Explicit template instantiation in Rust

3

Hm, I thought that this would actually just work. This means that my understanding of Rust's compilation model is wrong. I would very grateful if someone could explain what is wrong with my mental model :slight_smile:

Let's say I have two files/crates: container.rs and main.rs

// container.rs
use std::fmt::Display;

pub struct Container<T> {
    pub t: T
}

impl<T> Container<T> {
    pub fn print(&self) 
    where 
        T: Display
    {
        println!("{}", self.t);
    }
}

pub mod impls {
    use super::Container;

    pub fn make_i32_container() -> Container<i32> {
        Container { t: 92 }
    }
}

//main.rs
extern crate container;

use container::Container;

fn main() {
    let c: Container<i32> = Container { t: 92 };
    c.print();
}

And I compile then using the following two commands:

$ rustc --crate-type lib container.rs 
$ rustc main.rs --extern container=./libcontainer.rlib

I think that libcontainer.rlib contains (optimized, if --release is present) machine code or bit code for non-generic make_i32_container function. If this is true, it must contain code for Container<i32> instantiation as well.

Then, when we compile main.rs, it seems to me that the compiler can see that we already have the necessary instantiation, and avoid specializing Container for i32 the second time. Or are template instantiations per-crate, and then the duplicates get removed during linking, like with C++ inline?