How to satisfy clippy

[orbli]

I'm checking my code through cargo clippy. it shows a warning below:

warning: usage of `contains_key` followed by `insert` on a `HashMap`
   |
39 | /         if !orderbooks.contains_key(&inst) {
40 | |             orderbooks.insert(inst, T::new_update(update));
41 | |             return Ok(());
42 | |         }
   | |_________^ consider using `orderbooks.entry(inst)`
   |
   = note: `#[warn(clippy::map_entry)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_entry

warning: 1 warning emitted

    Finished dev [unoptimized + debuginfo] target(s) in 23.72s

As you can see, I need the function to return if i choose to add an entry. chaining entry with or_inserts in this case seems cant help much. Is there anyway more idiomatic?

[trentj]

Entry is an enum that you can match against in the normal fashion, and its variants contain values that support operations like insert. The methods like or_insert are just there for greater convenience. Here's what Clippy is trying to tell you to do:

use std::collections::hash_map::Entry;
if let Entry::Vacant(e) = orderbooks.entry(inst) {
    e.insert(T::new_update(update));
    return Ok(());
}

This is different from your original code because it does not look up inst twice (good) but it unconditionally consumes inst (possibly bad). On nightly you can use the hash_raw_entry feature to get access to raw_entry_mut. But if you don't want to use nightly, you're stuck with either doing the double lookup (and ignore/silence Clippy) or cloning inst.