I tried this code:

use std::collections::BTreeSet;

fn main() {
    let mut set = BTreeSet::<u32>::new();
    set.insert(0);
}

using the following command line invocation:

rmc test.rs --unwind 2

with RMC version: 8de5cd67609

CBMC memory usage went up to 20GB after 1 minute and reached ~30 GB shortly after.