Does a compiled Rust executable exclude unused code from dependencies?

Question

If I build a Rust application using Cargo with some crate dependencies, will any code in those dependencies that is unused by my application be eliminated from the final executable?

Answer 1

It looks like it. I made a test lib and bin crate side by side:

// hellobin/src/main.rs

extern crate hellolib;

fn main() {
    hellolib::func1();
}

For the lib:

// hellolib/src/main.rs

pub fn func1() {
    println!("Hello, world!");
}

pub fn func2() {
    println!("Hello, other world!");
}

Building my binary and then inspecting symbols with nm:

$ nm target/debug/helloworld | grep hello
0000000100001360 t __ZN10helloworld4main17h749f61fb726f0a10E
00000001000014b0 T __ZN8hellolib5func117hec0b5301559d46f6E

Only the used function has a symbol in the final binary.

You can compile with cargo rustc -- -C link-dead-code though and you will see both symbols are present, including the unused one:

$ nm target/debug/helloworld | grep hello
0000000100001270 t __ZN10helloworld4main17h3104b73b00fdd798E
00000001000013d0 T __ZN8hellolib5func117hec0b5301559d46f6E
0000000100001420 T __ZN8hellolib5func217hc9d0886874057b84E

I believe (but I'm not sure) that it's the linker removing the dead code, so it may have still been compiled and then removed during linking.