What Is Monomorphization and How Does It Affect Binary Size?

Monomorphization is the process where the Rust compiler generates a separate, specialized version of a generic function or struct for every concrete type used in the code, which increases binary size because each type combination results in duplicated machine code.

fn largest<T: PartialOrd>(list: &[T]) -> &T {
    let mut largest = &list[0];
    for item in list {
        if item > largest { largest = item; }
    }
    largest
}

fn main() {
    let ints = vec![1, 2, 3];
    let chars = vec!['a', 'b', 'c'];
    // Compiler generates: largest_i32 and largest_char
    let _ = largest(&ints);
    let _ = largest(&chars);
}

The compiler creates distinct implementations like largest_i32 and largest_char at compile time, meaning if you use a generic with ten different types, the final binary contains ten copies of that function's logic.

Monomorphization is like a factory that builds a custom tool for every specific material you give it, rather than using one universal tool. This ensures the final program runs very fast because the code is perfectly optimized for each data type, but it makes the final file larger because it includes a separate copy of the instructions for every type you used.