diff --git a/library/core/src/ffi/c_str.rs b/library/core/src/ffi/c_str.rs
index 92e38df4049..163a65c909e 100644
--- a/library/core/src/ffi/c_str.rs
+++ b/library/core/src/ffi/c_str.rs
@@ -253,7 +253,7 @@ impl CStr {
     /// ```
     ///
     /// [valid]: core::ptr#safety
-    #[inline]
+    #[inline] // inline is necessary for codegen to see strlen.
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_cstr_from_ptr", issue = "113219")]
@@ -280,6 +280,8 @@ const fn strlen_ct(s: *const c_char) -> usize {
                 len
             }
 
+            // `inline` is necessary for codegen to see strlen.
+            #[inline]
             fn strlen_rt(s: *const c_char) -> usize {
                 extern "C" {
                     /// Provided by libc or compiler_builtins.