diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index d217e1c5f8f..f4133dfbc95 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -56,6 +56,8 @@ impl<'tcx> MonoItem<'tcx> {
         }
     }
 
+    // Note: if you change how item size estimates work, you might need to
+    // change NON_INCR_MIN_CGU_SIZE as well.
     pub fn size_estimate(&self, tcx: TyCtxt<'tcx>) -> usize {
         match *self {
             MonoItem::Fn(instance) => {