Conditionally apply funtime::timed

Cargo.toml

[package]
name = "sandbox"
version = "0.1.0"
authors = ["User Name <foo@bar>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[features]
profiling = ["funtime"]

[dependencies]
funtime = { version = "0.3", optional = true }

src_slash_main.rs

#[cfg_attr(feature = "profiling", funtime::timed)]
fn foo() {
    std::thread::sleep(std::time::Duration::new(1, 0));
}

fn main() {
    foo();
}

$ cargo run
$ cargo run --features profiling