ty, "__fat_ptr");
-    memcpy_ty(bcx, scratch.val, lval.val, scratch.ty);
-
-    DatumBlock::new(bcx, scratch.to_expr_datum())
-}
-
 fn trans_addr_of<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
                              expr: &hir::Expr,
                              subexpr: &hir::Expr)
                              -> DatumBlock<'blk, 'tcx, Expr> {
     let _icx = push_ctxt("trans_addr_of");
     let mut bcx = bcx;
     let sub_datum = unpack_datum!(bcx, trans_to_lvalue(bcx, subexpr, "addr_of"));
+    let ty = expr_ty(bcx, expr);
     if !type_is_sized(bcx.tcx(), sub_datum.ty) {
-        // DST lvalue, close to a fat pointer
-        ref_fat_ptr(bcx, sub_datum)
+        // Always generate an lvalue datum, because this pointer doesn't own
+        // the data and cleanup is scheduled elsewhere.
+        DatumBlock::new(bcx, Datum::new(sub_datum.val, ty, LvalueExpr(sub_datum.kind)))
     } else {
         // Sized value, ref to a thin pointer
-        let ty = expr_ty(bcx, expr);
         immediate_rvalue_bcx(bcx, sub_datum.val, ty).to_expr_datumblock()
     }
 }

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;
+}