diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index fe2df226115..948ad104cdf 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -736,7 +736,7 @@ macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
     #[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 b36473d9b75..de46fedaebb 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -631,7 +631,7 @@ macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
     #[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