New lint: map_unwrap_or_bool

[jhwgh1968]

What it does

This lint discourages a less readable idiom when checking an operation which returns an Option<T> chained with a boolean predicate.

My main motivation is to avoid a bad habit of mine while I was still learning all the parts of the APIs. I imagine I am not the only one who learned it.

Categories

• Kind: clippy::pedantic

What is the advantage of the recommended code over the original code

• Makes the intention clearer
• Uses a function that is literally designed for this
• Allows additional access to the returned value, such as let Some(...) clauses

Drawbacks

Could be superseded by rust-lang/rust#75298

Example

fn main() {
    let https_proxy: Option<String> = std::env::var("HTTPS_PROXY").ok();

    if https_proxy.map(|v| v.starts_with("https://")).unwrap_or(false) {
        eprintln!("You have an HTTPS proxy set in the environment");
    } else {
        eprintln!("You do not have an HTTPS proxy set in the environment");
    }
}

Could be written as:

fn main() {
    let https_proxy: Option<String> = std::env::var("HTTPS_PROXY").ok();

    if https_proxy.filter(|v| v.starts_with("https://")).is_some() {
        eprintln!("You have an HTTPS proxy set in the environment");
    } else {
        eprintln!("You do not have an HTTPS proxy set in the environment");
    }
}

[jhwgh1968]

Hmm, that doesn't quite work, does it? Surely there must be some way to handle that operator chaining better...

[jhwgh1968]

Ah, it's filter. I'll update the issue.

[ghost]

This seems like it belongs in pedantic to me.

I prefer both to comparison with Some(true) which I see every now and then.

if https_proxy.map(|v| v.starts_with("https://")) == Some(true) {
     //...
}

Also Clippy will ask you to use map_or in the first example.
map_unwrap_or

[jhwgh1968]

This seems like it belongs in pedantic to me.

I did not update that part after my edit. Perhaps you are right. I will fix that.

Also Clippy will ask you to use map_or in the first example.
map_unwrap_or

I saw that lint, but even specifying -D clippy::pedantic I couldn't get it to trigger on my code example. I presumed it had an exception for booleans.

Even if I am wrong, though, I feel there is a better suggestion than unwrap_or(false, |c| c.condition()). If I don't get much agreement that my suggestion is an improvement, I would be willing to take others.

---

Related, which I just found: rust-lang/rust#75298

Ironically, the only alternative not mentioned in that discussion is what I suggested linting this to.

[ghost]

I actually think map_or looks the best. I like that it's compact and the default is close to the mapping function. Let's see what others think.

I saw that lint, but even specifying -D clippy::pedantic I couldn't get it to trigger on my code example. I presumed it had an exception for booleans.

It works for me. Playground