diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index 122baec8e58..770651b61a5 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -728,7 +728,7 @@ mod builtin {
     #[cfg(dox)]
     macro_rules! module_path { () => ({ /* compiler built-in */ }) }
 
-    /// Boolean evaluation of configuration flags.
+    /// Boolean evaluation of configuration flags, at compile-time.
     ///
     /// For more information, see the documentation for [`std::cfg!`].
     ///
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 9d0373404aa..25d59d912e0 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -598,7 +598,7 @@ pub mod builtin {
     #[macro_export]
     macro_rules! module_path { () => ({ /* compiler built-in */ }) }
 
-    /// Boolean evaluation of configuration flags.
+    /// Boolean evaluation of configuration flags, at compile-time.
     ///
     /// In addition to the `#[cfg]` attribute, this macro is provided to allow
     /// boolean expression evaluation of configuration flags. This frequently