Make uninit_vector function unsafe

[kitcatier]

Hello, I found a soundness issue in this crate.

distaff/src/utils/mod.rs

Lines 10 to 14 in fad92ce

	pub fn uninit_vector<T>(length: usize) -> Vec<T> {
	let mut vector = Vec::with_capacity(length);
	unsafe { vector.set_len(length); }
	return vector;
	}

The unsafe function called needs to ensure that the parameter must be:

• new_len must be less than or equal to capacity().
• The elements at old_len..new_len must be initialized.

https://doc.rust-lang.org/std/vec/struct.Vec.html#method.set_len
and the developer who calls the uninit_vector function may not notice this safety requirement.
Marking them unsafe also means that callers must make sure they know what they're doing.
A similar situation to the above is as follows:
https://github.com/facebook/winterfell/pull/30

[kitcatier]

Here's a safer implementation that uses std::mem::MaybeUninit to create an uninitialized Vec:

pub fn uninit_vector<T>(length: usize) -> Vec<T> { 
     let mut vector = Vec::with_capacity(length); 
     unsafe { 
         let data_ptr = vector.as_mut_ptr(); 
         std::ptr::write_bytes(data_ptr, 0, length); 
         vector.set_len(length); 
     } 
     return vector; 
 }

This implementation uses std::ptr::write_bytes to initialize each element in the vector. This ensures that all elements in the vector are properly initialized, avoiding potential memory safety issues.

[bobbinth]

@kitcatier - thank you for noticing this! This codebase isn't actively maintained, but there is a similar function here: https://github.com/facebook/winterfell/blob/main/utils/core/src/lib.rs#L78. You may want to create an issue about it in the Winterfell repo (though, I think there the function is marked as unsafe).

[kitcatier]

thanks