Cargo activates features by itself - rust

Question

I have a binary that depends on a library. In the library I specified these features:
[features]
default = []
fet = []
I would expect feature fet would ONLY be activated when it is compiled with the feature flag. In my binary the Cargo.toml looks like this:
[dependencies]
a = { path = "./a" }
[features]
default = []
I haven't specified the fet feature in the Cargo.toml, however this gives me a compilation error in my binary:
fn main() {
if cfg!(feature = "fet") {
compile_error!("not expected");
}
}
Why is feature fet still activated in my binary? I executed these commands and get the same error:
cargo run
cargo run --features=default
Ideally, I want my binary to activate certain features from dependencies if they are explicitly mentioned in the cargo run command (in the --features flag). Is this possible? I was hoping this would work in my binary Cargo.toml:
[features]
default = []
fet = ["a/fet"]
and feature fet would be activated if I ran this command:
cargo run --features=fet

Answer 1

The cfg! macro will be expanded to true or false depending on if the condition is true or false, so if the feature flag is not set, it will still expand to this:
fn main() {
if false {
compile_error!("not expected");
}
}
The compile_error! macro will still be invoked, so you'll get the compiler error either way.
The cfg! macro is mostly useful when you can rely on the compiler optimizing away branches that will never be taken, such as an if false branch. The code still needs to compile before optimization, though, so it's less useful when the code will only compile if the condition is true or false.
What you'd want to use instead is either a #[cfg] attribute or the cfg_if::cfg_if! macro:
// with #[cfg] attribute
fn main() {
#[cfg(feature = "fet")]
compile_error!("not expected");
}
// with cfg_if!
use cfg_if::cfg_if;
fn main() {
cfg_if!{
if #[cfg(feature = "fet")] {
compile_error!("not expected");
}
}
}