diff --git a/alloc/src/vec/mod.rs b/alloc/src/vec/mod.rs
index 29ddf939499a8..a7e736299bed6 100644
--- a/alloc/src/vec/mod.rs
+++ b/alloc/src/vec/mod.rs
@@ -477,6 +477,7 @@ impl<T> Vec<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "vec_with_capacity")]
     pub fn with_capacity(capacity: usize) -> Self {
         Self::with_capacity_in(capacity, Global)
     }