diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1953,6 +1953,15 @@
     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, unwind,
+    or interact with the environment in an observable way e.g. via a
+    volatile memory access, I/O, or other synchronization. 
+    The ``mustprogress`` attribute 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, that means a callee might not make progress.
+    Note that `willreturn` implies `mustprogress`.
 
 Call Site Attributes
 ----------------------