clippy::uninit_vec: 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()

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

#[allow(clippy::uninit_vec)]
fn my_function() { /* ... */ }

[lints.clippy]
uninit_vec = "allow"

cargo clippy -- -A clippy::uninit-vec

Clippy's own documentation

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
   

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

Related lints

• transmute_null_to_fntransmuting a known null pointer into a function pointer
• transmuting_nulltransmuting a known null pointer into a reference
• uninit_assumed_initthis call for this type may be undefined behavior
• unit_cmpChecks for comparisons to unit
• unit_hashDetects ().hash(_)
• unit_return_expecting_ordthis closure returns the unit type which also implements Ord

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker