diff --git a/compiler/rustc_mir/src/dataflow/framework/engine.rs b/compiler/rustc_mir/src/dataflow/framework/engine.rs
index 07f5ab5e7bd..d3ad42f6bbc 100644
--- a/compiler/rustc_mir/src/dataflow/framework/engine.rs
+++ b/compiler/rustc_mir/src/dataflow/framework/engine.rs
@@ -87,6 +87,11 @@ where
     analysis: A,
 
     /// Cached, cumulative transfer functions for each block.
+    //
+    // FIXME(ecstaticmorse): This boxed `Fn` trait object is invoked inside a tight loop for
+    // gen/kill problems on cyclic CFGs. This is not ideal, but it doesn't seem to degrade
+    // performance in practice. I've tried a few ways to avoid this, but they have downsides. See
+    // the message for the commit that added this FIXME for more information.
     apply_trans_for_block: Option<Box<dyn Fn(BasicBlock, &mut A::Domain)>>,
 }