The first time I wanted a NLL, I did a hash lookup, matched on the option that came back, and wanted to insert into the hash in the None match arm. Since then I think they've added convenience methods for getting or inserting. I still hope that IndexAddAssign (which would have addressed my use case) becomes an overloadable operator someday.

Recently I was parsing a line-oriented, whitespace delimited file and had to introduce a temporary ("line", below):

    let f = File::open(path)?;
    let reader = BufReader::new(f);
    for line_result in reader.lines() {
        let line = line_result?;
        let mut word_iter = line.split_whitespace();

because I think something didn't live long enough when I did line_result?.split_whitespace(). I forget exactly what the code was like before I got it past the borrow checker, though.

For me, it seems like every time I dip a toe into rust I run into borrow/NLL issues right off the bat.