Add SpanContext trait. (#10)

This changeset adds the SpanContext trait with support for
baggage item iteration.

Completes #6

.gitignore

@@ -1,2 +1,4 @@
-target/
+*.swp
 Cargo.lock
+
+target/

opentracing-api/src/context.rs

@@ -0,0 +1,46 @@
+/// SpanContext represents Span state that must propagate to
+/// descendant Spans and across process boundaries.
+///
+/// SpanContext is logically divided into two pieces: (1) the user-level "Baggage" that
+/// propagates across Span boundaries and (2) any Tracer-implementation-specific fields
+/// that are needed to identify or otherwise contextualize the associated Span instance
+/// (e.g., a `(trace_id, span_id, sampled)` tuple).
+pub trait SpanContext<'a> {
+    /// Associated type defining how to iterate over baggage items.
+    type Iter: Iterator<Item = (&'a String, &'a String)>;
+
+    /// Iterate over baggage items.
+    ///
+    /// Baggage items are key/value pairs that are propagated from
+    /// the associated `Span` throught the trace.
+    fn baggage_items(&'a self) -> Self::Iter;
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashMap;
+    use std::collections::hash_map::Iter as HashMapIter;
+    use super::SpanContext;
+
+    struct TestContext {
+        items: HashMap<String, String>,
+    }
+    impl<'a> SpanContext<'a> for TestContext {
+        type Iter = HashMapIter<'a, String, String>;
+
+        fn baggage_items(&'a self) -> Self::Iter {
+            self.items.iter()
+        }
+    }
+
+    #[test]
+    fn get_items() {
+        let mut items = HashMap::new();
+        items.insert("key".into(), "value".into());
+        let context = TestContext {
+            items: items
+        };
+        let items: Vec<(&String, &String)> = context.baggage_items().collect();
+        assert_eq!(items, [(&"key".into(), &"value".into())])
+    }
+}

opentracing-api/src/lib.rs

@@ -1,7 +1,9 @@
 #![doc(html_root_url = "https://docs.rs/opentracing-api/0.1.0")]
 
+mod context;
 mod tag;
 mod field;
 
+pub use context::SpanContext;
 pub use tag::{Tags, ParseTagsError};
 pub use field::{Fields, ParseFieldsError};