Rust Generic Function Size Trick

2024-07-01

When using generics in Rust (and any language that supports this), what happens under the hood is that the compiler generates a function implementation when it finds a call for each combination of different types used in the generic parameters.

This can produce a lot of code if the function body is big.

pub fn work_with_path<T: AsRef<Path>>(path: T) {
    // Here the body is small, but imagine this method has 100+ lines.
    println!("{}", path.as_ref().display());
}

For every call to work_with_path with a different type T a implementation is generated.

Instead, we can do this

pub fn work_with_path<T: AsRef<Path>>(path: T) {
    #[inline(never)] // inline is needed in this case because the inner method is small in the example.
    fn work_with_path(path: &Path) {
        println!("{}", path.display())
    }

    let path = path.as_ref();
    work_with_path(path);
}

And the big meaty function will only be generated once, while the code to turn the type T into a valid reference to a path will be whats generated multiple times, depending on the T. Which usually is a small part of the method.

Godbolt URL (using inline(never) on the outer functions to avoid inlining them in main)

The std does this in multiple places, like this one or this one

Example from the std:

#[stable(feature = "rust1", since = "1.0.0")]
pub fn with_extension<S: AsRef<OsStr>>(&self, extension: S) -> PathBuf {
    self._with_extension(extension.as_ref())
}

fn _with_extension(&self, extension: &OsStr) -> PathBuf {
    let self_len = self.as_os_str().len();
    let self_bytes = self.as_os_str().as_encoded_bytes();

    let (new_capacity, slice_to_copy) = match self.extension() {
        None => {
            // Enough capacity for the extension and the dot
            let capacity = self_len + extension.len() + 1;
            let whole_path = self_bytes.iter();
            (capacity, whole_path)
        }
        Some(previous_extension) => {
            let capacity = self_len + extension.len() - previous_extension.len();
            let path_till_dot = self_bytes[..self_len - previous_extension.len()].iter();
            (capacity, path_till_dot)
        }
    };

    let mut new_path = PathBuf::with_capacity(new_capacity);
    new_path.as_mut_vec().extend(slice_to_copy);
    new_path.set_extension(extension);
    new_path
}