diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 636fe115746fb..21459c9730491 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -20,6 +20,7 @@ use mir::interpret::{ConstValue, EvalErrorKind, Scalar};
 use mir::visit::MirVisitable;
 use rustc_apfloat::ieee::{Double, Single};
 use rustc_apfloat::Float;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::{dominators, Dominators};
 use rustc_data_structures::graph::{self, GraphPredecessors, GraphSuccessors};
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
@@ -2706,6 +2707,36 @@ impl Location {
         }
     }
 
+    /// Returns `true` if `other` is earlier in the control flow graph than `self`.
+    pub fn is_predecessor_of<'tcx>(&self, other: Location, mir: &Mir<'tcx>) -> bool {
+        // If we are in the same block as the other location and are an earlier statement
+        // then we are a predecessor of `other`.
+        if self.block == other.block && self.statement_index < other.statement_index {
+            return true;
+        }
+
+        // If we're in another block, then we want to check that block is a predecessor of `other`.
+        let mut queue: Vec<BasicBlock> = mir.predecessors_for(other.block).clone();
+        let mut visited = FxHashSet::default();
+
+        while let Some(block) = queue.pop() {
+            // If we haven't visited this block before, then make sure we visit it's predecessors.
+            if visited.insert(block) {
+                queue.append(&mut mir.predecessors_for(block).clone());
+            } else {
+                continue;
+            }
+
+            // If we found the block that `self` is in, then we are a predecessor of `other` (since
+            // we found that block by looking at the predecessors of `other`).
+            if self.block == block {
+                return true;
+            }
+        }
+
+        false
+    }
+
     pub fn dominates(&self, other: Location, dominators: &Dominators<BasicBlock>) -> bool {
         if self.block == other.block {
             self.statement_index <= other.statement_index
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index b440cf87a24c1..625423baecec2 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -1784,12 +1784,14 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 // of the union - we should error in that case.
                 let tcx = this.infcx.tcx;
                 if let ty::TyKind::Adt(def, _) = base.ty(this.mir, tcx).to_ty(tcx).sty {
-                    let moved_before_this = this.move_data.path_map[mpi].iter().any(|moi| {
-                        this.move_data.moves[*moi].source < context.loc
-                    });
-
-                    if def.is_union() && moved_before_this {
-                        return;
+                    if def.is_union() {
+                        if this.move_data.path_map[mpi].iter().any(|moi| {
+                            this.move_data.moves[*moi].source.is_predecessor_of(
+                                context.loc, this.mir,
+                            )
+                        }) {
+                            return;
+                        }
                     }
                 }