let groups = ranks.into_iter().fold(HashMap::new(), |mut acc, x| {
            match acc.get(&x) {
                Some(n) => acc.insert(x, n + 1),
                None => acc.insert(x, 1)
            };

            acc
        });

warning: cannot borrow `acc` as mutable because it is also borrowed as immutable
   --> src/combination.rs:286:28
    |
285 |             match acc.get(&x) {
    |                   --- immutable borrow occurs here
286 |                 Some(n) => acc.insert(x, n + 1),
    |                            ^^^           - immutable borrow later used here
    |                            |
    |                            mutable borrow occurs here
    |
    = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

let mut letters = HashMap::new();

for ch in "a short treatise on fungi".chars() {
    let counter = letters.entry(ch).or_insert(0);
    *counter += 1;
}

#[derive(StructOpt)]
struct Args {
  x: bool,
  y: bool
}

cargo r -- --x #<- ok
cargo r -- --y #<- ok
cargo r -- --x --y #<- ne ok

use structopt::clap::ArgGroup;
use structopt::StructOpt;

#[derive(Debug, StructOpt)]
#[structopt(
    group = ArgGroup::with_name("verbose"),
    group = ArgGroup::with_name("quiet").conflicts_with("verbose"),
)]
struct Opt {
    #[structopt(long, group = "verbose")]
    verbose: bool,
    #[structopt(long, group = "quiet")]
    quiet: bool,
}

fn main() {
    dbg!(Opt::from_args());
}