doc(array,vec): add notes about side effects when empty-initializing

library/alloc/src/macros.rs

@@ -29,6 +29,10 @@
 /// to the same boxed integer value, not five references pointing to independently
 /// boxed integers.
 ///
+/// Also, note that `vec![expr; 0]` is allowed, and produces an empty vector.
+/// This will still evaluate `expr`, however, and immediately drop the resulting value, so
+/// be mindful of side effects.
+///
 /// [`Vec`]: crate::vec::Vec
 #[cfg(not(test))]
 #[macro_export]

library/std/src/primitive_docs.rs

@@ -489,6 +489,10 @@ mod prim_pointer {}
 /// * A repeat expression `[x; N]`, which produces an array with `N` copies of `x`.
 ///   The type of `x` must be [`Copy`].
 ///
+/// Note that `[expr; 0]` is allowed, and produces an empty array.
+/// This will still evaluate `expr`, however, and immediately drop the resulting value, so
+/// be mindful of side effects.
+///
 /// Arrays of *any* size implement the following traits if the element type allows it:
 ///
 /// - [`Copy`]