diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1953,6 +1953,14 @@
     the function. The instrumentation checks that the return address for the
     function has not changed between the function prolog and epilog. It is
     currently x86_64-specific.
+``mustprogress``
+    This attribute indicates that the function is required to return
+    or interact with the environment. They must eventually return or
+    interact with the environment in some way, e.g. via a side effect or
+    synchronization. ``mustprogress`` is intended to model the
+    requirements of the first section of `[intro.progress] of the C++
+    Standard <http://eel.is/c++draft/intro.progress>`_. This attribute does
+    not apply transitively to callees.
 
 Call Site Attributes
 ----------------------