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

Author: woodruffw

Diff for: library/alloc/src/macros.rs

@@ -29,6 +29,9 @@
 /// to the same boxed integer value, not five references pointing to independently
 /// boxed integers.
 ///
+/// Also, note that `[T; 0]` is a valid initializer. This will initialize (or call)
+/// `T` but not populate the vector with it, so be mindful of side effects.
+///
 /// [`Vec`]: crate::vec::Vec
 #[cfg(not(test))]
 #[macro_export]

Diff for: library/std/src/primitive_docs.rs

@@ -489,6 +489,9 @@ 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 `[x; 0]` is a valid repeat expression. This will produce an empty array
+/// but will also initialize (or call) `x`, which may produce side effects.
+///
 /// Arrays of *any* size implement the following traits if the element type allows it:
 ///
 /// - [`Copy`]