When developing software in Rust, leveraging generics is a great way to write flexible and reusable code. However, when it comes to debug builds, managing code expansion due to heavy usage of generics can become challenging. In this article, we’ll explore techniques to effectively manage code size in debug builds, particularly when you are making extensive use of generics in Rust.
Understanding Code Expansion
Code expansion refers to the increase in binary size when different generic instances are monomorphized, which means that the Rust compiler generates specific concrete types for each instance of a generic in your code. This process is beneficial for performance in release builds as it brings optimizations, but it may unnecessarily bloat debug builds.
Why It's an Issue in Debug Builds
In a debug build, our main focus is on ensuring correctness and facilitating debugging rather than optimizing execution speed or binary size. However, the monomorphization that occurs due to heavy generic usage can lead to significantly larger binary sizes, which can slow down compile times and increase memory usage.
Techniques for Managing Code Expansion
1. Limit Generic Parameters
One of the simplest ways to control code expansion is to limit the number of generic parameters used. Instead of using multiple generic parameters where possible, try consolidating them.
fn process(item1: T, item2: U) {
println!("{} - {}", item1, item2);
}
Consider refactoring to reduce generics:
fn process(item1: T, item2: T) {
println!("{} - {}", item1, item2);
}
2. Using Traits Object Instead
Instead of generics, use trait objects. This can minimize the code instances that are created.
fn process_items(items: Vec<&dyn Display>) {
for item in items {
println!("{}", item);
}
}
By using &dyn Trait, you can avoid monomorphization, though this comes with the cost of dynamic dispatch, which might be slower in execution.
3. Specializing for Common Cases
If certain uses of your generic functions are exceedingly common, consider providing specialized implementations for them.
trait Processor {
fn process(&self);
}
impl Processor for i32 {
fn process(&self) {
println!("Processing i32: {}", self);
}
}
impl Processor for T where T: Display {
fn process(&self) {
println!("Processing generic: {}", self);
}
}
Debugging Expanded Code
If your binary size is still an issue in debugging, consider using cargo bloat, a tool designed to analyze where expansion occurs:
cargo install cargo-bloat
cargo bloat --release -n 10
This will give you insights on what items in your code contribute most to the binary's size so you can decide where to refactor.
Conclusion
While generics offer great flexibility and promote reusable code patterns in Rust, they can lead to large binary sizes, particularly in debug builds where this is less desired. By strategically using trait objects, limiting generic types, and specializations, one can manage and potentially reduce the size increase caused by code expansion. Moreover, tools like cargo bloat can assist developers to pinpoint and manage these expansions effectively.
