tracing-core/src/context_carrier.rs

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// http://www.apache.org/licenses/LICENSE-2.0
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/// The Injectable implementation supports inject the give key/value for further propagation,
/// Such as putting a key/value into the HTTP header.
pub trait Injectable {
    /// Inject the given key/value into the implementation.
    /// The way of injection is determined by the implementation, no panic! should happens even injection fails.
    fn inject(&self, key: String, value: String);
}

/// The Extractable implementations extract propagated context out the implementation.
/// Such as fetching the key/value from the HTTP header.
pub trait Extractable {
    /// Fetch the value by the given key.
    fn extract(&self, key: String) -> &str;
}