Detect hidden malloc in comparison of Option<String>

What it does

When comparing Option<String>, we may create an intermediate object for the sole purpose of the comparison, and for that we allocate the inner String object. Instead we can compare the inner str object without allocation:

fn main() {
    let string = Some("Hello, world!".to_owned());
    // There is an unnecessary malloc hidden in this statement in `.to_owned()`
    assert!(string == Some("Hello, world!".to_owned()));
    // This comparison is done without allocation
    assert!(string.as_deref() == Some("Hello, world!"));
}

Lint Name

string-allocation-during-comparison

Category

perf

Advantage

• Saves redundant memory allocation
• Faster

Drawbacks

None that I can think of

Example

assert!(string == Some("Hello, world!".to_owned()));

Could be written as:

assert!(string.as_deref() == Some("Hello, world!"));

Search rust-clippy for the lint name string-allocation-during-comparison and inspect existing lints that analyze comparisons and suggest rewrites. Use the provided Option<String> example as the behavioral baseline; done means the allocating comparison is detected and the as_deref() form is recommended without false positives.