Features
- Rust course
- Intermediate
- Lesson 80/115
- 4 min
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")].
Practice 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.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn bicim() -> &'static str {
if cfg!(feature = "json") {
"json"
} else {
"metin"
}
}
fn main() {
println!("biçim: {}", bicim());
}Expected output
biçim: metin
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- It's a compiler rule
- For performance
Explanation: Removing something from the API or inverting behaviour when a feature is on can break another user's build.
2 Conditional item
#[____(feature = "json")]
pub fn json_yaz() {}#[cfg(feature = "json")]
pub fn json_yaz() {}Explanation: #[cfg(..)] removes the item from the build if the condition fails.
3 Syntax and meaning
| Term | Meaning |
|---|---|
default = ["metin"] |
On by default |
optional = true |
Optional dependency |
#[cfg(feature = "x")] |
Conditional item |
cfg!(feature = "x") |
Compile-time bool |