Generate n distinct random numbers in rust

I am trying to test a program which checks for correct use of brackets. To do this I want to generate Long strings of numbers with correct use of brackets (to test incorrect uses I can easily just swap around individual characters or groups of characters)

I can easily generate a vector of non-bracket characters. However, To place brackets correctly I need n unique indices. I do not know of a way to efficiently generate random indices (a list of unique random numbers in a range) so I would like either a way to do that or a way of converting random numbers into unique indices.

The two best approaches I can think of for this are:

1. generate a vector of all indices 0-N , shuffle it and then take the first n
2. generate a random number, test if it is in a hashset, if isn't add it and if it is regenerate until it isn't. Repeat until the hashset has n members then convert it to a vector

The first approach is efficient for N = n + $\epsilon$ but very inefficient for N >> n while the second approach is very inefficient for N = n + $\epsilon$ but efficient for N >> n.

Note:

1. n is the number of brackets N is the number of characters
2. for those unfamiliar with the notation $\epsilon$ is some small positive number