1 files changed, 22 insertions, 0 deletions
diff --git a/src/let_else.rs b/src/let_else.rs
index da17a68..a1627f1 100644
--- a/src/let_else.rs
+++ b/src/let_else.rs
@@ -21,3 +21,25 @@ right: {:?}",
 }
 
 pub(crate) use assume;
+
+/// Binds the given expression to the given pattern, or else executes
+/// `unreachable!();` with a helpful panic message and returns.
+macro_rules! know {
+    ($pattern:pat, $value:expr) => {
+        // The expression might change each time it's evaluated, so we
+        // have to bind it so that we can reuse it in the panic message.
+        let _value = $value;
+
+        let $pattern = _value else {
+            unreachable!(
+                "assertion `left matches right` failed:
+ left: {}
+right: {:?}",
+                stringify!($pattern),
+                _value
+            );
+        };
+    };
+}
+
+pub(crate) use know;