lower move_val_init during MIR construction

Because of its "magic" order-of-evaluation semantics, `move_val_init`
must be lowered during MIR construction in order to work.

src/librustc_mir/build/expr/as_temp.rs

@@ -38,9 +38,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         debug!("expr_as_temp(block={:?}, expr={:?})", block, expr);
         let this = self;
 
-        if let ExprKind::Scope { .. } = expr.kind {
-            span_bug!(expr.span, "unexpected scope expression in as_temp: {:?}",
-                      expr);
+        if let ExprKind::Scope { extent, value } = expr.kind {
+            return this.in_scope(extent, block, |this| {
+                this.as_temp(block, temp_lifetime, value)
+            });
         }
 
         let expr_ty = expr.ty.clone();

src/librustc_mir/build/expr/into.rs

@@ -16,6 +16,8 @@ use hair::*;
 use rustc::ty;
 use rustc::mir::*;
 
+use syntax::abi::Abi;
+
 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
     /// Compile `expr`, storing the result into `destination`, which
     /// is assumed to be uninitialized.
@@ -206,25 +208,49 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     }
                     _ => false
                 };
+                let intrinsic = match ty.sty {
+                    ty::TyFnDef(def_id, _, ref f) if
+                        f.abi() == Abi::RustIntrinsic ||
+                        f.abi() == Abi::PlatformIntrinsic =>
+                    {
+                        Some(this.hir.tcx().item_name(def_id).as_str())
+                    }
+                    _ => None
+                };
+                let intrinsic = intrinsic.as_ref().map(|s| &s[..]);
                 let fun = unpack!(block = this.as_local_operand(block, fun));
-                let args: Vec<_> =
-                    args.into_iter()
-                        .map(|arg| unpack!(block = this.as_local_operand(block, arg)))
-                        .collect();
+                if intrinsic == Some("move_val_init") {
+                    // `move_val_init` has "magic" semantics - the second argument is
+                    // always evaluated "directly" into the first one.
 
-                let success = this.cfg.start_new_block();
-                let cleanup = this.diverge_cleanup();
-                this.cfg.terminate(block, source_info, TerminatorKind::Call {
-                    func: fun,
-                    args: args,
-                    cleanup: cleanup,
-                    destination: if diverges {
-                        None
-                    } else {
-                        Some ((destination.clone(), success))
-                    }
-                });
-                success.unit()
+                    let mut args = args.into_iter();
+                    let ptr = args.next().expect("0 arguments to `move_val_init`");
+                    let val = args.next().expect("1 argument to `move_val_init`");
+                    assert!(args.next().is_none(), ">2 arguments to `move_val_init`");
+
+                    let topmost_scope = this.topmost_scope();
+                    let ptr = unpack!(block = this.as_temp(block, Some(topmost_scope), ptr));
+                    this.into(&ptr.deref(), block, val)
+                } else {
+                    let args: Vec<_> =
+                        args.into_iter()
+                            .map(|arg| unpack!(block = this.as_local_operand(block, arg)))
+                            .collect();
+
+                    let success = this.cfg.start_new_block();
+                    let cleanup = this.diverge_cleanup();
+                    this.cfg.terminate(block, source_info, TerminatorKind::Call {
+                        func: fun,
+                        args: args,
+                        cleanup: cleanup,
+                        destination: if diverges {
+                            None
+                        } else {
+                            Some ((destination.clone(), success))
+                        }
+                    });
+                    success.unit()
+                }
             }
 
             // These cases don't actually need a destination

src/librustc_trans/mir/block.rs

@@ -418,16 +418,6 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                 };
                 let intrinsic = intrinsic.as_ref().map(|s| &s[..]);
 
-                if intrinsic == Some("move_val_init") {
-                    let &(_, target) = destination.as_ref().unwrap();
-                    // The first argument is a thin destination pointer.
-                    let llptr = self.trans_operand(&bcx, &args[0]).immediate();
-                    let val = self.trans_operand(&bcx, &args[1]);
-                    self.store_operand(&bcx, llptr, None, val);
-                    funclet_br(self, bcx, target);
-                    return;
-                }
-
                 if intrinsic == Some("transmute") {
                     let &(ref dest, target) = destination.as_ref().unwrap();
                     self.trans_transmute(&bcx, &args[0], dest);

src/test/codegen/move-val-init.rs

@@ -0,0 +1,29 @@
+// Copyright 2017 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
+
+#![feature(core_intrinsics)]
+#![crate_type = "lib"]
+
+// test that `move_val_init` actually avoids big allocas
+
+use std::intrinsics::move_val_init;
+
+pub struct Big {
+    pub data: [u8; 65536]
+}
+
+// CHECK-LABEL: @test_mvi
+#[no_mangle]
+pub unsafe fn test_mvi(target: *mut Big, make_big: fn() -> Big) {
+    // CHECK: call void %1(%Big*{{[^%]*}} %0)
+    move_val_init(target, make_big());
+}