Function try_with_capacity_in 

pub fn try_with_capacity_in<T, A: Allocator>(
    size: usize,
    alloc: A,
) -> Result<Vec<T, A>, TryReserveError>

Constructs a new, empty Vec<T, A> with the specified capacity with the provided allocator.

The vector will be able to hold exactly capacity elements without reallocating. If capacity is 0, the vector will not allocate.

It is important to note that although the returned vector has the capacity specified, the vector will have a zero length. For an explanation of the difference between length and capacity, see Capacity and reallocation.

Examples

use std::alloc::System;

let mut vec = try_with_capacity_in(10, System)?;

// The vector contains no items, even though it has capacity for more
assert_eq!(vec.len(), 0);
assert_eq!(vec.capacity(), 10);

// These are all done without reallocating...
for i in 0..10 {
    vec.try_push(i)?;
}
assert_eq!(vec.len(), 10);
assert_eq!(vec.capacity(), 10);

// ...but this may make the vector reallocate
vec.try_push(11)?;
assert_eq!(vec.len(), 11);
assert!(vec.capacity() >= 11);