Performance Rules

excessive-clone

Severity: Warning

Detects .clone() calls on large structs, especially inside loops. Cloning in hot paths can cause significant performance degradation.

// Bad
for item in &items {
    let owned = item.clone();
    process(owned);
}

// Good
for item in &items {
    process(item);
}

string-from-literal

Severity: Warning

Detects String::from("literal") which is slower than "literal".to_owned() or "literal".to_string().

// Bad
let s = String::from("hello");

// Good
let s = "hello".to_owned();

collect-then-iterate

Severity: Warning

Detects .collect::<Vec<_>>() followed by .iter() or .into_iter(). The intermediate collection is unnecessary.

// Bad
let results: Vec<_> = items.iter().map(transform).collect();
for r in results.iter() { /* ... */ }

// Good
for r in items.iter().map(transform) { /* ... */ }

large-enum-variant

Severity: Warning

Detects enum variants where the size difference exceeds 200 bytes. Large variants waste memory for every instance of the enum.

// Bad
enum Message {
    Small(u32),
    Large([u8; 1024]),
}

// Good
enum Message {
    Small(u32),
    Large(Box<[u8; 1024]>),
}

unnecessary-allocation

Severity: Warning

Detects heap allocations where a stack reference would suffice.