RS.CLIPPY.UNINIT_VEC

Vec with uninitialized data

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: uninit_vec. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for set_len() call that creates Vec with uninitialized elements. This is commonly caused by calling set_len() right after allocating or reserving a buffer with new(), default(), with_capacity(), or reserve().

Why is this bad?

It creates a Vec with uninitialized data, which leads to undefined behavior with most safe operations. Notably, uninitialized Vec<u8> must not be used with generic Read.

Moreover, calling set_len() on a Vec created with new() or default() creates out-of-bound values that lead to heap memory corruption when used.

Known Problems

This lint only checks directly adjacent statements.

Example

let mut vec: Vec<u8> = Vec::with_capacity(1000);
unsafe { vec.set_len(1000); }
reader.read(&mut vec); // undefined behavior!

How to fix?

  1. Use an initialized buffer: 
    let mut vec: Vec<u8> = vec![0; 1000];
reader.read(&mut vec);
  2. Wrap the content in MaybeUninit: 
    let mut vec: Vec<MaybeUninit<T>> = Vec::with_capacity(1000);
vec.set_len(1000);  // `MaybeUninit` can be uninitialized
  3. If you are on 1.60.0 or later, Vec::spare_capacity_mut() is available: 
    let mut vec: Vec<u8> = Vec::with_capacity(1000);
let remaining = vec.spare_capacity_mut();  // `&mut [MaybeUninit<u8>]`
// perform initialization with `remaining`
vec.set_len(...);  // Safe to call `set_len()` on initialized part