Rust
Features
Do conditional compilation with Cargo features.
Features
Features let you turn optional parts of a crate on and off. Unused code isn't compiled, so compile time and binary size shrink:
# Cargo.toml
[features]
default = ["metin"]
metin = []
json = ["dep:serde_json"]
[dependencies]
serde_json = { version = "1", optional = true }
The user chooses: fatura = { version = "1", features = ["json"] } or on the command line cargo build --features json.
Conditional compilation
Code looks at features with the #[cfg(..)] attribute or the cfg!(..) macro:
#[cfg(feature = "json")]
pub fn json_yaz(..) { .. } // compiled only if json is on
fn bicim() -> &'static str {
if cfg!(feature = "json") { "json" } else { "metin" }
}
#[cfg] removes the item entirely; cfg! is an expression that is true/false at compile time, and both branches must compile. The same mechanism is used for the operating system: #[cfg(target_os = "windows")].
Exercise Format choice
Write fn bicim() -> &'static str returning "json" or "metin" using cfg!(feature = "json"), and print the result as biçim: .... No features are enabled in the exercise environment.
Show example solution
fn bicim() -> &'static str {
if cfg!(feature = "json") {
"json"
} else {
"metin"
}
}
fn main() {
println!("biçim: {}", bicim());
}
Test yourself
-
Why should features be only <strong>additive</strong>?
- It isn't needed
- If two crates in the tree ask for different features, Cargo unions them; one feature must not break another
- It's a compiler rule
- For performance
Show answer
If two crates in the tree ask for different features, Cargo unions them; one feature must not break another — Removing something from the API or inverting behaviour when a feature is on can break another user's build.
-
Conditional item
#[____(feature = "json")]
pub fn json_yaz() {}
Show answer
#[cfg(feature = "json")]
pub fn json_yaz() {}
-
Syntax and meaning
- default = ["metin"]
- On by default
- optional = true
- Optional dependency
- #[cfg(feature = "x")]
- Conditional item
- cfg!(feature = "x")
- Compile-time bool
Next lesson: Workspace
