1 files changed, 34 insertions, 0 deletions
diff --git a/assertions/src/mechanism.rs b/assertions/src/mechanism.rs
new file mode 100644
index 0000000..e14b91a
--- /dev/null
+++ b/assertions/src/mechanism.rs
@@ -0,0 +1,34 @@
+// Copyright 2018 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::marker::PhantomData;
+
+pub struct True;
+pub struct False;
+
+pub trait Expr {
+    type Value;
+}
+
+impl Expr for [(); 0] {
+    type Value = False;
+}
+
+impl Expr for [(); 1] {
+    type Value = True;
+}
+
+// If the macro instantiates this with `T = [(); 1]` then it compiles successfully.
+//
+// On the other hand if `T = [(); 0]` the user receives an error like the following:
+//
+//    error[E0271]: type mismatch resolving `<[(); 0] as assertions::Expr>::Value == assertions::True`
+//     --> src/main.rs:5:5
+//      |
+//    5 |     const_assert!(std::mem::size_of::<String>() == 8);
+//      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `assertions::True`, found struct `assertions::False`
+//
+pub struct Assert<T: Expr<Value = True>> {
+    marker: PhantomData<T>,
+}