Generic method cannot invoke specialized implementation?

Question

It is possible to make a call to a specific implementation of a generic method on a struct, ex:

impl Printer<i16> {
    pub fn run() { println!("Specific i16"); }
}

Printer::<i16>::run();

It is also possible to make a blanket, default implementation of that method, ex:

trait Blanket<T: Printable> {
    fn run() { println!("Blanket"); }
}

impl<T: Printable> Blanket<T> for Printer<T> {}

Why is it that attempting to call the specific implementation from a generic function does not work?

fn generic_run<T: Printable>() {
    Printer::<T>::run();
}

generic_run::<i16>::(); // Prints "Blanket"

Is there a way to allow a generic function to dispatch to specific implementations?

Playground

user4815162342 · Accepted Answer · 2025-11-21 08:03:59Z

Your "specific" implementation is just an inherent method on Printer<i16>, which has nothing to do with the method on Blanket. The fact that an inherent method shares name with a method defined by the trait won't make generic code that depends on the trait invoke the inherent method - Rust generics aren't "duck typed" like that. The feature that would enable¹ what you're looking for is called specialization, and is experimentally available in nightly Rust. Unfortunately the experiment has a number of issues and won't make it into stable Rust any time soon.

In the meantime, in simpler cases you can emulate specialization using TypeId::of(). For example, defining Blanket::run() like this results in the expected output of "Blanket" followed by twice "Specific - i16":

use std::any::{Any, TypeId};

// ...Printable and generic_run() defined as before...

trait Blanket<T: Printable> {
    fn run() {
        if TypeId::of::<T>() == TypeId::of::<i16>() {
            return Printer::<i16>::run();
        }
        println!("Blanket");
    }
}

Playground

In the above code you can expect the == check to be optimized away in release builds because, due to monomorphization, the compiler knows the type of T when generating the code for a specific Blanket::<T>::run().

¹ Even the unstable specialization still doesn't allow C++ style "duck typing" of generic code - see here or here for examples of how it works.

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