diff --git a/src/librustc/mir/repr.rs b/src/librustc/mir/repr.rs
index 70b0b810c6c4..f236c4ebb977 100644
--- a/src/librustc/mir/repr.rs
+++ b/src/librustc/mir/repr.rs
@@ -93,6 +93,12 @@ pub struct Mir<'tcx> {
     /// the first argument is either the closure or a reference to it.
     pub upvar_decls: Vec<UpvarDecl>,
 
+    /// A boolean indicating whether the last argument (which must be a tuple)
+    /// is passed as its individual components at the LLVM level.
+    ///
+    /// This is used for the "rust-call" ABI.
+    pub spread_last_arg: bool,
+
     /// A span representing this MIR, for error reporting
     pub span: Span,
 
@@ -123,6 +129,7 @@ impl<'tcx> Mir<'tcx> {
             arg_decls: arg_decls,
             temp_decls: temp_decls,
             upvar_decls: upvar_decls,
+            spread_last_arg: false,
             span: span,
             cache: Cache::new()
         }
@@ -341,10 +348,6 @@ pub struct TempDecl<'tcx> {
 pub struct ArgDecl<'tcx> {
     pub ty: Ty<'tcx>,
 
-    /// If true, this argument is a tuple after monomorphization,
-    /// and has to be collected from multiple actual arguments.
-    pub spread: bool,
-
     /// Either keywords::Invalid or the name of a single-binding
     /// pattern associated with this argument. Useful for debuginfo.
     pub debug_name: Name
diff --git a/src/librustc/mir/visit.rs b/src/librustc/mir/visit.rs
index 2c58d35973e7..bd42eb3211a8 100644
--- a/src/librustc/mir/visit.rs
+++ b/src/librustc/mir/visit.rs
@@ -665,7 +665,6 @@ macro_rules! make_mir_visitor {
                               arg_decl: & $($mutability)* ArgDecl<'tcx>) {
                 let ArgDecl {
                     ref $($mutability)* ty,
-                    spread: _,
                     debug_name: _
                 } = *arg_decl;
 
diff --git a/src/librustc_mir/build/mod.rs b/src/librustc_mir/build/mod.rs
index 23591f05b877..1f8dffe58717 100644
--- a/src/librustc_mir/build/mod.rs
+++ b/src/librustc_mir/build/mod.rs
@@ -169,7 +169,7 @@ pub fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
         tcx.region_maps.lookup_code_extent(
             CodeExtentData::ParameterScope { fn_id: fn_id, body_id: body_id });
     let mut block = START_BLOCK;
-    let mut arg_decls = unpack!(block = builder.in_scope(call_site_extent, block, |builder| {
+    let arg_decls = unpack!(block = builder.in_scope(call_site_extent, block, |builder| {
         let arg_decls = unpack!(block = builder.in_scope(arg_extent, block, |builder| {
             builder.args_and_body(block, return_ty, arguments, arg_extent, ast_block)
         }));
@@ -184,12 +184,11 @@ pub fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
     }));
     assert_eq!(block, builder.return_block());
 
+    let mut spread_last_arg = false;
     match tcx.node_id_to_type(fn_id).sty {
         ty::TyFnDef(_, _, f) if f.abi == Abi::RustCall => {
             // RustCall pseudo-ABI untuples the last argument.
-            if let Some(last_arg) = arg_decls.last() {
-                arg_decls[last_arg].spread = true;
-            }
+            spread_last_arg = true;
         }
         _ => {}
     }
@@ -218,7 +217,9 @@ pub fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
         }).collect()
     });
 
-    builder.finish(upvar_decls, arg_decls, return_ty)
+    let (mut mir, aux) = builder.finish(upvar_decls, arg_decls, return_ty);
+    mir.spread_last_arg = spread_last_arg;
+    (mir, aux)
 }
 
 pub fn construct_const<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
@@ -331,7 +332,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
             ArgDecl {
                 ty: ty,
-                spread: false,
                 debug_name: name
             }
         }).collect();
diff --git a/src/librustc_trans/mir/mod.rs b/src/librustc_trans/mir/mod.rs
index 1934f7b870d1..efbab3a3b4dc 100644
--- a/src/librustc_trans/mir/mod.rs
+++ b/src/librustc_trans/mir/mod.rs
@@ -349,7 +349,7 @@ fn arg_local_refs<'bcx, 'tcx>(bcx: &BlockAndBuilder<'bcx, 'tcx>,
     mir.arg_decls.iter().enumerate().map(|(arg_index, arg_decl)| {
         let arg_ty = bcx.monomorphize(&arg_decl.ty);
         let local = mir.local_index(&mir::Lvalue::Arg(mir::Arg::new(arg_index))).unwrap();
-        if arg_decl.spread {
+        if mir.spread_last_arg && arg_index == mir.arg_decls.len() - 1 {
             // This argument (e.g. the last argument in the "rust-call" ABI)
             // is a tuple that was spread at the ABI level and now we have
             // to reconstruct it into a tuple local variable, from multiple